Physical and Biological Properties of Bioaerosols

Abstract

Bioaerosols include bacterial cells and spores, viruses, pollen, fungi, algae, detritus, allergens and cell fragments. Bioaerosol particles are usually a small fraction of all aerosol particles in our surroundings, but their impact can be critical. They are a means for transmission of disease, they cause allergic reactions and they have effects on the global climate, ecology and biodiversity. This chapter provides an overview of the main types of bioaerosol particles, their sources, transport and sinks, and their potential effects on health and atmosphere.

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Chapter 3
Physical and Biological Properties of Bioaerosols
Jakob Löndahl
P. Jonsson et al. (eds.), Bioaerosol Detection Technologies, Integrated Analytical Systems,
DOI 10.1007/978-1-4419-5582-1_3, © Springer-Verlag New York 2014
J. Löndahl ()
Division of Ergonomics and Aerosol Technology, Department of Design Sciences,
Lund University, Box 118, 221 00 Lund, Sweden
e-mail: jakob.londahl@design.lth.se
Introduction
Bioaerosols make up a diverse group of airborne particles with biological origins.
They include bacterial cells and spores, viruses, pollen, fungi, algae, detritus, al-
lergens and cell fragments. Sometimes the term is also used to include secondary
particles in the atmosphere that are formed from the condensation of gaseous mol-
ecules released by biological organisms. In this chapter, though, bioaerosols refer to
primary (directly emitted) aerosol particles.
Bioaerosol particles are usually a small fraction of all aerosol particles in our
surroundings, but their impact can be critical—a single viable microbial pathogen
could, for instance, be sufficient to spread an infection. Bioaerosols are a means for
transmission of disease between humans, crops and livestock. They can cause aller-
gic reactions and affect indoor air quality. They have to be controlled and minimized
in hospital operating theatres, the food industry, pharmaceutical manufacturing and
cleanrooms. They are important in ecology and biodiversity. They can damage our
cultural heritage and in the worst case, be used for biological terrorism or warfare.
The physical characteristics of bioaerosols are highly varied. The smallest par-
ticles, such as cell fragments, may consist of a few molecules and be 0.001 µm in
diameter while pollen and algae can have sizes of 100 µm or more (Figs. 3.1 and
3.2). This range, which spans over five orders of magnitude, is comparable to the
difference between a pinhead and the St. Peter’s Basilica in Rome.
Aerosol particles, from 0.001–100 µm, have different mechanisms for transport
in the air, deposition on surfaces and light scattering—properties that are important
both for their effects and for their detection. The size also has implications for the
amount of material carried by the particle, since the volume is proportional to the
cubed diameter (∝ dp
3) and thus spans over 15 orders of magnitude.
Characteristics other than physical size are significant for aerosol movement.
These include density, shape and electrical charge. The density of biological

J. Löndahl34
material typically is around 1.0–1.5 g cm−3, but bioaerosol particles are often mixed
with other matter with higher density, such as mineral dust or sea salt. The shape
of bioaerosols may vary from spherical to elongated structures, from a single cell
to chains or complex aggregates. The electrical charge is presumably close to the
“neutral” Boltzmann charge distribution. Some studies have suggested that airborne
bacteria can carry a high charge, but this is likely due to the methods used for spray-
ing them [1].
A key to understanding the impact of microorganisms in the air is their viability,
that is, their capacity to survive and grow. Many microbes have developed systems
that protect them against the harsh conditions in the atmosphere where exposure to
UV radiation may be high, temperature and oxygen pressure low, and the availabil-
ity of liquid water and nutrition limited [2]. Formation of resting stages, pigmenta-
tion, biofilms and psychrotolerance are among the mechanisms that are involved to
preserve viability.
By mass, the major part of microbial cells generally consists of water (~ 70 %),
although at atmospheric conditions they may desiccate. The remainder mainly is
comprised of proteins, lipids, carbohydrates and other macromolecules.
Ambient concentrations of bioaerosol particles remain uncertain due to a limited
amount of data and the lack of measurement standard. Until recently, culture-based
techniques have been most common and it is well known that these methods cap-
ture only a fraction of the total diversity of the biological particles present in the
air. Culture-independent methods are rapidly developing, but data from these on
environmental bioaerosols are still scarce.
This chapter provides an overview of the main types of bioaerosol particles, their
sources, transport and sinks, and their potential effects on health and atmosphere.
Fig. 3.1 Approximate size range of various individual bioaerosol particles. Fragments include, for
instance, parts of cells, plants and biofilms or the crust on sand and rocks

