Interpretation of APHA standard for APC in air

Hey guys, need your help in understanding the meaning of this value 30 CFU.cm-2.week-1. 

Honestly, I've wracked my mind trying to figure out what that would be in plain numbers. I've been studying Charles' "Compilation regarding  Micro.Standards,etc for env. air,compressed air,filters in food plants" and now am trying to understand if my corporate standard for Y&M in the air is correct (less than 100 cfu/plate). 

P.S. Never heard of negative exponent when studying math - but I've never been very good at technical subjects, being more inclined to literature and languages:)

Thanks in advance! 

I think your figure of 30 CFUcm^-2week^-1 would translate to what I've attached to my post, it's hard to draw fractions on this forum! As a written sentence, I believe this would be:

30 CFU per square centimeter per week.

Here is a link on how negative exponents work: https://www.cuemath....tive-exponents/, basically they put figures underneath a fraction (can you tell I'm not a math teacher? :sorcerer: ) So:

5^-1 = 1/5¹ = 1/5.

5^-2 = 1/5² = 1/25

5^-3 = 1/5³ = 1/125

When working with units, it's common to use this notation to show relationships between figures. For instance, the unit of PSI (pounds of force per square inch) could be written as lbf*in^-2 OR as lbf/in². Engineering problems usually result in needing convert units into other units, and this notation can make it easier to keep track of you conversions. 

Would be great to have someone else chime in too, because it's been a while since I've done any of this as well.

Many thanks, Brothbro! That figures - however, what's the meaning of "per week"? I'm collecting air sample into a plate (exposing agar media to the environment for 30 min as per the lab spec), then send it for Y&M testing - and in 5 days I receive, say, 50 cfu/plate result. OK, taking into account an APHA limit "30 CFU per square centimeter" and knowing that surface of a standard petri dish is 145 square centimeters, we can calculate 50 : 145 = 0.34 cfu/sq cm - so, presence of Y&M in my air is way below the limit. And opposite - APHA limit for a standard petri dish is 30 x 145 = 4,350 cfu/plate. Do you think, my calculations make any sense?

I'm not totally clear on the "per week" aspect of your original figure either, you're right that typically the settle plates are left exposed for a certain amount of time. Then they are close, incubated, and analyzed. Doing some digging I see these figured referenced a lot in the attached paper. The methods described here appear similar to yours, where a plate is exposed to air for 15-30 mins then tested. But the paper then goes to state the units for the result are CFU.cm-2.week-1. How did they jump from a 15-30 min exposure time to listing a week in their units? I don't see their work on converting raw counts to the final result. I may be missing something here too. My help may only go as far as explaining negative exponents!

I guess if we simply converted your figure of 50CFU.cm^-2.30min into CFU.cm^-2.week^-1 I believe we'd end up with 114 CFU.cm^-2.week^-1_. This results from converting the minutes into weeks by multiplying by 60min/1hr, then 24hr/1day, then 7day/1week.

The APHA Standard refers to a sedimentation Plate and the observed number of colonies on the Plate (ie cfu/plate). The observed result is required to be "Standardized" for comparison purposes. So data needs to be converted mathematically into cfu / (standard unit area of Plate) / (standard unit length of time).

Assume actual Plate area = A (cm2) (145cm2)

Assume actual Time = T mins (30min)

Assume actual colony count in T min = X cfu (50cfu)

"Reference" Count per cm2, per week = (X /A)  x  ([T /60] x 24 x 7) = 50/145 x ([30/60] x 168) = 0.345 x 84 = 29.0

Seems (just) inside the (recommended) APHA Standard.

The APHA Standard refers to a sedimentation Plate and the observed number of colonies on the Plate (ie cfu/plate). The observed result is required to be "Standardized" for comparison purposes. So data needs to be converted mathematically into cfu / (standard unit area of Plate) / (standard unit length of time).

 

Assume actual Plate area = A (cm2) (145cm2)

Assume actual Time = T mins (30min)

Assume actual colony count in T min = X cfu (50cfu)

 

"Reference" Count per cm2, per week = (X /A)  x  ([T /60] x 24 x 7) = 50/145 x ([30/60] x 168) = 0.345 x 84 = 29.0

 

Seems (just) inside the (recommended) APHA Standard.

Thanks Charles! Though, I still don't understand a meaning of "per week": what week has to do with it?

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. :smile:

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   25 downloads

Quantifying airborne fungi using Omeliansky formula.pdf   276.22KB   16 downloads

PS -

The APHA Standard refers to a sedimentation Plate and the observed number of colonies on the Plate (ie cfu/plate). The observed result is required to be "Standardized" for comparison purposes. So data needs to be converted mathematically into cfu / (standard unit area of Plate) / (standard unit length of time).

 

Assume actual Plate area = A (cm2) (145cm2)

Assume actual Time = T mins (30min)

Assume actual colony count in T min = X cfu (50cfu)

 

"Reference" Count per cm2, per week = (X /A)  x  ([T /60] x 24 x 7) = 50/145 x ([30/60] x 168) = 0.345 x 84 = 29.0

 

Seems (just) inside the (recommended) APHA Standard.

Excuse me, but I'm curious about the calculation part, the APHA is using the "cm^-2.week^-1" but the petri dish area is 145mm^2. Should we convert the A from 145 mm to 14.5mm? So, their result should be "290 (289.65) CFU.cm-2.week-1. " instead? Am I missing something? I'm trying to implement this and a bit confused on this part as well.