Transforming Our Cities: Best Practices Towards Clean Air and Active Transportation

Abstract

Purpose of Review
By 2050, 70% of the global population will live in urban areas, exposing a greater number of people to specific city-related health risks that will only be exacerbated by climate change. Two prominent health risks are poor air quality and physical inactivity. We aim to review the literature and state the best practices for clean air and active transportation in urban areas.

Recent Findings
Cities have been targeting reductions in air pollution and physical inactivity to improve population health. Oslo, Paris, and Madrid plan on banning cars from their city centers to mitigate climate change, reduce vehicle emissions, and increase walking and cycling. Urban streets are being redesigned to accommodate and integrate various modes of transportation to ensure individuals can become actively mobile and healthy. Investments in pedestrian, cycling, and public transport infrastructure and services can both improve air quality and support active transportation. Emerging technologies like electric and autonomous vehicles are being developed and may reduce air pollution but have limited impact on physical activity. Green spaces too can mitigate air pollution and encourage physical activity.

Summary
Clean air and active transportation overlap considerably as they are both functions of mobility. The best practices of clean air and active transportation have produced impressive results, which are improved when enacted simultaneously in integrated policy packages. Further research is needed in middle- and low-income countries, using measurements from real-world interventions, tracing air pollution back to the sources responsible, and holistically addressing the entire spectrum of exposures and health outcomes related to transportation.

Full text

AIR POLLUTION AND HEALTH (S ADAR AND B HOFFMANN, SECTION EDITORS)
Transforming Our Cities: Best Practices Towards Clean Air and Active
Transportation
Andrew Glazener
1,2
&Haneen Khreis
1,3,4,5
#Springer Nature Switzerland AG 2019
Abstract
Purpose of Review By 2050, 70% of the global population will live in urban areas, exposing a greater number of people to
specific city-related health risks that will only be exacerbated by climate change. Two prominent health risks are poor air quality
and physical inactivity. We aim to review the literature and state the best practices for clean air and active transportation in urban
areas.
Recent Findings Cities have been targeting reductions in air pollution and physical inactivity to improve population health. Oslo,
Paris, and Madrid plan on banning cars from their city centers to mitigate climate change, reduce vehicle emissions, and increase
walking and cycling. Urban streets are being redesigned to accommodate and integrate various modes oftransportation to ensure
individuals can become actively mobile and healthy. Investments in pedestrian, cycling, and public transport infrastructure and
services can both improve air quality and support active transportation. Emerging technologies like electric and autonomous
vehicles are being developed and may reduce air pollution but have limited impact on physical activity. Green spaces too can
mitigate air pollution and encourage physical activity.
Summary Clean air and active transportation overlap considerably as they are both functions of mobility. The best practices of
clean air and active transportation have produced impressive results, which are improved when enacted simultaneously in
integrated policy packages. Further research is needed in middle- and low-income countries, using measurements from real-
world interventions, tracing air pollution back to the sources responsible, and holistically addressing the entire spectrum of
exposures and health outcomes related to transportation.
Keywords Cities .Air pollution .Active transportation .Clean air .Physical activity .Best practices .Public health
Introduction
Ambient air pollution (PM
2.5
) was the fifth-ranking mor-
tality risk factor in 2015 (after high systolic blood pressure,
smoking, high fasting plasma glucose, and high total cho-
lesterol), causing 4.2 million premature global deaths
(7.6%ofalldeaths)[1,2]. Over 90% of ambient air pollu-
tion fatalities occur in low- and middle-income countries,
mainly in Asia, Africa, and the Eastern Mediterranean re-
gion [1]. Those aged 50 and older are the most susceptible;
a trend that is particularly relevant as lifespans continue to
lengthen and adults aged 60 and older represent an increas-
ing proportion of the global population [1,3]. Additionally,
the associated premature mortality and morbidity reduces
countries’labor incomes and gross domestic product
(GDP) growth. In 2016, for example, the cost associated
with the burden of disease from ambient PM
2.5
worldwide
was USD$5.7 trillion: 4.4% of the global GDP [4].
This article is part of the Topical Collection on Air Pollution and Health
*Haneen Khreis
H-Khreis@tti.tamu.edu
Andrew Glazener
andrewglazener@gmail.com
1
Center for Advancing Research in Transportation, Emissions,
Energy, and Health (CARTEEH), Texas A&M Transportation
Institute (TTI), 2929 Research Parkway, 3135 TAMU, College
Station, TX 77843-3135, USA
2
Langford College of Architecture, Texas A&M University, College
Station, TX, USA
3
ISGlobal, Centre for Research in Environmental Epidemiology
(CREAL), Barcelona, Spain
4
Universitat Pompeu Fabra (UPF), Barcelona, Spain
5
CIBER Epidemiologia y Salud Publica (CIBERESP), Madrid, Spain
Current Environmental Health Reports
https://doi.org/10.1007/s40572-019-0228-1