3 Physical and Biological Properties of Bioaerosols 35
Types of Bioaerosols
Bacteria and Archea
Bacteria are unicellular, prokaryotic, microorganisms found in almost all environ-
ments on Earth. Humans carry ten times more bacteria by number on their skin and
in their gut flora than there are cells in the body. Airborne bacteria may be transport-
ed as single cells, but typically they are attached to other particles such as leaf, soil
or skin fragments [3]. Spherical bacteria ( cocci, e.g. Micrococcus or Streptococcus)
often occur in pairs, chains or clusters. Rod-shaped bacteria ( bacilli, e.g. Bacillus
anthrasis) may be single or form chains.
Based on the structure of the cell wall, most bacteria can be grouped as either
Gram-positive or Gram-negative. The cell walls of Gram-positive bacteria general-
Fig. 3.2 Electron microscope images of (a) Mycobacterium tuberculosis (image credit: Janice
Carr), (b) Flavivirus, (c) Redbud pollen and (d) Chlamydomonas sp., green microalgae

J. Löndahl36
ly have a thick layer of peptidoglycan, while Gram-negative have several thin layers
of peptidoglycan surrounded by an outer membrane containing lipopolysaccharides
(endotoxin). Some Gram-positive bacteria, such as Bacillus and Clostridium, can
develop endospores. These are dormant forms with high resistance to radiation,
heat, freezing and other types of environmental stress. Endospores can be both
smaller and larger than the vegetative cell.
Outdoor particles with bacteria are normally found to have an aerodynamic di-
ameter of approximately 4 µm except at coastal sites and over oceans where the
diameter is about half of this or less [3]. Continental bacteria are normally larger
than marine bacteria, but the difference may be partly due to the formation process,
where airborne bacteria produced from the water sometimes carry less residual ma-
terial [4]. The reported sizes of indoor bacteria are about 1–3 µm [5].
Archaea were previously believed to be specific type of bacteria, but are now
recognized as a separate domain of life. Originally, archaea were found mostly in
extreme environments, but over time they have been detected in many common
habitats. The knowledge about archaea in the air, however, is very limited.
Viruses
Viruses are parasitic infectious agents that only can reproduce in living cells of ani-
mals, plants or bacteria. Those specialized on infecting bacteria are called bacterio-
phages. Since viruses need a host cell to multiply and carry out metabolic processes,
they are usually not considered organisms. Viruses are the most common biological
entity in ecosystems. A viral particle that is released from a cell and capable of in-
fecting other cells is called a virion.
All viruses carry nucleic acid, either as DNA or RNA (but not both). A protein
shell called a capsid surrounds the genetic material, which is the source of infec-
tion. The capsid allows recognition of receptors on the host cell and helps the virion
in some cases to enter the host cell. In addition, some viruses have an envelope of
lipoproteins originating from the cell membrane.
Viruses are normally found in the size range 0.02–0.3 µm, although a few excep-
tions exist such as mimivirus around 0.4 µm and the exotic Megavirus chilensis,
which is even larger [6, 7]. Viruses are typically shaped either as rods (helical) or
spheres (icosahedral).
The literature on airborne viruses is comparatively scarce. This is most likely
due to inefficient measurement methods—especially before the development of
molecular techniques such as PCR. It is assumed that viruses, at least those that
are viable, normally are transported attached to larger particles. Since they have no
repair systems they are usually more easily inactivated by heat, radiation and other
types of environmental stress than live microorganisms. The relationship between
relative humidity and survival of viruses in the air is complex and varying depend-
ing on virus type [8].

