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What is the Acceptable Level of Mold in Buildings? InspectAPedia®  -

• What is the Acceptable Level of Mold in Buildings? Mold Clearance Inspecting and Testing Guide
• How does a building "pass" a clearance inspection?

Here we define the acceptable level of mold in buildings following mold testing or post-cleanup mold clearance tests in buildings. This document is a chapter of the Mold Action Guide which provides an easy to understand step-by-step guide for dealing with toxic or allergenic indoor mold and other indoor contaminants: what to do about mold "mildew," moisture, in your house or office, building-related illness, involving your physician, treatment, sick building investigators, reduction of irritants, and special products to help clean buildings and air.

Extensive, in-depth articles about mold and other indoor air quality concerns are organized at our Mold Information Center © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

ACCEPTABLE MOLD LEVEL What are Acceptable Mold Levels in Buildings

Problems with indoor versus outdoor spore counts

Comparing indoor mold spore levels with outdoor mold spore levels

A common mold remediation clearance test plan specifies that the indoor mold level or spore counts should be no more than 50% of the outdoor level. This approach can generate nonsense, particularly in mold sampling reports I've reviewed in which the genera and species of the indoor mold spores were completely different from the outdoor mold spores. For example an outdoor "Pen/Asp" spore count of 1000 spores/M³ of air might be compared with an indoor "Pen/Asp" spore count of 500 spores/M³ of air.

But often the outdoor "Penicillium/Aspergillus" spores are not the same species as the indoor species, making such count comparisons completely meaningless. Worse, some labs include small basidiospores in their "Pen/Asp" count since often many small amerospores (small round featureless mold spores) like certain Penicillium spores and certain Basidiomycetes are difficult or even impossible to differentiate microscopically. If the indoor and outdoor Penicillium or "Pen/Asp" spores are different species from one another, you are evaluating the effectiveness of a mold remediation project by comparing "apples and oranges."

Examine the indoor mold genera and species

It is important therefore to look qualitatively as well as quantitatively at indoor mold spores after a mold remediation project. If there is a high level of indoor spores of the same genera (and species) as the problem-mold which was originally identified then the cleanup may have been incomplete, regardless of "outdoor spore count comparisons.

A common example of incomplete work in which we find problem-levels of Penicillium or Aspergillus after a mold remediation is when mold-infected fiberglass insulation has not been removed because it "looked clean" to the naked eye of the mold cleanup crew. A simple vacuum sample of that material can indicate whether or not the insulation needs to be removed.

What indoor mold levels indicate a problem

After a mold remediation? It depends partly on site conditions. In a sealed room where moldy debris has been removed, testing immediately may disclose an abnormally high level of indoor problem mold which has remained airborne even though the cleaning has removed all of the original mold reservoir. In that case some additional surface cleaning and fresh air exchange may be all that's needed.

Mold counts versus mold Species: Even more questionable is the use of "generic" airborne mold spore counts without further attention to genera and species. A count of 200 Stachybotrys chartarum mold spores/M³ of air would be unusually high as this mold is not normally airborne. I'd be worried about where those spores came from. Conversely, a count of 200 Aspergillus sp. spores/M³ of air in the same circumstances might be considered very "clean".

What indoor mold spore level is considered "contaminated" then depends in part on what mold genera or genera and species have been identified. But if the indoor mold spore count/M3 of air is high enough, we may decide that more investigation or cleaning is needed regardless. While there is no well-established quantitative standard for fungal spores on surfaces or in air, mold contamination is considered present in a building when the total mold spore concentration per cubic meter of air is above 10,000. (Baxter, ETS). Acceptable levels for individual species vary since species toxicity varies widely as does spore size, weight, and other features which affect risk to building occupants. E.g. Aspergillus/Penicillium in a "clean" residential building study was at a mean of 230, in buildings known to have a moisture or flooding problem it was at 2235 and in mold contaminated buildings the figure was 36,037.

Mold sample processing lab variation

Some writers also warn that mold labs have reliability problems. We agree that two experts counting the same slide will not provide precisely the same counts. Our lab participates in a "round robin" quality assurance program in which a group of expert mold sample processing labs around the country "count" spores on the very same slide (we mail it around). We compare our count results as a method for checking on and refining our procedures.

While counts vary among labs, it is unusual for counts by expert labs following the same (agreed-on) count procedures to vary by as much as one order of magnitude. If the mold sample processing lab is actually using an expert to process the samples (rather high-volume part-time minimally-educated workers), then the variability in lab results is very much less than the variation in mold sample results caused by variations in sampling conditions I discussed below at "Mold Sampling Conditions Cause Wide Variation in Results".

Surface samples of mold:

The presence of toxic or allergenic mold as a dominant particle in any sample (surface or air) is usually a cause for further investigation or remediation. The presence of incidental occurrences of toxic or allergenic material in surface samples requires interpretation in light of other building conditions, type of particle (spore chains), and other factors.

Cultures of mold for Clearance Testing

Since only a small percentage of all molds (perhaps 10%) will grow in any culture at all under any condition, using a culture to screen for problematic mold is a questionable practice. There are uses for mold cultures but we question their application for building screening.

The mold level target is never zero except in special applications such as medical and drug facilities which operate in a "clean room" environment where no stray particles are permitted. Mold is a natural ingredient in outdoor air most of the time. We do not want to find higher indoor mold levels than outdoor, and we do not want to find high levels of problematic mold spores indoors.

Mold Sampling Conditions Cause Wide Variation in Mold Test Results

Warning: interpret all quantitative mold clearance test data with caution, particularly air samples. Individual samples of airborne mold (or any other airborne particles in a building) show tremendous variation from minute to minute, usually by many orders of magnitude, making "ok" mold clearance sampling test results a thing to view with care.

If the mold remediation clearance investigator did not document the sampling method, including where, when, and under what conditions a sample was collected, interpretation of sample results can be tenuous. We re building fans on or off? Were windows open or shut? Was work going on in the area? How long after completion of the mold remediation project did the investigator wait before testing?

Turning on a ceiling fan in a room can increase the airborne particle level by a factor of 1000! In situations of particular risk (such as sensitive occupants) or ambiguously-conducted post mold remediation testing, additional or periodic testing should be considered.

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MOLD ACTION GUIDE - WHAT TO DO ABOUT MOLD
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MOLD ATLAS & PARTICLES INDEX
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More Reading on Mold Levels, Mold Test Interpretation, & Mold Prevention Methods

Mold Levels: allergenic or toxic mold: how much means a problem? It depends.
Validity of Cultures (settlement plates, culture plates, Anderson samplers, or mold test swabs) to find toxic mold in buildings
Mold in Fiberglass Insulation
Mold Prevention: Avoiding Mold Problems in Buildings by Using Mold-resistant Construction Products & Practices
MOLD PREVENTION GUIDE list articles describing steps to reduce the chances of future mold growth in buildings.

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