Worldwide, trends of motor vehicle dependency have con-
tributed to the emission of greenhouse gases (GHGs) and air
pollutants, which adversely impact urban air quality. GHGs
produced by motor vehicle traffic further contribute to global
warming and climate changes that can independently exacer-
bate poor air quality and public health outcomes [5,6]. Motor
vehicle dependency has also resulted in physical inactivity for
many, incurring a significant burden of disease which is re-
sponsible for 3.2 million deaths (6% of all deaths) per year [7,
8]. With these emerging impacts and concerns, more transpor-
tation planners and public health officials have been collabo-
rating to integrate physical activity and active travel, in what is
known as active transportation, to reduce physical inactivity.
Active transportation decreases the risk of type 2 diabetes,
weight gain, respiratory disease, cardiovascular disease, cere-
brovascular disease, dementia, depression, obesity, cancer,
and impaired mental health all resulting in an increased risk
of premature mortality [9]. Active transportation could pro-
vide means to integrate physical activity habits into daily rou-
tines and can improve ambient air quality if modal shifts from
motor vehicles occur at a sufficient scale [10].
The mortality and disease burdens associated with ambient
air pollution and physical inactivity can have profound effects
on the quality of life and the social and economic progression
of cities and their citizens. The global urban population is
expected to grow by 2.5 billion people before 2050 (90% of
which will be in Asian and African cities), exposing more
people to the health burdens associated with urban air pollu-
tion and physical inactivity [11]. In this paper, we first over-
view the current status of air quality and active transportation
in cities and countries around the world alongside relevant
standards and recommendations. We then review the best
practices of clean air and active transportation to highlight
current interventions addressing poor air quality and physical
inactivity in cities. Most of the literature we source was pub-
lished within the last 5 years.Existing literature about clean air
and active transportation, however, is based on research from
Western countries. Many middle- and low-income countries
do not have extensive data sets on air quality; therefore, liter-
ature from these regions is currently sparse [12]. For this rea-
son, we mainly focus on Western countries, even though the
greatest urban growth and health risks are concentrated in
Africa, Asia, and Eastern Mediterranean countries [1,8,13].
Air Pollution
The Current State of Air Quality and Health Effects
In 2017, over 80% of the world’s urban population was ex-
posed to air pollution levels that exceeded World Health
Organization (WHO) recommendations [1]. Ninety-seven
percent of cities with populations greater than 100,000 in
middle- and low-income countries do not meet the WHO air
quality guidelines [14]. Globally, there is high variability in air
pollution levels across different cities, as shown in the WHO
air quality database which stores the most recently recorded
annual particulate matter (PM) means. In 2017, Bakersfield,
California, had the highest annual PM
2.5
mean—18.2 μg/
m
3
—in the United States (U.S.) [15]. In 2016, the highest
annual PM
2.5
means for the European Union (EU) were all
from Polish cities, ranging between 35 and 38 μg/m
3
[14].
Similarly, the ten most polluted cities in Asia were in India
and had annual PM
2.5
means ranging from 113 to 173 μg/m
3
in 2016 [16]. Most middle- and low-income countries have
just begun measuring urban air quality while high-income
countries have spent decades exploring a variety of methods
to measure and reduce air pollution. Air pollution in low-
income countries has been increasing while Western countries
have seen gradual decreases in air pollution, due, in part, to
clean air programs and policies that can provide insight on the
best practices to expedite air pollution reductions in areas with
worsening air quality.
A key contributor to ambient air pollution is road transpor-
tation. Motor vehicles are responsible for air pollution that
degrades urban air quality, specifically PM
2.5
,PM
10
, black
carbon (BC), ultrafine particles (UFP), nitrogen oxide
(NO
x
), and carbon monoxide (CO), each detrimental for hu-
man health [17–19]. These pollutants are released into the air
via exhaust pipes, physical decomposition of vehicle and road
parts, and resuspension caused by motor vehicle traffic. With
the placement of roads near homes, offices, and schools, an
increasing proportion of the population is near busy highways
and urban roads. Further, the number of registered vehicles
and vehicle miles traveled is increasing which results in more
people being exposed to air pollution and the riskof numerous
adverse health effects [20].
In conservative estimates, motor vehicle-related air pol-
lution causes 1% (184,000 deaths) of all air pollution-
related deaths worldwide. Ambient air pollution exposure
has been linked to premature mortality and cardiovascular
disease [21], stroke [22], respiratory diseases [21]andin-
flammations including lung cancer [23], pneumonia [24],
childhood asthma [25], and chronic obstuctive pulmonary
disease [26], as well as congenital anomalies [27], deep
vein thrombosis [28], type 2 diabetes [29], obesity [30],
autism and child behavior problems [31], dementia [32],
and pregnancy complications [33]. Road transportation in
Europe is responsible for 19% of CO, 36% of NO
x
,and
10% and 8% of PM
2.5
and PM
10
emissions, respectively,
and 30% of NO
x
, 10% of total PM, 54% of CO, and 14% of
CO
2
emissions worldwide, although the contribution of
road transportation to overall air pollution will vary de-
pending on traffic characteristics, site layout, and the pol-
lutant studied [17,34–36]. It is difficult to tease out the
contribution of traffic-related air pollution to overall
Curr Envir Health Rpt

ambient air pollution and associated health effects, but
some studies have attempted doing so in various ways.
For example, some studies use (proximity to) traffic flow
in a GIS to examine the health of individuals living in low
verus high traffic intensity areas [37,38]. Others used dis-
persion modeling to explicility model the contribution of
traffic to the overall air pollution and associated health
effects [39,40]. Similary, source apportionment methods
have been used to identify the contribution of traffic to
overall air pollution and associated health effects [41,
42]. Other studies consider certain pollutants, such as
NO
2
, as markers of traffic-related air pollutants. In com-
parison to the literature on the health effects of overall
ambient air pollution, studies specifically investigating
the contribution of different air pollution sources to specif-
ic health effects are less available. Looking forward, the
specific assessment of health effects of traffic-related air
pollution is important for policy decision-making as it
can inform effective mitigation policies [43].
Air Quality and Vehicle Emission Standards
Air Quality Standards
Periodically, the WHO will assemble a group of experts
from a variety of fields and institutions to review the
latest literature concerning the status of air quality and
health effects of air pollution and produce international
guidelines that aim to reduce air pollution levels to ensure
healthy living environments. The latest guidelines were
published in 2005 and are detailed in Table 1.Currently,
over 80% of the global population lives in areas where
these guidelines are not met, which combined with emerg-
ing evidence suggesting that there are health risks from
PM and ozone (O
3
) exposure at levels below the WHO
guidelines has prompted revision to the guidelines that are
expected to be published in 2020 [14,44–46]. The WHO
guidelines are stricter than air pollution standards in the
U.S. or EU, where the most progressive air quality stan-
dards are in place [47,48]. For this reason, we consider
the WHO air quality guidelines as the best available stan-
dards that all countries should aim for. In line with the
context of this paper, we next specifically discuss vehicle
emission standards and how they contribute to overall air
pollution reductions and compliance with the WHO air
quality guidelines.
Vehicle Emission Standards
Vehicle emission standards are in various stages of develop-
ment around the world. A recent review of the 19 G20 coun-
tries revealed how developing and underdeveloped countries
are lagging behind in their efforts to regulate GHG emissions
produced by motor vehicles [49]. Six of these countries—
Australia, Brazil, China, India, Mexico, and Russia—
currently employ outdated vehicle emission standards, while
adopting EU vehicle emission standards could reduce tailpipe
PM
2.5
emissions by 67% [49].
In Europe, vehicle emission regulations are based on
the Euro Standards that began in 1992. Since then, there
have been six updates, each more stringent on emissions
[50]. Although there has been significant progress in lim-
iting transportation emissions throughout Europe, about
90% of European city dwellers were still exposed to pol-
lution levels above WHO air quality guidelines in 2017
[51]. Vehicle emissions can be widely dispersed in the air
becoming a transboundary issue requiring collaboration
among neighboring countries and states. Following the
recent Volkswagen emissions scandal, a study found that
nine other vehicle manufacturers in Europe had been sell-
ing diesel cars that produced more tailpipe emissions of
NO
x
on the road than their laboratory tests reported [52].
The volume of excessive NO
x
emissions ranged from
14 gigagrams (Gg) in Sweden to 140 Gg in Germany
and France, causing a substantial health burden [52].
On the other hand, the U.S. Environmental Protection
Agency enforced The Clean Air Act (CAA) of 1970 to ad-
dress the entire spectrum of air pollution sources. The CAA
has produced automobile-specific standards to regulate vehi-
cle emissions in the U.S. which are currently in the “Tier 3”
phase that is set to last until 2025 [53]. A comparison of
vehicle emissions regulation in the U.S. versus Europe is
shown in Table 2.
Table 1 WHO air quality guidelines
Pollutant WHO guidelines
PM
2.5
10 μg/m
3
per cubic meter annual mean
OR
25 μg/m
3
24-h mean
PM
10
20 μg/m
3
annual mean
OR
50 μg/m
3
24-h mean
O
3
100 μg/m
3
8-h mean
Nitrogen dioxide (NO
2
)40μg/m
3
annual mean
OR
200 μg/m
3
1-h mean
SO
2
20 μg/m
3
24-h mean
OR
500 μg/m
3
10-min mean
CO 30 mg/m
3
1-h mean
OR
10 mg/m
3
8-h mean
Source: [37]
Curr Envir Health Rpt