3 Physical and Biological Properties of Bioaerosols 37
Pollen
Pollen is produced by plants to transfer the male gametes (sperm cells) to the
pistils or female structures where fertilization takes place. The size range of the
pollen grains is approximately 10–100 µm, although many that are found in the
atmosphere are between 20 and 60 µm. Most pollen that contains allergens is an-
emophilous (“wind-loving”). Anemophilous plants (e.g. grass, conifers, birches and
ragweed) use wind as their primary dispersal vector. However, not all pollens are
transferred through the air.
Pollen grains vary not only in size, but also in surface structure, density and
shape. Normally they have a hard shell that protects the seed and prevents deteriora-
tion of the genetic material present in the nuclei from environmental stress during
transport through the atmosphere. The shell may rupture into smaller fragments
between 0.1 and 5 µm at high humidity [9]. The shape and structure of the pollen
grains are characteristic for different plant species and their origin can often be
identified under microscope.
The dispersal, resuspension and transport of pollen into the atmosphere are large-
ly determined by meteorological factors. For instance, emissions decrease when
wind speeds are low and during rainfall. There also appears to be an association
between transport distances and temperature, with an increase in distances travelled
as the temperature increases [10].
The concentration of pollen in the air follows a seasonal cycle that primarily is
a function of climate and has a limited shift due to meteorological factors. Climate
change may adjust the onset of these pollination seasons and alter their duration.
Fungi
Fungi are among the most common organisms on Earth and include molds, rusts,
smuts, yeasts, mildews and mushrooms. In many ecosystems they play a key
role as decomposers. They may be independent or live in symbiotic or parasitic
associations with plants and animals. Fungi are a source of severe problems when
they cause crop diseases, infectious disease or allergies, but can also be beneficial
when used in biological pesticides or for the production of antibiotics.
Some fungi are unicellular, such as yeasts, but usually they consist of multi-
cellular tubular filaments called hyphae. The hyphae branch into a network called
mycelium, which may develop reproductive spores. These spores can be released
by passive processes, using wind or other external forces, or by active processes
where an ejection is produced by, for instance, osmotic pressure or surface tension.
It is commonly assumed that spores constitute the bulk of the bioaerosols pro-
duced from fungi. These are found in the size range 1–50 µm, but more typically
between 2 and 10 µm [11, 12]. Frequent genera in the atmosphere are Cladospo-
rium, Alternaria and the actively released ascospores and basidiospores. It has been
shown that emission of the spores may be accompanied by significant amounts of
fungal fragments [13].

J. Löndahl38
Some fungi produce mycotoxins, which are secondary metabolites associated
with both acute and chronic health effects. Fungal spores are coated with ergosterol
and sometimes sugar alcohols (e.g. arabitol and mannitol). These compounds can be
used as chemical tracers for the total spore concentration in the air.
Microalgae and Cyanobacteria
Microalgae are eukaryotic and predominantly photosynthetic organisms that are
present in both aquatic and many non-aquatic habitats. In aquatic environments,
microalgae play an important role in the microbial food web.
Cyanobacteria, also called blue-green algae, are bacteria which, similar to most
algae, can produce chemical energy through photosynthesis. Algae and cyanobacte-
ria together produce the major part of the atmospheric oxygen.
Algae can be both unicellular and multicellular and range in size from about
1 µm to several meters. Few studies have investigated airborne microalgae and
cyanobacteria, but they have nevertheless been found in both indoor and outdoor
air around the globe [14, 15].
Allergens, Lichens, Fragments and Other Bioaerosols
A variety of biologically derived particles in the air can be added to the list. Crusts on
rocks and sand often carry biological material that may be suspended into the air as
a result of erosion from wind and rainfall [16]. Lichens, which are a symbiotic union
between fungi and microalgae or cyanobacteria, are widespread on Earth and have
also been detected in the atmosphere [17]. Plant fragments are, by mass, among the
most common bioaerosol materials in the atmosphere [18]. Other particle types that
can be important, especially from a health point of view as they may cause allergic
reactions, are skin fragments, fur fibers, insect secretions and dandruffs.
Sources, Transport and Sinks
Dispersal is fundamental to life. After survival and reproduction it can be regarded
as the third most important need for an organism. Since the air provides an efficient
pathway for fast transport over long distances, it is thus not surprising that many
microbes have developed mechanisms to spread into the atmosphere, stay viable for
long periods in its inhospitable environment and deposit on a new location to begin
life. It has even been suggested that through millions of years of evolution microbes
have adapted at the group level to become airborne by, for example, changing the
local climate and increasing wind speeds [19, 20]. Transport by the wind may very
well be the most common route for microbial dispersal [21]. This section reviews
emission, aging and deposition of bioaerosols (Fig. 3.3).

