How to Read Mold Test Results

How to Read Mold Test Results

May 05, 2022

How to Read Mold Test Results 

You finally have your results back from the mold test and you open the file to discover none of it makes sense. This is where we come in. We answer hundreds of calls a month asking about how to interpret mold test results. This is our specialty as we have over 15 years experience of interpreting mold data. In that time period we estimate that we have reviewed/consulted over 100,000 reports. We have served as an expert witness in court cases concerning mold and we understand how confusing some of this terminology can be. So Lets get to it.

 Determine the type of Test result you have

Your first step is to determine the type of results you have. The majority of mold samples will either be surface ( collected with a tape slide or swab direct from a surface) or an air test (collected using a pump and a spore trap cassette). We will go over each type below in detail.

 Surface Sampling

Surface sampling typically yields a clear yes (Fungal Growth) or no (No Fungal Growth) result. On our reports specifically on the second page there is a row that says Impression (this word is highlighted in green) to the right of that will be a result that says either: No Fungal Growth, Minimal Fungal Growth, or Fungal Growth.

No Fungal Growth = not enough mold to indicate a growth on this surface

Minimal Fungal Growth = Enough mold to indicate a growth but it is usually at the beginning of a growth ie. The area has only recently became wet or was only getting wet for a short period of time. This could also mean that the area gets wet but not that much water (condensation)

Fungal Growth = Mold has grown or is growing on this surface. The area gets enough or at one time was getting enough water to support a mold growth.

Below the impression line you will notice that the mold types are listed out on the left of the report. If the space to the right of the mold type is blank this will mean that we have tested for that mold type but did not find any. If the space is not blank it can possible contain : Scattered,Very Low, Low, Medium, High , or Very High.

Scattered = A very small amount of mold spores found and they were Scattered over the sample in no particular pattern (mold grows in patterens like branches on a tree.)

Very Low = 1/8 of the surface of the sample is covered with spores of this mold type.

Low =   ¼  of the surface of the sample is covered with spores of this mold type.

Medium =  ½ of the surface of the sample is covered with spores of this mold type.

High =  ¾ of the surface of the sample is covered with spores of this mold type.

Very High = Almost the entire surface of the sample is covered with spores of this mold type.

In the picture above Kitchen #2 and Kitchen # 3 are positive

 So basically if you have a positive result your samples will say Fungal Growth and one of the mold types below will have very low to very high listed. A positive results represents an issue on this surface that will require remediation. The first thing to consider is how is this area getting wet. Mold needs water to grow so any surface that gets a positive result is getting wet somehow. The surface could be getting wet from a leak or from condensation.

 While any growth requires cleanup certain mold types can be more dangerous : Chaetomium, Stachbybotrys, and Pen/Asp.

These mold types are also commonly found when you have a leak or a flood because they need more water to grow.

So while typically Scattered is a non issue. If one of the above listed mold type shows up ,even with a Scattered result, you should visibly investigate further to determine if there is a hidden leak or water source. 

 Air Test Sampling

Air Sampling is normally a little harder to interpret and it doesn’t allows yield a simple yes or no result. Sometimes your results can land in a grey area that does not give us a definitive answer. Hopefully you will gain some understanding on how to read the results and also the limitations of this type of sampling.

 Air tests will give actual numbers associated with the different mold types we found. Again if the space is blank that simply means that we tested for this type but did not see it. There are 2 sets of numbers that you will see reported on air test results: Raw Count and Spores / M3 (spores per meter cubed).

Raw Count = the actual number of spores seen and counted by the analyst

Spores / M3 = An industry reporting standard that we calculate using a formula to convert litres of air into meters of air.

To calculate spores / M3 we have to first identify the volume you collected. This is typically 25 L or 75 L when using our kits. Total volume is the flow rate of the pump x how many minutes ran ( 15L flowrate x 5 minutes will give us a volume of 75 L)  Once we have the total volume we can insert it into the formula

 1,000 / Total Volume = Detection Limit  

 (1,000 divided by Total Volume= Detection Limit)

Detection Limit = The lowest number of spores per meters cubed that can be found according to the total volume.

So here is an example

Raw Count = 2   Total Volume = 75   Detection Limit = 13   Spores/M3 = 26

Raw Count x Detection Limit = Spores / M3

So I can see that maybe we are getting a little too technical with this, but I want to give a brief understanding of how the numbers are calculated so that all of these words are not confusing you.

Another important note is that generally you want to have a control outdoor sample ( a sample to compare to) to be able to judge against. If you do not have a control sample you can still gather info from the results but those results will not be as good as they could be if you had taken the outdoor control sample.

Background Debris = the amount of non mold particles seen on the sample. Background Debris is given a rating from 1-5 with 1 being the lowest. The higher the amount of debris the more difficult the sample is to read because some of the debris could be obscuring mold spores. So a sample with a high background debris will be less accurate that one with a low amount of debris. 3 is the average and is considered good.

Now that we understand what the numbers are we can dive into what levels indicate an issue. All of the numbers discussed below will be represented in spores / M3.

 A good starting point is to see if any of your samples has a total spore count of more than 1,000 spores/M3.

600-999 Total spores/M3 is that grey area I mentioned earlier. This is where an outside control can help tremendously.

1,000 spores/M3 is where we would start to mention that there is a possibility of an issue and that further visual investigation is needed to rule out a hidden leak etc. This is a sample that should be compared to outside to see if there are more spores outdoors than indoors.

The samples should be compared to the outdoor control to rule out false positives. If the outdoor control has similar mold types and has more spores than the inside then we can assume a good many of the indoor spores are simply coming from outside. So the outside spore totals will negate some of the indoor totals. This should all be considered when making a determination.

In the report above the Kitchen and Basement samples are positive. Even though their total spore counts are lower than outside you must also consider the types of spores when comparing inside to outside. You can not simply look at the total and make a judgement.

There are also some specific mold types that also hold more weight when it comes to interpretation.

Penicillium/Aspergillus = If you find more than 600 spores/M3 of this mold type we would consider this a positive result

Chaetomium =  If you find more than 300 spores/M3 of this mold type we would consider this a positive result.

Stachybotrys = = If you find more than 1 spores/M3 of this mold type we would consider this a positive result.  Stachybotrys is a very dense spore because it needs more AW(water activity) to grow. With it being so dense it does not often appear in air results. This is why there is such a low number that triggers a positive result.

 This is a general overview of how the results can be interpreted. You should not use this guide as a substitute for professional advice. If you have looked over your results and this guide and are still not sure what your report means then please reach out to us directly and we will help you.



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