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Fact Sheet: Bioaerosols (Airborne Microorganisms)

Air movement provides an important and common mechanism for biological dispersal (movement from one location to another).  Each cubic meter of indoor or outdoor air may contain thousands or even millions of microorganisms and biological particles. These airborne particles are collectively referred to as Bioaerosols.  Examples of bioaerosols include viruses, bacteria, fungi, pollen, fragmented particles from microbial cells or insects, and by-products of living organisms (e.g. animal dander, insect excrement).  The size of aerosolized particles varies between a fraction of a micrometer (µm) to approximately 30 µm.  Particles larger than 30 µm also become airborne but for shorter periods of time.

Sources of Bioaerosols

Bioaerosols may originate from almost any natural or man-made surface and each source may give rise to an entirely unique assemblage of bioaerosols.  Below are examples of bioaerosol concentrations from different environments.  These concentrations are not necessarily representative of normal or extreme levels. 

Category Activity Type Bacteria (CFU/m3) Fungi (CFU/m3)
Agricultural animal facilities 103 - 105 102 - 108
composting 103 - 106 102 - 107
harvesting, storage 102 - 103 103 - 109
Air systems HVAC 10 - 103
10 - 104		 102 - 107
10 - 103
Indoor surfaces ceilings & walls 10 - 103 10 - 104
carpet 103- 106 102 - 105
house plants 10 - 104 102 - 105
operating room 10 - 102 10 - 102
Industrial saw mill 10 - 103 104 - 108
food processing 10 - 103 102 - 104
manufacturing 102 - 106 102 - 106
Water treatment aeration tanks 102 - 103 10 - 102
activated sludge 102 - 106 10 - 103

Cell or Particle Size

Cell or particle size is an important factor in determining risks associated with microbial contamination.  In general, the smaller the particle, the greater the risk.  This relationship is due to the fact that smaller cells and spores become trapped within lung tissue and are not easily expelled.  As previously mentioned, cell fragments can be many times smaller than the intact cell or spore and their smaller size poses even greater health risks. 

1: Nasopharynx or Head

Deposition of spores > 20 µm

2: Larynx, Trachea, Bronchi

Deposition of spores < 20 µm

3: Bronchioles and Alveoli

Deposition of spores  <1 - 4  µm

Cell Size in Micrometers or Microns (µm)*
0.025 0.05 0.1 0.25 0.5 1 2 5 10 20 40
                     
                     
viruses

most bacteria

most fungi, algae, protozoa

* cell fragments can be several times smaller than the original cell or spore 

 

Monitoring Bioaerosols in Indoor Environments

The goal of air monitoring is to provide a representative sample of microorganisms and particles present within the air that we breath.  Although this objective may appear straightforward, the process is actually riddled with potential problems.  Sampling methods must account for a number of factors such as viable and nonviable organisms, differences in cell size and shape, temporal variances, spatial variances, occupant use, sampled air volumes, location of reference samples, and infiltration from outdoor air - just to name a few.  Unfortunately, no single air sampling method addresses all of the above issues without some assumptions.  To minimize these assumptions, an air monitoring strategy must be carefully designed.  Such studies are extremely expensive - often cost prohibitive.  Even if an adequate design is achieved, the results represent only a snap-shot for that given sampling event.  

Modern airborne sampling employs one of three protocols: 1) impactor sampling, 2) liquid impinger sampling, or 3) filtration sampling.  Each of these methods pulls a measured volume of air with the aid of an electric or battery-powered pump.  The air is then directed through a chamber (or a series of chambers), guiding the spores on a specific trajectory to a solid agar disc or adhesive medium (impactor samplers), a liquid buffer (impinger samplers), or a filter (filtration samplers).  With the impactor method, cells or spores are usually cultured on a suitable nutrient medium.  Each organism is then identified and reported as colony forming units m-3 (CFUs per cubic meter).  One significant disadvantage of viable spore sampling is that it grossly underestimates the number of total cells/spores (viable and nonviable).  Nonviable cells, although no longer infectious, still exhibit the same allergenic, irritant, toxigenic properties as viable cells/spores. 

Interpreting Bioaerosol Data 

Because state and federal standards for most bioaerosols do not exist, the most common practice compares indoor cell concentrations to concentrations measured outdoors during the same sampling event.  This unofficial benchmark implies that indoor counts should not be significantly greater than outdoor counts.  When indoor concentrations are significantly greater, it is generally assumed that an indoor amplification source exists.  In other words, there is likely microbial contamination present on indoor building materials. 

Such basic approaches do not apply to all microorganisms.  Relatively high levels of one airborne microbe may represent very low risks, while extremely low levels of more dangerous contaminants should trigger immediate action.    Other important factors must also be considered, including the location of air samples, frequency of detection, frequency of sampling, type of air sampler (viable or nonviable), and the environmental conditions.  In light of such complex variables, interpretation of air testing results are best left to specialists with appropriate training and experience.   

Fungal Bioaerosols Originating From Building Cavities

One of the most persistent misconceptions regarding bioaerosols is that fungal contamination within wall, ceiling, and floor cavities poses little if any risk to building occupants.  In reality, movement of cells, spores, and cell fragments may occur via through-wall openings and gaps at structural joints.  Examples of through-wall openings include electrical outlets, cable boxes, light fixtures, and heating/cooling vents.  Infiltration is affected by a number of variables such as particle size, gap/crack size, building pressure, and wall construction. 

To appreciate the potential for cross contamination from building cavities, consider the following:

  • Cell diameters for bacteria and small fungal spores range between 0.5 to 4 micrometers (microns).  For example, a common fungal contaminant Aspergillus versicolor produces spores with a diameter of approximately 2 microns.

  • The human eye can distinguish objects in the size range of 100 micrometers and above.  So the smallest visible cracks or gaps within building surfaces would be at least 100 micrometer in width.  Most building cracks are actually are at least 1 millimeter in width or 1,000 micrometers.

  • If visible building cracks are at least 100 to 1,000 micrometers wide, then it can be assumed that 50 to 500 spores of Aspergillus versicolor (set side by side) would fit within the space of 100 to 1,000 micrometers.  In non-technical terms, this would be the equivalent of rolling a very small marble through a very large open door-way. 

In addition to bioaerosols, many microbes also produce metabolites called Volatile Organic Compounds (VOCs), which are capable of migrating directly through building materials such as wood sheathing, concrete, drywall, and even polyethylene vapor barriers.  Microbial VOCs are potent irritants and are often implicated as the cause of headaches, eye irritation, sore throat, sinusitis (inflammation of sinuses), and rhinitis (runny nose).

 

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