3 Physical and Biological Properties of Bioaerosols 39
Sources—Primary Emission
Bioaerosols are produced by a range of processes including bubble bursting, ero-
sion, active dispersal and mechanical interaction between surfaces. The particles
may be airborne as individual cells, but often they are attached to other material
such as dust, plant fragments or as a group of cells trapped in biofilm.
Globally, open water is a major source of ambient bioaerosols, since water cov-
ers about 70 % of the Earth. The surface of the water has an almost omnipresent mi-
crolayer, up to some hundreds of µm thick, which is in contact with the atmosphere.
This surface microlayer is a unique habitat and compared to subsurface waters, it
is enriched by orders of magnitude with bacteria, viruses and other microorgan-
isms as well as lipids, carbohydrates, polysaccharides, amino acids and proteins.
The aerosol particles dispersed from the surface are even further concentrated with
biological material [23, 24].
There are three main types of droplets formed from bodies of water: spume
drops, film drops and jet drops (Fig. 3.4) [25]. Spume drops, which generally have
a minimum diameter of about 40 µm, are torn off from wave crests when wind
speed exceeds 7–11 m s−1. Film and jet drops originate from the bursting of bubbles.
Film drops, usually with diameters between 1 and 10 µm, are released horizontally
when the thin film on top of the bubble ruptures, while jet drops, with diameters of
6–100 µm or about one-tenth of the initial bubble, are ejected in an upward direction
when the bubble collapses. Bubbles larger than 8 µm in diameter are required for
jet drops [26]. Major sources of bubbles are breaking waves, rain and boat traffic.
Over dry land, plants release bioaerosol particles as a result of decay processes or
for reproduction—the latter including pollen and spores. Particles may be emitted
from surfaces of vegetation, soil and rock because of erosion, wind or other types of
mechanical interaction. Droppings from birds and rodents can transmit viral and mi-
croorganism material. In populated areas, the concentration of bioaerosols is often
Fig. 3.3 Cycling of primary
aerosols in the atmosphere
(modified from [22])

J. Löndahl40
enhanced due to agriculture, horticulture, waste disposal, sewage treatment works,
traffic and other human activities.
The outdoor emissions of bioaerosols vary with time and location [18]. Over wa-
ter, the production of bioaerosols is influenced by ice cover and wind speed, which
can vary over the year. Over land, the emissions largely depend on the vegetation
cycles and meteorological conditions, which changes daily and weekly as well as
seasonally. Temporal variation is most pronounced in rural and remote regions. In
densely populated areas, the variation over time is limited as human activity is rela-
tively constant. Geographically, bioaerosol emissions in uninhabited regions may
be homogenous over vast areas, while the emissions in cities often vary strongly
with spatial location since they often are dominated by a few point sources.
Indoor bioaerosols may originate from the outdoor air, but often other sources
are more significant. Especially if water or moisture is available, microbial growth
can be excessive in a building structure, or its ventilation and air conditioning sys-
tems. Particles directly emitted from humans and animals also constitute a large
fraction of the indoor bioaerosols.
In some indoor environments, bioaerosol particles may be detrimental, such as in
operating theatres, industrial cleanrooms, isolation rooms and the food industry. In
these locations efficient ventilation, clean procedures and airlocks usually minimize
most particle sources. However, some particles generated inside the facilities such
as skin fragments or emissions from specific processes may be difficult to remove
completely and can cause severe problems.
Biological warfare and terrorism have been behind much of the development
of new detection techniques for bioaerosols in the past decades. Agents intended
for airborne transmission include bacteria such as Bacillus anthracis, Francisella
tularensis and Yersinia pestis, viruses such as Variola major (smallpox) and Ebola,
toxins such as botulinum toxin (produced by the bacteria Clostridium botulinum)
and many mycotoxins. The consequences can be disastrous if a virus that is not nor-
mally spread through the air is modified to allow airborne transmission—a scenario
which has become less unlikely with the advancement of biotechnology [27].
Transformation and Aging during Transport
Aerosol particles often stay for weeks in the atmosphere before deposition on the
ground. Some particles reach the stratosphere, which further prolongs their atmo-
Fig. 3.4 Aerosol production from water and typical droplet diameters

