Tuesday, October 23, 2007


Indoor or outdoor air may contain thousands or even millions of microorganisms and biological particles in just one cubic meter of air. 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 these particles generally varies between a fraction of a micrometer (µm) to approximately 30 µm. Bioaerosols may originate from numerous natural or man-made sources such as agriculture (harvesting, storage, composting etc) and industrial activities (manufacturing, food processing etc), indoor surfaces (ceiling, wall, carpets, house plants) and water treatment plants etc.
Particle size is an important factor in determining risks associated with microbial contamination. In general, particles > 20 µm (fungi, algae, pollen etc) affect Region 1, Particles < 20 µm (mostly bacteria and some fungi, algae) affect Region 2 and particles < 1or 2 µm (mostly viruses and some bacteria) affect Region 3.

Modern airborne sampling of bioaerosols 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 (particles) 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).
Since state and federal standards for most of the 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, and the environmental conditions while assessing the effects of bioaerosols.
For more information and understanding, please refer the following book and other literature

Macher, Janet (Ed.), Bioaerosols: Assessment and Control, American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, 1999.
ISBN: 978-1-882417-29-2


Falguni Patadia said...

Very interesting article Neeraj. Gives a different perspective of aerosols. I had never come across bioaerosols and after reading the article I could relate to something interesting. I had watched in a television serial once about how fungi had infected a patient's brain and how the experts had no clue what was going on. The image in this blog post showing regions infected and the associated text unfolds the mystery to me better now

Neeraj Rastogi said...

Hi Falguni,
Thanks for your comment. I also find it very interesting when I came across it and therefore, decided to write a small blog on it.