Originally Posted By: Caoimh?n P. Connell
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OK here?s the scoop- There are a number of ways this could pan out where a lab result of 5,000 CFU/g indicates a lower mould concentration than 1,000 CFUs/g. But, I?m only going to select two scenarios (long-windedness notwithstanding). Importantly, I would like to repeat something I said on May 2, on this board ( http://www.nachi.org/bbsystem/viewtopic.php?t=10216
) regarding data quality objectives (DQOs) which was: ?Without DQOs, you don?t have data, you have numbers or names on a lab report that possibly CANNOT be interpreted by anyone, since ?data? has no intrinsic meaning outside of a priori decision criteria
.? Such is the case here.
The units of expression, a CFU/g is a unit that is used for the convenience of the lab, not necessarily for that of the consultant or, in this case even meaningful, since the unit is missing tons of information not included in the report (possibly not even provided to the lab, known by the lab, or even known by the ?mould inspector.?)
Here is how this pans out; two carpets were sampled in exactly the same manner with exactly the same proper sampling and collection equipment, and submitted to a reputable lab for analysis.
Carpet #1 is in a brand new model house, in which no family has ever lived, and with brand new carpets, and no history of water damage and no mould problem. Sampling parameters: Sample area = 12 inches by 12 inches, sample result =5,000 CFUs/g. The lab, in order to report the value, must weigh the material collected in the sample, thus providing the denominator of the unit. In this case, the cassette contained 0.01 grams of vacuumed material (which is about right for a brand new carpet).
Carpet #2 is from a ten year old family home with dogs, cats, kids spilling food and real people living real lives. Sampling parameters: Sample area = 12 inches by 12 inches, sample result =1,000 CFUs/g. In this case, the cassette contained 0.35 grams of vacuumed material, which is about right since Dad didn?t have time to run the vacuum cleaner last Saturday.
We look at our DQOs and determine what is the question that is being asked? Which is ?Which carpet is most heavily contaminated with mould?? But the answer being given by the lab is to the question ?Which amount of removable debris contains the most mould?? That is a different question and does not meet one of our DQOs.
So our first step is to convert the reported lab units into a number that will speak to our question; that number is CFU density in the carpet, which is expressed as ?CFU/g/area of carpet (in square centimeters, cm2)? which algebraically becomes CFU/cm2 and which expresses the amount of mould in the carpet per gram of material removed per unit of area sampled.
In this example, Carpet #1 with 5,000 CFUs/g = 0.05 CFU/cm2; and Carpet #2 with 1,000 CFUs/g is equal to 0.38 CFU/cm2. NOW we have two sample results which can be compared with each other. As you can see 0.38 is much greater than 0.05 even though it came from a sample that had a lower result reported by the lab. Now we take our numbers and determine the final answer to our question.
Both of the carpets measure 12 feet by 20 feet. Carpet #1 (5,000 CFUs/g) contains an estimated total fungal loading of only 12,000 viable spores; while Carpet #2 (1,000 CFUs/g) contains an estimated total fungal loading of 84,000 viable spores; seven times more contaminated than the carpet that contained 5,000 CFUs/g.
Now, which sample result indicates more contamination? Answer 1,000 CFU/g ? OBVIOUSLY! (The poor home inspector buries face in hands and weeps gently in the back of the court room, as the two million dollar law suit against him inches just a little closer to reality?)
Here is the second scenario:
The lab tells you, the HI, that you need at least half a gram of material for them to analyze the sample. So in Carpet #1, being a brand new carpet, in order to get the requisite 0.5 grams, you have to sample a carpet area of 60 inches by 120 inches (It?s a pig! Trust me, I?ve had to do it!) But in the Smith residence, (you know, dogs, cats, kids), you only need to sample an area of 14 inches by 15 inches to get the half a gram of debris needed by the lab.
Now, in each case, you have the same amount of vacuum debris, 0.5 grams, but in one case you had to sample 50 square feet (the nice new clean carpet) to get it, and in the other case you only had to sample 1.5 square feet to get the sample. But now there is a disparity in areas, so, again, we convert the units into density, and we see that the results still come out the same, and the nice clean never walked on carpet contains 5,000 CFUs/g, and the dirty mould infested carpet contains 1,000 CFUs/g.
A person, like myself, who makes a living out of collecting samples, and interpreting lab data, and then defending that data in court, won?t get tripped up or fooled into thinking that 5,000 is greater contamination than 1,000 because we use our DQOs to guide us, and the intermediate numbers are not the answer to the question; often they aren?t the answer to ANY question? they're just numbers! However, other consultants who think interpreting a lab result is as easy as assuming the higher value means more and the lower value means less ? meets me in court, and looses their shirt (and their reputation) and possibly some finances along the way.
By the way ? these are just two examples of why 5,000 is less than 1,000 ? there are others even when one has accounted for debris and area. This is just the tip of the industrial hygiene ice burg that will sink a ?certified mould inspector? or other untrained mould "expert" every time. These aren?t tricks, these are the realities of the expertise of sampling (for anything).
By the way these same concepts just came up in a case that had nothing to do with mould - a bunch of engineers, who had no competency in sampling tried to interpret chemical concentrations in air based on the lab report. Unfortunately, they lacked the technical expertise to understand the conversion of a nanogram per liter of air into a part per million- it is likely that their firm is going to have to part with several hundreds of thousands of dollars in damages, based on my report, all because they were engineers pretending to be industrial hygienists investigating indoor air quality issues.
I hope this sheds some light on issue. By the way, I didn?t even touch on sampling error, (which is one of my fortes), and could even sink a PhD CIH ? and which did, in a real life trial, because even being a CIH is no axiomatic badge to competency in microbial issues (and the PhD didn't help her one bit).
Again ? my bottom line ? if one didn?t know all this before, and one can?t anticipate the other scenarios to which I alluded, its my humble opinion that one should not be out pretending they are performing mould sampling and/or mould assessments, and they shouldn?t be pretending they are interpreting data.
Enlightenment before agreement.
Caoimh?n P. Connell
(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)