3 Physical and Biological Properties of Bioaerosols 41
spheric lifetime. During this time, they can be transported thousands of kilometers,
but also transformed and inactivated since particles in the air are exposed to both
physical and chemical aging processes.
The size and structure of aerosols are modified mainly by coagulation, restruc-
turing and evaporation or condensation. During coagulation, particles collide and
form larger units. The coagulation rate increases proportionally to the square of
the number concentration (~ N2) and is thus most rapid in polluted environments.
Particle restructuring may especially occur as a result of water uptake at high
humidity because of the increasing surface tension or due to droplet formation. The
exchange of semivolatile compounds by evaporation and condensation is continu-
ous and highly related to the chemical reactions in the air. Common chemical reac-
tions in the air include oxidation, nitration, hydrolysis and photolysis. The reactions
are often driven by UV radiation as well as oxidants such as OH, ozone, NO3 or
acids such as H2SO4 and water.
Despite the environmental stress in the air, it is possible for many microbes to
remain metabolically active, grow and maintain reproductive capacity [28, 29]. As
mentioned in the introduction, these microbes often have protection mechanisms
that help them preserve viability. Many microorganisms survive by formation of en-
dospores, but also non-spore-forming bacteria have the means to maintain viability
such as repair mechanisms and pigmentation. Some biological agents without repair
mechanism, such as viruses and fungal spores, may be shielded by attachment to
dust, pollen, biofilms or other material. For instance, the foot-and-mouth disease
virus was found to cross the English Channel and the concentration of the influenza
virus increased when dust from China reached Taiwan [30, 31]. It has even been
claimed that bacteria can remain viable more than 50 km above the Earth’s surface
[32]. The atmosphere may very well be a habitat of its own for some microorgan-
isms and not only a means for transportation [2].
Deposition
Particles are removed from the air by either wet or dry deposition on a surface. Wet
deposition, when particles are washed out from the atmosphere by precipitation, is
the main sink for atmospheric particles (between ~ 0.1 and 10 µm). The particles
may be scavenged in a cloud by formation of or collision with cloud droplets, or
below the cloud if they come in contact with rain or snowfall.
In dry deposition, particles collide and stick to the ground as a result of sedimen-
tation, diffusion, impaction or other processes such as interception, turbulent eddies
and thermophoresis. Particles larger than about 0.5 µm, as most bioaerosols, trans-
port relative to the air primarily due to gravitational settling. The settling velocity
is proportional to the squared diameter (∝ dp
2) and thus increases dramatically with
size. Large particles that are not removed by sedimentation may land on a surface
by impaction when the air stream changes direction, or be intercepted on it if some
part of the particle comes in contact. Particles smaller than 0.5 µm move mainly by
diffusion due to Brownian motion. The diffusion speed is approximately propor-
tional to the inverse of the squared diameter (∝ 1/dp
2). In narrow air spaces, such