Active Transportation
The Current State of Active Transportation
Walking and cycling are the most common forms of active
transportation, but public transportation can be considered ac-
tive transportation as walking or cycling typically bookend
public transportation trips [54]. Overall, the built environment
can dictate the suitability of various transportation modes in a
city. Key aspects of the built environment that are conducive
to active transportation are infrastructure separated from mo-
tor vehicle traffic, short travel distance, density, parks and
recreational space, and overall accessibility [55]. However, a
defining characteristic of urban growth over the past several
decades is sprawl. Urban sprawl describes the outward expan-
sion of cities and increases the distance between destinations,
reinforcing reliance on private motor vehicles. Sprawl is anti-
thetical to high-density development which is instrumental in
the utilization of active transportation [56]. Beyond the built
environment, other factors such as price, comfort, and timeli-
ness are conducive to active transportation use [57,58].
Active Transportation Guidelines and Policies
As previously mentioned, physical activity can be routinely
attained via active transportation. The WHO recommends at
least 150 min of moderate-intensity physical activity through-
out the week, or at least 75 min of vigorous-intensity physical
activity throughout the week, or an equivalent combination of
moderate- and vigorous-intensity activities, for adults, to re-
duce serious health risks [7]. The attainability of this recom-
mendation varies around the globe: South East Asia hosts the
lowest percentage of physically inactive adults (15% of men
and 19% of women), while 50% of women and 40% and 36%
of men fail to meet physical activity recommendations in the
Americas and Eastern Mediterranean regions, respectively
[8]. Globally, 31% of adults are not physically active for
150 min/week [8].
Because the built environment, and its respective transpor-
tation infrastructure, largely dictate transportation mode
choices, active transportation rates reflect the urban planning
and design policies that mold the built environment. There are
four levels of policy interventions that influence active trans-
portation: society-level policies (reducing speed limits,
restricting car use, or limiting parking spaces to discourage
motor vehicle travel), city-level policies (land use design
changes to increase density and walking and cycling), route-
level policies (providing the required infrastructure to enable
active transportation), and individual-oriented policies (edu-
cational initiatives to encourage changing travel behaviors).
To elicit significant behavioral change, interventions must be
made in each of these areas [59].
Intersection Between Active Transportation
and Clean Air
Clean air and active transportation overlap considerably as
they are both functions of mobility. Shifting from pollution-
intensive energy generation and transportation modes to ac-
tive transportation will result in air quality and physical activ-
ity benefits that contribute to healthier urban environments
[45]. A systematic review evaluating Health Impact
Assessments (HIA) of transportation mode shifts reported
overall air quality benefits due to reduced motor vehicle use
but also found increased risk from air pollution exposure dur-
ing active transportation [60]. However, these results can be
context specific, as described below.
Risk-Benef it Trade-off
Many studies addressed the risk-benefit trade-off of additional
air pollution exposure due to active transportation. Two stud-
ies concluded that the benefits of physical activity from active
transportation outweigh the risks of air pollution exposure in
most scenarios [9,61]. Less than 1% of cities globally have
ambient PM
2.5
levels (95 μg/m
3
)highenoughtooffsetthe
physical activity benefits of cycling for 30 min a day, accord-
ing to the WHO air quality database [61]. One of the most
polluted cities in the database was Delhi (153 μg/m
3
PM
2.5
)
where an individual could bike for up to 45 min each day
before air pollution exposure health risks surpassed physical
activity benefits [61]. At the global annual urban background
PM
2.5
mean (22 μg/m
3
), individuals could walk or cycle for
16and7heachday,beforetheynolongerreceivephysical
activity benefits due to air pollution exposure [61].
Differences in Transportation Mode Exposure
A study of air pollution exposure based on transportation
mode in cities in the UK, Belgium, Netherlands, Spain,
Ireland, and Switzerland concluded that motorists are exposed
to more PM
2.5
, BC, and CO than both cyclists (20%, 70%, and
90%) and pedestrians (40%, 300%, and 300%)—who are the
Table 2 Comparison of vehicle emission regulations per pollutant
Vehicle emission pollutants USA European Union
Nitrogen oxide (NO
x
) 0.04 g/mi 0.06/0.08* g/mi
Carbon monoxide (CO) 2.61 g/mi 1.0/0.5* g/mi
Carbon dioxide (CO
2
) 155 g/mi 130 g/mi
Particulate matter (PM) 0.003 g/m 0.008 g/mi
Non-methane organic gases 0.06 g/mi 0.07/na* g/mi
Sources: [39,41]
*Petrol/diesel fuels
Curr Envir Health Rpt

