Thanks Charles! Though, I still don't understand a meaning of "per week": what week has to do with it?
Hi Olena,
Reference time unit is obviously arbitrary, I suspect the choice of "week" was probably since it eventually yields "conveniently" handled numbers. 
I daresay that, as "attempted" in Post 3, if using a consistent plate size and exposure time, one could appropriately simplify the APHA calculation (as [probably] illustrated in your earlier example, ie -
https://www.ifsqn.co...od/#entry163062
There are various "qualifications" in the Literature regarding the sedimentation method, eg -
The use of settle plates is not recommended when sampling air for fungal spores, because single spores can remain suspended in air indefinitely. Settle plates have been used mainly to sample for particulates and bacteria either in research studies or during epidemiologic investigations. Results of sedimentation sampling are typically expressed as numbers of viable particles or viable bacteria per unit area per the duration of sampling time (i.e. CFU/area/time); this method can not quantify the volume of air sampled. Because the survival of microorganisms during air sampling is inversely proportional to the velocity at which the air is taken into the sampler, one advantage of using a settle plate is its reliance on gravity to bring organisms and particles into contact with its surface, thus enhancing the potential for optimal survival of collected organisms. This process, however, takes several hours to complete and may be impractical for some situations.
29.5.1 Limitation
Though the method has the advantage of simplicity, it has certain limits.
- In this method only the rate of deposition of large particles from the air, not the total number of bacteria carrying particles per volume, is measured.
- Growth of bacteria in the settled particles may be affected by the medium used since not all microorganisms are growing well on all media.
- Moreover since air currents and any temporary disturbances in the sampling area can affect the count, many plates have to be used.
- Since only particles of certain dimensions tend to settle on to the agar surface and, also, the volume of air entering inside the Petri dish is not known, this technique gives only a rough estimate and can be used only to isolate air-borne microorganisms.
- However, one can gather information about the kind of air-borne microbes occurring in a particular area by repeated use of settling plate technique for a fixed period of time
http://ecoursesonlin...w.php?id=101971
JFI another analytical evaluation of sedimentation data utilizes Omeliansky’s equation -
N = 5a × 104 (bt)−1, Where N = microbial CFU/m3 of indoor air; a = number of colonies per petri dish; b = dish surface (cm2); t = exposure time (minutes)
This methodology, since a different unit, utilizes different result objectives such as illustrated in these documents -
Assessment indoor air quality.pdf 655.81KB
45 downloads
Quantifying airborne fungi using Omeliansky formula.pdf 276.22KB
26 downloads
PS -
Edited by Charles.C, 18 May 2023 - 06:10 AM.
expanded