J. Löndahl42
as the respiratory tract, deposition due to diffusion can efficiently remove the main
fraction of particles below 0.1 µm within a few seconds.
In the range 0.1–1 µm, neither diffusion nor sedimentation or impaction act
strongly on the aerosol particles. Thus, particles in this size span accumulate and
stay longest in the air. It can also be noted that certain bioaerosols, such as some
pollen and fungal spores, are aerodynamically shaped to be buoyant in the air, which
facilitates long-distance transport.
From a health point of view, deposition of bioaerosol particles in the respiratory
tract is of special concern. Figure 3.5 illustrates the regional deposition of inhaled
particles in a normal nose breathing adult during relaxed conditions as calculated
with the Multiple-Path Particle Dosimetry Model (MPPD, version 2.11, Chemical
Industry Institute of Toxicology, Research Triangle Park, NC). As expected, the de-
position probability has a minimum for particles between 0.1 and 1 µm. Bioaerosol
particles larger than 10 µm—such as many pollen, fungal spores and algae—most
likely deposit in the nose and mouth, although some may reach further down into
the respiratory tract. Bacteria, viruses, cell fragments and other small bioaerosol par-
ticles can easily reach the alveoli where gas exchange takes place: around 30–60 %
of the particles in the range 0.01–2 µm that deposit in the respiratory tract end up in
the pulmonary region. The pulmonary region is the most sensitive part of the lungs.
Health Effects
Historical Background
Observations of adverse health outcomes from bioaerosols have accompanied hu-
manity throughout history. An early notion of harmful contagions, possibly molds,
and measures for decontamination is found in Leviticus 14 in the Bible. More devas-
tating was the bubonic plague, caused by the bacterium Yersinia pestis, which likely
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3 Physical and Biological Properties of Bioaerosols 43
was the reason for the Black Death that killed at least one third of the population of
Europe in the fourteenth century [33]. Bubonic plague is a zoonotic disease, trans-
mitted from animals to humans, but can also spread between humans via coughing,
for instance. Later, in the mid-nineteenth century, the Irish potato famine gave rise
to one million deaths and large-scale emigration. The starvation was caused by a
potato blight from the fungus-like microorganism Phytophthora infestans, which
can be rapidly transmitted through the air.
Over the last century, several major outbreaks of influenza have been noted [34].
The pandemic of 1918–1920, known as the Spanish flu, with a total of 40–50 mil-
lion deaths was by far the worst. Notorious are also the 1957 Asian flu and the
1968 Hong Kong flu, each with a total mortality of about one million, and the less
destructive but more recent Swine flu in 2009. Judging from history, we will have
more influenza pandemics in the future—perhaps next time if the highly pathogenic
H5N1 virus, known as bird flu, develops an ability to spread in the air from person
to person.
Health Effects from Exposure
There are many health effects associated with bioaerosols related to toxic and al-
lergenic materials, and to infectious diseases from pathogenic biological agents.
Respiratory infections, which often can be linked to inhaled pathogens, are the most
severe diseases worldwide in terms of mortality. They are the source of about 7 % of
the total deaths, which in 2002 corresponded to almost 4 million people [35]. Tu-
berculosis, a lung disease usually caused by airborne Mycobacterium tuberculosis
(Fig. 3.2a), is the reason for another 3 % of the yearly loss of lives in the world,
primarily in low- and middle-income countries [36]. Infectious diseases linked
to bioaerosols are essentially caused by viruses (e.g. influenza, measles), bacte-
ria (e.g. tuberculosis, Legionnaire’s disease) or fungal spores (e.g. aspergillosis,
blastomycosis).
A biological agent must be viable in order to cause an infection, but non-
viable bioaerosols can also have implications for health if they carry toxic or
allergenic material. Allergens comprise a range of molecular structures from
chemicals with low molecular weight to proteins with high molecular weight
[37]. Common allergens include: proteases (enzymes important in protein ca-
tabolism), some proteins present in dander, saliva and urine from cats and dogs,
glycopeptides from fungi, and industrially produced enzymes from fungi and
bacteria.
Asthma, which is a chronic inflammation of the airways, is sometimes associated
with exposure to allergens from dust mites, dander, cockroaches and mold. Onset
can also be due to exposure to non-allergenic biological material such as endotoxins.
Many toxins with well-known health effects are produced from bacteria and
fungi. Bacterial endotoxins can be strongly pro-inflammatory and inhalation stud-
ies have linked them to symptoms such as fever, neutrophilic airway inflammation,
cough and lung function impairment [38]. Knowledge about airborne exposure to

J. Löndahl44
fungal toxins—mycotoxins—is still limited. Notable is the carcinogenic aflatoxin
from Aspergillus flavus, but the primary route for exposure is ingestion. Other com-
pounds that can be used as biomarkers for toxic effects are ergosterol, which is a
component in fungal membranes and yeasts, and β-(1→3)-D-glucans, which are
cell wall components of most fungi and some bacteria.
Indoor and Workplace Bioaerosols
A building and its ventilation system can facilitate survival of microorganisms since
it provides a closed environment where many of the natural inactivation mecha-
nisms are inhibited, such as UV-light, ozone and large temperature variations. This
is amplified by efforts to save energy by decreased ventilation and increased insu-
lation. Many indoor environments also accommodate potent sources of biological
agents from industrial processes, waste handling, animals or people.
Nonspecific health symptoms related to a building where people reside are of-
ten described as sick building syndrome (SBS). The cause of the symptoms is not
known, but often linked with poor air quality and nearby biological or chemical
contaminants. SBS is probably common, but in absence of clear criteria for diagno-
sis, the prevalence remains uncertain.
Workers may sometimes be exposed to high concentrations of bioaerosols. Oc-
cupations at high risk for infectious diseases are health care workers, farmers and
veterinarians. Of increasing concern is, for instance, multidrug-resistant Staphylo-
coccus aureus (MRSA), which can affect hospital workers and patients. Around
5–10 % of all patients admitted to hospitals in Europe and the United States are af-
fected by health care associated or nosocomial infections, whereof many are spread
through the air [39].
Hygiene Hypothesis
It has long been noted that a certain exposure early in life to microbial agents may
decrease the risk of developing allergies [40, 41]. This is referred to as the hy-
giene hypothesis and the basic idea is that contact with a diverse range of biological
agents during childhood is essential for the development of the immune system.
Although the hypothesis is well supported by epidemiological findings, there are
contradictory data that remain to be explained.
Atmospheric and Ecosystem Impact
Biological material is one of the most important groups of aerosols in the atmo-
sphere, although both its concentration and impact on the environment remains
largely unclear. It has been suggested that up to 1000 Tg of bioaerosols are emitted