least exposed [62]. However, a study of Sacramento,
California, found that cyclists have the greatest exposure to
PM
2.5
, BC, and UFP per mile [63]. The results from these
studies highlight the importance of study context as air pollu-
tion exposure varies considerably between transportation mi-
croenvironments and between regions/cities. [64•]reviewed
39 studies that investigated the relationship between transpor-
tation mode and air pollution exposure and concluded that
motorists consistently experience the highest exposure to air
pollutants including CO, NO
2
, BC, and PM. However, active
transportation users, mainly cyclists, have greater ventilation
rates than motorized transportation commuters and therefore
inhale more air pollutants. Despite inhaling more air pollution,
a study of air pollution exposure in Montreal concluded that
cyclists burn 3.63 times as many calories as motorist and reap
physical activity benefits [65].
An important determinant of air pollution exposure for ac-
tive transportation users is the time of day and route chosen to
travel. In Sacramento, California, 15–75% of air pollution
exposure was influenced by the time of day and route choice
[63]. In Boston, Massachusetts, BC and NO
2
concentrations
were 33% higher near street bike lanes compared to bike lanes
that were distanced from motor vehicles, with a reduction of
BC and NO
2
concentrations by 3.4% and 11.6%, respectively,
as vegetation density increased [66]. Urban background sites
in many major European cities consistently presented lower
air pollution concentrations than main traffic arteries, with
fluctuations recorded at different day times [62].
Best Practices
The status of air quality and active transportation in cities and
countries around the world, as well as their overlap, has been
summarized in the previous sections. The policies and prac-
tices detailed in the subsequent sections are considered the
best practices for achieving clean air and improved active
transportation, each being identified from literature published
within the last 5 years. These practices—car-free policies, ve-
hicle technologies, urban design interventions, green space
provision, and public transportation provision—are rarely mu-
tually exclusive and can make significant additive improve-
ments when enforced simultaneously.
Car-Free Policies
Car-free policies aim to restrict the use of motor vehicles in
cities. Some policies ban the use of motor vehicles in certain
areas, while others may temporarily close streets so that only
pedestrians and cyclists can use them. Other methods to re-
strict the use of cars include road pricing, environmental
zones, and taxation which are meant to disincentivize the
use of motor vehicles. The intentions of these policies vary
as some are designed to reduce specific pollutants, while
others may focus on reducing GHGs or limiting traffic con-
gestion. While the air quality effects of these policies inher-
ently overlap, their success is variable and the extent to which
they encourage active transportation also fluctuates, as will be
overviewed next.
The idea of a “car-free city”is revolutionary considering
how drastically automobiles have altered the urban landscape,
yet several European and Asian cities aim to achieve a car-free
city [67]. The C40 Cities Climate Leadership Group has pro-
duced a pledge to transition to zero emission vehicles while
developing cities in a way that reduces the use of vehicles
overall [68]. As a member of the C40 initiative to reduce fossil
fuel reliance and the Carbon Neutral Cities Alliance, Oslo,
Norway, expects to boast a car-free city center within the next
year and hopes to achieve a 95% reduction in GHG emissions
by 2030 [69,70]. The removal of private vehicles in Oslo
could eliminate a significant portion of GHG (60% of GHG
emissions in Oslo are produced by road transportation) and
other air pollutants, as seen in several European cities already
[70]. For example, NO
2
concentrations decreased by 40% on a
car-free day in Paris and 20% in Leeds during a leg ofthe Tour
de France [67]. Other cities have already successfully imple-
mented various degrees of car restrictions. Copenhagen and
Brussels are home to the first and second largest car-free areas
in Europe while Basel has its historic medieval city center car-
free [71]. Paris and Madrid have made permanent bans on
diesel vehicles and placed other restrictions on private vehi-
cles [71]. Berlin and other German cities have utilized low-
emission zones that restrict the use of vehicles which do not
attain specific emission standards in certain areas. Mexico
City has been enforcing car-free days, a method that
Nairobi, the world’s second most congested city, has proposed
as a solution to its traffic and air pollution problems [71,72].
London is preparing to enlist a ban on diesel vehicles starting
in 2020 and San Francisco is converting one of its busiest
streets into a car-free space [71].
Other car-free policies may be more restricted in their spa-
tial and temporal coverage but can nevertheless improve air
quality and serve as a first step in an incremental process for
change and public acceptance. In Bogota, Colombia, 75 miles
of roads, in addition to the existing 200 miles of bike-only
lanes, are closed to cars weekly in an event known as
Ciclovia. Participants in Ciclovia are three times as likely to
meet physical activity requirements compared to the rest of the
population in Bogota, when measuring leisure time physical
activity [73]. There are now over 400 Ciclovias around the
world due to the initial success of the event in Bogota [74]. In
Los Angeles, California, on-road UFP and PM
2.5
concentra-
tions were decreased by 21% and 49%, respectively, during
CicLAvia 2014 [75]. Alternatively known as “open street”
events throughout the U.S., the physical activity and health
benefits of bike-only activities are typically limited due to
Curr Envir Health Rpt

