:: Section 9 of 10
V. SAMPLING TIME
Since many viable bioaerosols are cultivated with nutrient agar in petri dishes to determine the viable count, it is important for there to be a suitable number. This is a major limitation of bioaerosol sampling. When sampling bacteria, for instance,if there is too high of a surface density on the petri dish, the colonies might overlap, making it difficult to distinguish one colony from the next. If the surface density of collected bacteria is too low, the values become statistically insignificant. For bacteria, a surface density of approximately one colony per cm2 is a good goal8. A viable bioaerosol that has the potential to multiply is known as a colony forming unit (CFU).
The following equation aids in determining a suitable sampling time to achieve this surface density8.
where t is time, s is the surface density (CFU per unit area), CN is the average number concentration of bioaerosol particles, and Q is the sampler flow rate.
Recall that many different environments have different concentrations of bioaerosols. For instance, a livestock structure might have a very high concentration of culturable bioaerosols - perhaps 105 viable units per cubic meter of air or more. Outdoor environments tend to have lower concentrations - perhaps in the range of 102-103 viable units per cubic meter of air.
Use the following webcalculator to determine appropriate sampling times for varying airborne bacteria concentrations using a single-stage impactor to achieve the goal surface density (1 CFU/cm2). The default impactor has a total deposition area of 75 cm2 and operates at a flow rate of 20 Lpm (0.02 m3/min). Try different values to get an idea of how each variable affects the sampling time.
8Chapter 19: Bioaerosols, Aerosol Technology, Hinds, 1998