3 Physical and Biological Properties of Bioaerosols 45
each year (compared to 3300 Tg for sea salt and 2000 Tg for mineral dust) [42, 18]
(Table 3.1). Proteins, amino acids and related amino compounds alone may consti-
tute up to 10 % of the fine particulate matter in the atmosphere [43].
The most critical effects of atmospheric aerosols are their influence on the global
mean temperature. Airborne particles are essential to the Earth’s energy balance—
the absorption and reflection of incoming radiation from the sun and outgoing ra-
diation from the Earth’s system. Overall, aerosol particles cool the planet, mainly by
backscattering of light from the sun. The backscattering may be either direct (i.e.,
reflection of the light directly by the particles) or indirect, in which case the aero-
sol particles alter light scattering through interactions with clouds. At atmospheric
conditions, cloud droplets are formed by condensation of water vapor on already
existing particles, referred to as cloud condensation nuclei (CCN) or ice nuclei (IN).
Aerosol particles are, in other words, necessary for cloud formation. The average
albedo of Earth aerosols—the fraction of light reflected back into space—is about
20 % and most of this is from clouds.
Bioaerosols represent a relatively small fraction of the total amount of particles
in the atmosphere and thus have a limited effect on the directly scattered light on
a global scale. However, an increasing number of scientific studies are showing
that bioaerosols may have a significant influence on clouds and precipitation [18,
19]. As sources of cloud condensation nuclei biological particles seem to behave
similarly to other particles of equal size, but as ice nuclei they may be extremely
efficient.
The formation of ice crystals in clouds at temperatures between 0 and − 38 °C
originates on ice nuclei, which only are a minor fraction of all aerosol particles.
When the ice crystals have formed, they may quickly grow and thereby initiate
precipitation. The most efficient ice nuclei particles in the atmosphere appear to
be bioaerosols, as some bacterial strains and fungi are found to form ice at high
subzero temperatures [45, 46]. Notable are Pseudomonas syringae, Pseudomonas
fluorescens and Erwinia herbicola.
Table 3.1 Estimated global emissions of primary bioaerosol particles and magnitude of concen-
trations over vegetated regions [18, 44]. Primary emitted aerosols from sea salt, mineral dust and
fossil fuels are inserted for comparison
Global emissions
[Tg year−1]
median (min–max)
Number concentration
[m−3]
Mass concentration
[µg m−3]
Total bioaerosol 132 (< 10–1000)
Bacteria 1.66 (0.7–28) ~ 104~ 0.1
Viral particles ~ 104~ 10−3
Pollen 66 (47–84) ~ 10 (up to 103)~ 1
Fungal spores 31 (8–186) ~ 103–104~ 0.1–1
Algae ~ 100 (up to 103)~ 10−3
Sea salt 3340
Mineral dust 2150
Fossil fuel 35

J. Löndahl46
Other suggested atmospheric effects of bioaerosols include modification of
cloud chemistry. Metabolically active bacteria in cloud droplets can contribute to
the transformation of chemical compounds in the air [43]. The extent of this effect
is uncertain.
In addition, the transport of biological particles through the atmosphere can
sometimes have important consequences for ecosystems by changing biodiversity.
The introduction of new species can affect community diversity and modify the
population’s genetic structure (e.g. [47]).
Conclusion
Bioaerosols play a key role for our health and environment through a variety of
processes. Inversely, anthropogenic activity also has an extensive impact on bio-
aerosols. We do not yet have the full picture of all effects from airborne particles
with biological origins, but the developments in molecular microbiology and aero-
sol technology over the past years have provided new possibilities to unravel this
fascinating world.
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