their sparse occurrence throughout the year due to funding and
permitting issues [76]. Such events show promise and their
popularity has grown with 75% of open street programs in the
U.S. being conceived after 2010 and average several thousand
participants [76].
In addition to car-free policies, there are interventions that
can reduce car usage without explicitly banning the use of
motor vehicles. Berlin, Germany, has established an “environ-
mental zone”that is only accessible by low-emission vehicles
[71]. Cities across Germany have established environmental
zones, some of which were studied and found to decrease
measured annual mean PM
10
by an additional 2 μg/m
3
com-
pared to the reductions in PM
10
outside the environmental
zones [77]. These environmental zones, however, were typi-
cally focused on reducing concentrations of PM. Their effect
on other pollutants, like NO
2
, has largely only been modeled,
so while reductions in vehicle emissions can be expected, the
extent of real-world vehicle emission reductions is relatively
unclear [67].
Road pricing is another measure implemented in cities
across Europe and throughout other regions of the world to
deter the use of motor vehicles. There are different methods of
road pricing, notably Congestion Charge Zones (CCZ) and
distance-based road pricing. The effects of each method can
vary and are influenced by the tax levied, but there are mea-
surable reductions in air pollution [78]. A CCZ in London
successfully reduced CO
2
emissions by 19%. Distance-
based road pricing is less common but was estimated to pro-
duce larger CO
2
emission reductions (36%—Cambridge, UK)
[78]. A modeled CCZ in Beijing could reduce CO and NO
x
emissions by 60% and 35%, respectively [79].
While road pricing aims to discourage travel, parking pric-
ing aims to increase the efficiency of traffic flow. Estimates
attribute 30 to 50% of vehicle traffic in urban downtowns to
vehicles cruising for parking [80]. This activity not only con-
tributes to traffic congestion and longer travel time but in-
creases vehicle miles traveled and the pollution footprint of
motor vehicles as well. In San Francisco, SFpark is a parking
pricing system that manages the availability of on-street
parking spaces. As the occupancy of on-street parking spaces
increases,so does the price per hour whereas if parking spaces
are considerably unoccupied, the price per hour decreases.
After monitoring 256 street blocks for a pilot period of 2 years,
the adjusted parking rates managed to keep parking occupan-
cy below 80%, reducing cruising by more than 50% [80]. Pay-
to-park schemes that enable smoother traffic flow can provide
air quality benefits and also open streets to active transporta-
tion modes.
A third financial tool that has successfully improved air
quality is taxation. In 2007, Norway reformed its vehicle tax
structure [81]. The tax was based on the level of CO
2
emis-
sions per kilometer, instead of engine size. This incentivized
the purchase of cars that were more fuel efficient and resulted
in a shift away from high-polluting vehicles. Overall, this tax
reform decreased the market share of high-pollution vehicles
by 12% and the CO
2
intensity per vehicle was reduced by
6gCO
2
/km [81]. Taxes can also be applied to fuels, as was
modeled in a study of the EU in 2015. By increasing excise
taxes for diesel vehicles to equal excise taxes for petrol vehi-
cles, CO
2
,NO
x
,andPM
2.5
could be reduced by up to 14%,
17%, and 22%, although the impact of this scheme depends
on the disparity between diesel and petrol excise taxes [82]. If
a tax was levied based on CO
2
content of fuels (50€/tCO
2
),
reductions are more evenly spread and CO
2
,NO
x
,andPM
2.5
were decreased by an average of 8.5%, 9%, and 10% across
the EU [82]. Taxation is a tool that is largely dependent on
public and political support, however, which in the past has
proven to be an obstacle for vehicle and fuel tax advocates
[82].
Vehicle Technologies
Vehicle technologies can also play an important role in im-
proving urban air quality, although there is evidence that the
scale of adaptation needs to be wide for benefits to occur [83].
This section overviews emergent and disruptive technologies
that may make an impact on air quality and active transporta-
tion in urban areas. Specifically, this section discusses electric
and autonomous vehicles and how they might change trans-
portation systems and emissions.
Regarding urban air quality, the benefits of electric vehicle
(EV) and autonomous vehicle (AV) technologies are more
specific to GHG. Studies modeling EV market penetration
have projected reduced on-road emissions of CO
2
in China
by 20%, CO and NO
x
in Taiwan by 85% and 27%, CO in
Ireland by 14%, and NO
2
and NO
x
in Milan by 5.5% and
14.1%, respectively [17]. Many models, however, do not ad-
dress the health impacts of these reductions, and the GHG
reductions that EVs provide by replacing motor vehicles will
be offset by the GHGs that result from increased electricity
generation rendering little, if any, change in air quality and
health outcomes [84]. This zero-sum reality of GHG reduc-
tions associated with EVs is reliant on how electricity is de-
rived [17]. In the U.S., electricity generation produces 28% of
all GHG emissions and to meet the energy demand of a large
EV fleet would likely result in shifting the current burden of
GHG emissions from the transportation sector to the energy
sector [85]. However, there are countries leading the way in
producing clean energy to electrify their vehicle fleet. EVs
compose 19% of the passenger vehicle market in Norway,
the largest market share in the world. This was achieved
through a variety of financial incentives like limiting road
taxes and tolls and exempting EVs from purchase and parking
fees [86]. Norway can also rely on clean hydroelectric energy
as all 70,000 EVs are powered almost exclusively by hydro-
electricity (responsible for 96% of Norway’selectricity)[87].
Curr Envir Health Rpt

