The Meaning of Outdoor Air Microbial Levels: IAQ Tech Tip #36 Solving indoor air quality problems through their outdoor sources

When performing indoor air quality investigations that require microbial sampling an important consideration is where and when to take and outdoor samples. A key component of any IAQ investigation is the outdoor air sample. It provides crucial information that helps determine not only whether there is an indoor air amplification site or reservoir, but also potential sources for the indoor air problem.

Outdoor air is the ultimate source for many bioaerosols and for organisms that can eventually contaminate indoor air. The outdoor air may contain pollen, plant spores, fungal spores, bacteria and bacterial spores, algae, and viruses. These microscopic contaminates can enter structures through a number of sources including: doors, windows, structural cracks and ventilation intakes.
When an indoor air quality investigation is performed it is crucial to have data from outdoor samples to help determine whether indoor contaminants are being generated within the structure or are from infiltration of outdoor contaminants. Outdoor samples should be taken at approximately the same time as indoor air samples. In most circumstances the levels of indoor contaminants can be expected to be around 25% to 95% of that of the outdoor levels. If indoor aerosol concentrations are significantly higher than outdoor concentrations, or if different species are present in indoor samples versus outdoor samples, then an indoor reservoirs and/or amplification sites are likely present. An effective interpretation can be based on the comparison of these indoor and outdoor samples.
There are a number of considerations to be made with regards to taking and interpreting outdoor samples during an IAQ investigation. Key among these considerations is weather, time of day, and the location of the sampler. When sampling outdoors the temperature, wind, and humidity can have dramatic effects on collection efficiency. Sampling on days when there are strong winds can cause outside counts to be significantly higher than on non-windy days. High outdoor counts may mask small to moderate indoor mold problems since the interpretation is made on the basis of a ratio of indoor/outdoor spore counts. When sampling in windy conditions the location and orientation of sampling equipment is crucial. Ideally when using suction samplers (ex. Aerotech 6/Andersen N6/Air-O-Cell Cassettes) the inlet should be orientated into the direction of the wind, varying this orientation increases the chance of under sampling larger particles.
Sampling during and after rain can skew outdoor data. Rain can remove many spore types while it assists in the dispersion of others. Sampling on rainy, foggy, or very humid days may result in outdoor counts that are low or have a significantly different distribution of spore types. Generally, rainy day microflora differs from dry, sunny microflora in that levels of ascospores and basidiospores may be increased. Non-viable methods will reflect this directly with increased counts of ascospores and basidiospores.
Temperature can also affect the levels of outdoor contaminants as well as the method of collection. Temperature can affect impaction surface retention and temperatures at or below 0 degrees Celsius should use pre-warmed air when using agar impaction methods. Temperature and light levels also affect the natural dispersal of different types of spores and pollens and must be taken into consideration when sampling.
Disturbances of exterior land can also have profound effects on the data. Outdoor activities such as landscaping and farming can cause dramatic increases in the dissemination of bioaerosols and sampling during these times should be factored into any conclusions derived from the data.
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