While tailpipe emissions can be reduced with the above
technologies, non-exhaust emissions, in the form of PM (road
surface, brake, and tire wear) and the production of batteries,
have been suggested to increase upon adoption of EVs [88,
89]. Overall, the effects of EVs on air quality are understudied
and non-exhaust pollutants could potentially worsen the situ-
ation, despite potentially reducing transportation GHG emis-
sions [6].
On the other hand, AVs propose an alternative future for
transportation and city planning. Currently, there are limita-
tions to the deployment of AVs, but there is potential for re-
duction in vehicle emissions upon AV integration. A system-
atic review of the potential air pollution and public health
implications of AVs estimates fuel consumption reductions
ranging from 11 to 47% based on differing systems of optimal
acceleration and deceleration, connected vehicle-to-
infrastructure technology, road designs, and AV penetration
in the vehicle fleet [90•]. An integration model of battery-
powered AVs shows GHG emission reductions of ~ 90%
and ~ 75% per mile when replacing internal combustion mo-
tor vehicles and hybrid EVs due to the reduced GHG intensity
of battery-powered vehicles [90•]. These scenarios, however,
are quite optimistic, and the adoption of both technologies
(EV and AV) are likely to lag in underdeveloped and rural
areas, resulting in differential impacts, potentially by socio-
economic status. Further, the impact of vehicle technologies
on physical inactivity is limited and understudied.
Urban Design Interventions
An important influence on active transportation and the
resulting physical activity is the built environment. In many
Western countries, sprawl and suburbanization occurred after
World War II. As the economy boomed in the U.S., private
vehicles became more widely available, families moved to
suburbs, and low-density development followed as extensive
networks of highways allowed longer commutes. In Europe,
vehicles and highways provided a similar result to that in the
U.S. European cities that were rebuilding after World War II
replaced their dense pre-automobiles cores with spacious ur-
ban designs conducive to automobiles that limited active
transportation. The following paragraphs about urban design
interventions will discuss how land use policies and infra-
structure development influence transportation mode choice,
physical activity, and air quality.
The legacy of sprawling development can be seen in
Europe where 50% of passenger car trips are shorter than
5 km, many of which could be shifted to active transportation
modes if densification, road safety, and accessibility improve
[91]. A study of 14 cities in ten countries concluded that up to
89 min of physical activity per week (60% of the WHO rec-
ommendation for weekly physical activity) can be attributed
to the built environment design [92•]. In Melbourne,
Australia, built environment characteristics that influence ac-
tive transportation behavior were identified. A housing densi-
ty of > 20 dwellings/ha reduced the likelihood of taking a
private vehicle trip by 45%. Mixed land use and a connected
street network that provided access to at least nine local living
destinations decreased the use of motor vehicles by more than
50%. Living within 500 m of a supermarket alone reduced the
use of motor vehicles by more than 25% [93].
Infrastructure improvements that focus on safety and ac-
cessibility were also reasoned to encourage active transporta-
tion in six European cities that were included in a HIA that
focused on the health benefits derived from increased active
transportation [94•]. The health benefits of walking and cy-
cling in Paris, Basel, Barcelona, Prague, Copenhagen, and
Warsaw were modeled based on improved walking and cy-
cling rates. The improved rates used were equivalent to the
highest rates of walking and cycling among the six cities: 50%
of trips by walking in Paris and 35% of trips by cycling in
Copenhagen. Shifting 35% of trips to cycling resulted in 113
(Warsaw) to 5 (Basel) less annual deaths, while reductions in
mortality due to shifting 50% of trips to walking ranged from
19 (Warsaw) to 3 (Barcelona) annual deaths. These mode
shifts also produced cumulative GHG reduction co-benefits
ranging from 2503 (Basel) to 26,423 (Warsaw) tons of CO
2
emissions avoided in each city annually [94•]. Furthermore, a
HIA of 167 European cities finds that there is a positive cor-
relation between the expansion of cycling networks and rider-
ship. By expanding the cycling network to 315 km/100,000
people, cycling could account for 24.7% of all trips,
preventing10,000 premature deaths, with the greatest benefits
occurring in cities that had underdevelopedcycling infrastruc-
ture [95].
While both modeled HIAs have calculated the health im-
pacts of modal shifts, the method of urban design to encourage
such a shift is a separate issue that needs to be addressed. To
encourage active transportation in metropolitan Barcelona, the
development of superblocks has made walking and cycling
safer for residents [96]. Superblocks are proportioned city
blocks oriented in a 3 × 3 grid meant to improve the environ-
mental quality of neighborhoods. Main roads that allow a
maximum speed of 50 km/h form the perimeter of super-
blocks, while the interior roads that dissect the grid into nine
blocks do not allow vehicles to travel across superblocks and
restrict speeds to 20 km/h. Reduced traffic speed and overall
congestion has decreased vehicle emissions and noise levels
within superblocks. Since 2007, measurements of walking
and cycling have shown increases of 10% and 30% while
traffic within superblocks has dropped off by 26% in the
Barcelona neighborhood of Gracia and Vitoria-Gasteiz
[97]. The superblocks are part of a larger Urban Mobility
Plan in Barcelona, which aims to reduce motor vehicle
traffic by 21% in order to achieve a 40% reduction of
CO
2
emissions by 2030 [98].
Curr Envir Health Rpt

By increasing infrastructure to encourage the use of active
transportation, integrating transportation modes becomes
more feasible and can further increase accessibility. In the
U.S., “complete streets”are being implemented to redesign
the built environment in a way that is amenable to diverse
mobility. Over 1200 complete street policies have been
adopted and their success increases as the programs grow.
Ninety-five percent of all complete street policies in the U.S.
were designed to be accessible by all abilities and ages via
walking, cycling, and public transportation [99]. Public trans-
portation use increased 35% on a redesigned complete street
in San Francisco while a repurposed road in Seattle has ac-
commodated 35% more cyclists [100]. Proximity to a com-
plete street in Salt Lake City, Utah, has a positive correlation
to participation in active transportation with those living clos-
est to the complete street, engaging in 50% more walking trips
than subjects who lived further [101]. A comparative study of
six streets in Los Angeles showed UFP and PM
2.5
concentra-
tions were decreased by 7% and 2%, respectively, in complete
street environments due to decreased motor vehicle traffic
[102].
Green Space Provision
Green spaces, land that contains parks, forests, nature re-
serves, and/or waterbodies, have been associated with physi-
cal activity, reductions inair pollution, and overall health ben-
efits [103–105]. This section discusses how cities have man-
aged and placed green spaces to benefit air quality, physical
activity, and active transportation by providing healthy and
safe places for transportation and recreation.
While the effect of green spaces on concentrations of
gaseous pollutants is often limited, the removal of PM is
considerable [106]. In Strasbourg, France, green spaces
remove about 7% of PM
10
emissions in the city annually
[107]. A study of various green infrastructure interventions
determined that trees provided the greatest reprieve of am-
bient air pollution in Melbourne, Australia, when com-
pared to green walls and green roofs. When increasing
the density of trees to 80 trees per hectare, the annual up-
take of NO
2
(964 kg), SO
2
(125 kg), PM
10
(1474 kg),
PM
2.5
(43 kg), CO (10 kg), and O
3
(1885 kg) was in-
creased [108]. A systematic review of studies on the effect
of trees on PM
2.5
removal concluded there is a reduction of
5μg/m
3
of PM
2.5
for 10.2 million people throughout the
245 cities included in the review [109]. In Hamburg,
Germany, 16.8% of the urban area consists of parks, rec-
reation areas, or woodland justifying its claim as one of the
greenest cities in Europe. There are plans to expand green
space to cover 40% of Hamburg, accessible by foot and
bike exclusively, by 2035 to reduce motor vehicle accessi-
bility and the city’sCO
2
footprint by 80% in the next
30 years [67].
For urban dwellers, green space offers a retreat from the
clamor of cities [110]. There is an emerging field of study on
the mental health benefits of commuting through green
spaces. A positive correlation between active transportation
through green spaces and self-reported mental health was
identified by one study, but further research is needed to cor-
roborate these findings [105]. However, associations between
green spaces and public health are not a simple causal rela-
tionship, but are products of a matrix consisting of function-
ality, size, location, and aesthetic [110]. Green space has been
shown to encourage active transportation only if the appropri-
ate infrastructure is in place and safety is perceived [111].
Several studies have shown that the quality (aesthetics and
amenities) determines the utility of green space more than
the sheer quantity (placement) [110,112].
Public Transportation
As mentioned at the outset of this paper, public transportation
often entails active transportation due to the likelihood that
individuals walk or cycle to fulfill the first- and last-mile por-
tions of their travel. The practices and policies contained in
this section focus on the physical activity and air quality ben-
efits of improved public transportation provision and
utilization.
A study in the UK shows that a higher percentage of public
transportation users (almost four times as high) met public
health physical activity guidelines than car commuters [113].
In Montreal, public transportation users burn 1.73 as many
calories during commute than car commuters [65]. Public
transportation users in England average 16 (bus users) to 28
(train users) min of physical activity daily, and one-fifth (bus
users) to a half (train users) of these users achieve 30 min of
physical activity daily [114]. Two studies in the UK focused
on changes in body mass index (BMI), an increase of which
poses a risk of heart disease, diabetes, and certain cancers due
to shifts from active and public transportation to private vehi-
cles and vice versa [115,116]. In both studies, BMI increased
by ~ 0.33 kg/m
2
for those who reduced active and public
transportation usage while BMI decreased ~ 0.31 kg/m
2
for
those who adopted new active and public transportation com-
muting habits.
The benefits of public transportation utilization go beyond
physical activity as air quality benefits can result from the
reduced motor vehicle usage [117]. A new public transporta-
tion system in Thessaloniki, Greece, was shown to reduce air
pollution-related deaths by 20% due to the reduction of mobile
air pollution sources [84]. In the U.S., heavy rail systems, light
rail systems, and buses produce 76%, 62%, and 33% less
GHG per passenger mile than a single occupancy vehicle
[118]. A study of public transportation strikes in Barcelona
on 208 days between 2005 and 2016 showed a 4.1% and
7.7% increase in NO
x
and BC concentrations, respectively,
Curr Envir Health Rpt

in the absence of public transportation [119]. A similar study
observed 71 public transportation strikes in Germany from
2002 to 2011 and found a 14% and 4% increase of PM
10
and NO
2
air pollution concentrations on the days with strikes
[120].
Integrated Policy Packages
This final section focuses on policy packages that incorporate
different interventions from the previous sections. By enacting
a variety of policies simultaneously such as improved vehicle
technologies, restricted vehicle use, improved active and pub-
lic transportation infrastructure, air pollution and physical in-
activity are addressed more holistically and effectively than if
they were being addressed with a single policy tool and great-
er health benefits are expected.
A study of over 20 policies enacted in California between
1993 and 2012 to reduce emissions from transportation and
other sources showed considerable success. Emissions of NO
x
were reduced by 54%, PM
2.5
by 21%, and PM
10
by 15%,
despite a 22% and 37% increase in population and motor
vehicle miles traveled, respectively [121]. In 2008, Beijing
installed several control measures to improve air quality for
the Olympics. These included updating vehicle emission stan-
dards, reducing the number of government vehicles driven,
banning all diesel and heavy-duty trucks from operating in
Beijing, license plate bans, closing gas stations, and improv-
ing public transportation service. The mean concentration for
each targeted pollutant was substantially reduced: SO
2
by
60%, CO by 48%, PM
2.5
by 27%, and NO
2
by 43% [122].
Another city that is altering its transportation landscape by
enacting integrated policy packages is Copenhagen,
Denmark. Copenhagen has reduced carbon emissions by
40% since 1990, despite a population increase of 50%, and
aims to be carbon neutral and fossil fuel free by 2050 [123].
This progress has been achieved through a combination of
car-free policies and investment in active transportation infra-
structure. A bike share system, segregated biking lanes, ade-
quate bike parking facilities, and a “Green Wave Route”(cy-
clists traveling 12.4 mph will hit all green lights) have resulted
in 35% of trips taken by bicycle within the city. Almost 50%
of all trips are completed via active transportation and further
development of bicycle-exclusive lanes are expected to in-
crease cycling by another 15–20% [124].
Finally, Woodcock et al. set up a model of health impacts of
various urban transportation scenarios in London and Delhi.
Using a comparative risk assessment, the change in physical
activity, exposure to air pollution, and risk of traffic injury in
each scenario were used to quantify health benefits in both
cities [125]. The authors investigated three scenarios: lower-
carbon-emission motor vehicles, increased active travel, and a
combination of the two. The results showed that the third
scenario (the combination of active travel and lower-
emission motor vehicles) would yield the largest health bene-
fits preventing 7439 disability-adjusted life years in London
and 12,995 inDelhi, mainly from a reduction in the number of
years of life lost from ischemic heart disease (10–19% in
London, 11–25% in Delhi) [125]. Alone, the increased use
of lower-emission motor vehicles scenario resulted in the least
health benefits [125].
Conclusion and Future Research Directions
The attempt to curtail air pollution has been an evolving issue.
Despite air quality improvements, road networks are still
expanding, and car ownership is increasing globally, exacer-
bating air pollution and physical inactivity in cities. The ne-
cessity for interdisciplinary collaboration to ensure that future
city expansion is done sustainably has become an important
focus area of recent research and practice. Some cities are
trying to reduce urban air pollution by making active transpor-
tation more accessible and feasible for citizens while others
have undertaken projects to alter the built environment to be
amenable for pedestrians and cyclists through increased safety
and accessibility measures. Several more cities have enlisted
bans or taxes on vehicles and fuels to improve air quality and
encourage modal shifts to walking, cycling, or public trans-
portation (the entire list of policies discussed in this paper are
outlined below in Table 3). The positive health impacts of
these measures can be maximized by their integration (or bun-
dling in policy packages) but will not be immediate and may
take years to be fully realized. Their successes, however, will
add momentum to a global movement to reduce car depen-
dency and make cities healthier and more active.
Future research can usefully focus on addressing the fol-
lowing gaps in the literature:
&There is a lack of research in middle- and low-income
countries on air pollution and physical inactivity. Ninety
percent of future urban growth will be in Asia and Africa,
necessitating research in these regions.
&There are no global guidelines for active transportation
like there are for air pollution. Accountability studies have
been published regarding air pollution policies, but ac-
countability studies focusing on active transportation prac-
tices could encourage improvements in active transporta-
tion and provide a base for active transportation
guidelines.
&There is an overall lack of measurement studies that quan-
tify the active transportation impacts of green space pro-
vision. Understanding the relationship between the two
could allow for more informed land use decisions and
provide health benefits.
&The air quality impacts of complete streets are not exten-
sively researched. Understanding the air quality impacts
Curr Envir Health Rpt

Table 3 Examples of practices to improve air quality and support active transportation in cities
Policy Primary target Improves air
quality?
Supportive of active
transportation?
Overall impact
Air quality and vehicle emission standards
WHO air quality standards [32] To set a universal standard for air quality
to protect public health.
Yes No Recommendations for urban air quality regarding specific pollutant
concentrations.
Vehicle emission standards [35] Limiting emissions from motor vehicles to
protect the environment and public
health.
Yes No Reduced air pollution from motor vehicle traffic.
Clean Air Act of 1970 [35] Legislation in the USA that focused on
researching and reducing air pollution.
Yes No Reduced air pollution from motor vehicle emissions in the USA.
Active transportation guidelines
WHO physical activity guidelines [4] To set a universal standard for physical
activity levels.
Maybe Yes The WHO set forth physical activity guidelines to encourage healthy
behaviors in individuals. This can overlap with active transportation
invoking air quality benefits simultaneously.
Car-free policies
Car-free policies [51] To reduce motor vehicle activity. Yes Yes Restricting motor vehicle travel in specific areas within a city reduces
vehicle emissions and promotes walking, cycling, and public
transportation use.
Ciclovia [53] To promote cycling within a community. Yes Yes Closure of streets to motor vehicles 1 day each week in Bogota,
Colombia, promotes cycling and reduces air pollution.
Modal shifts [42] Provide an alternative mean of
transportation to a motor vehicle.
Yes Yes Benefits of physical activity from active transportation outweigh the
risks of air pollution exposure in most scenarios and can reduce the
use of motor vehicles.
Open streets [106] Close road access to cars to encourage
individuals to walk and cycle for
transportation.
Yes Yes Closing urban roads to motor vehicle traffic and encouraging travel by
walking and cycling provides health benefits through physical
activity and air quality improvements.
Environmental/lowemissionzones[52] Restricting the use of highly polluting
vehicles.
Yes No Restricting access to specific areas of the city to motor vehicles that do
not meet stringent emission standards.
Road pricing [60] A financial disincentive for traveling via
motor vehicle.
Yes Maybe Operational tax for motor vehicles driving on designated roadways
which can reduce personal motor vehicle use and increase active and
public transportation use.
Parking pricing [62] A financial disincentive for traveling via
motor vehicle.
Yes Maybe Pay-to-park schemes managed on-street parking availability, reducing
the amount of time spent cruising for parking which accounts for a
significant portion of motor vehicle traffic in dense urban areas. Can
reduce personal motor vehicle use and increase active and public
transportation use.
Vehicle or fuel taxation [63] A financial disincentive for traveling via
motor vehicle.
Yes No Levying taxes that apply to GHG emission-intensive vehicles to
incentivize the purchase of fuel-efficient vehicles. Additional taxation
mayoccurbasedontheCO
2
content of fuel.
Vehicle technologies
Electric vehicles To eliminate motor vehicle exhaust
pollution.
Maybe No Reduce tailpipe emissions and associated air pollution but scale of
adaptation needs to be wide and energy sources need to be clean for
benefits to occur. Non-exhaust emissions might increase.
Autonomous vehicles Optimize driving styles for fuel efficiency. Maybe No Efficient driving patterns may increase fuel efficiency and could reduce
thenumberofvehiclesontheroad.
Urban design interventions
Built environment design [74,126•]MaybeYes
Curr Envir Health Rpt

Tab le 3 (continued)
Policy Primary target Improves air
quality?
Supportive of active
transportation?
Overall impact
To make active modes of transportation
more safe and sustainable.
Dense and mixed-use development is conducive to improved access to
nearby destinations, increasing the utilization of active transportation
modes to travel.
Investment in infrastructure to improve
safety and accessibility by walking
and cycling [106]
Promote active transportation. Yes Yes Perceived safety and appropriate accessibility increase the ease of
shifting travel to active modes instead of motorized transportation.
Segregated travel routes for pedestrians
and cyclists [47]
Make active transportation safer and more
convenient.
Yes Yes Distance between motor vehicle traffic and active transportation users
improves safety and decreases personal air pollution exposure.
Superblock development [77] To reduce traffic flow in residential areas. Yes Yes Superblocks can reduce motor vehicle traffic and enable individuals to
travel by walking and cycling easier than by motor vehicles to nearby
destinations.
Expanding cycling networks [81] Improves access and ease of travel for
cyclists.
Maybe Yes Expanding cycling networks correlates with higher cycling rates and
potentially modal shifts from motorized transportation
Bike share system/green wave route
[79,80]
Promote access and convenience of
cycling.
Yes Yes Increasing the accessibility to active modes of transportation improves
active transportation utility and correlate with a decrease in motor
vehicle usage improving air quality.
Complete streets [83,122] To provide infrastructure that enables
alternative modes of transportation.
Yes Yes Redesigning streetscapes to accommodate multiple modes of travel
increases physical activity and decreases use of motor vehicles
improving air quality.
Green space provision
Green space provision [87,93] Offer urban spaces that provide
recreational and health-promoting
opportunities.
Yes Yes Vegetation can remove pollutants and can provide a more
accommodating transportation environment for active transportation
users or leisure-time physical activity.
Increasing tree density [89,90•] Remove pollutants from the air. Yes No Removes air pollution, mainly PM, from the air.
Outfitting green spaces with active
transportation infrastructure [92•]
Provide quality spaces for individuals to
be active.
Yes Yes The quality of a green space is a greater determinant of active
transportation than the simple provision of green space
Public transportation
Improving public transportation [94•,
95–100,123,124]
Promote (modal shifts to) active
transportation.
Yes Yes Using public transportation typically requires walking or cycling to
public transportation hubs. The removal of low/single-occupancy
vehicles due to public transportation and/or “ride sharing”decreases
GHG and traffic emissions.
Curr Envir Health Rpt

of complete streets could allow for more informed land
use decisions and provide health benefits.
&There is a lack of full chain models and source-
apportionment studies that trace back air pollution levels
and potential changes to the actual sources responsible.
&The health impacts at low levels of air pollution are un-
known and may challenge the wisdom that air quality
guidelines represent safe levels.
&There is a lack of work holistically addressing the entire
spectrum of exposures and health outcomes due to trans-
portation. A paper currently in progress has identified 12
additional pathways, besides air pollution and physical
inactivity, that describe the relationship between public
health outcomes and mobility habits in urban areas [127].
Compliance with Ethical Standards
Conflict of Interest The authors declare that they have no conflict of
interest.
Human and Animal Rights and Informed Consent This article does not
contain any studies with human or animal subjects performed by any of
the authors.
Publisher’sNoteSpringer Nature remains neutral with regard to jurisdic-
tional claims in published maps and institutional affiliations.
References
Papers of particular interest, published recently, have been
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