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Mold Levels: allergenic or toxic mold: how much means a problem?
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• Mold exposure limits, contamination levels
• Variation in mold toxicity
• Mold exposure standards - various
• Effects of mycotoxins, toxicity of Stachybotrys, Penicillium, Aspergillus

What mold spore levels indicate contamination?

Mold spore counts and mold culture results are not reliable alone for evaluating a mold problem

Warning: fungal spore counts in indoor air, whether "viable" or "non-viable" counts, as well as swab and culture methods to identify just what mold is present in a building are questionable methods to characterize the presence or absence of a mold problem indoors. From experience carefully inspecting many buildings for mold problems and simultaneously collecting many types of field samples and examining them in my lab, my view is that anyone who makes one or a few indoor air measurements or relies on culture plates or swabs to "characterize" the mold level and mold species in a building is risking being way off target, particularly if the "test results" show low numbers.

This document provides information about airborne mold spore counts, surface density, and other measures of the exposure level of allergenic, infectious, and levels of toxic mold in residential buildings: At what level is allergenic or toxic mold considered a problem in buildings? How do we obtain an accurate and meaningful measurement of mold exposure indoors? © Copyright 2008 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.

Reasons Simple Airborne Mold Counts Alone Are Not Good Indicators of Risk

"Mold Levels" warning: single number "mold levels" are unreliable as a measure of health risk in buildings for several reasons including at least the following:

1. Particle levels vary: the actual level of indoor particles in air varies by several orders of magnitude over very short few-minute intervals, making reliance on any single measurement questionable, particularly if the measurement does not show evidence of a problem. Simple mechanical disturbance like turning on or off a fan can completely change individual measurement results. Short term measurements are therefore inaccurate.
2. Human sensitivity varies: because of wide variation in individual human reaction and risk to mold, variation in allergenicity, pathogenicity, and toxicity among mold genera and individual species, and even additional variation in these levels depending on specific environmental conditions such as the substrate upon which a mold is growing (its food), quantitative risk-levels are better used as an overall indication of building cleanliness before and after a mold cleanup, and not as an absolute level of risk to building occupants.
3. Particle toxicity varies: There is very wide variance among the toxicity of individual mold species, ranging from none to probably highly toxic, pathogenic, or even carcinogenic.
4. Spore size or mass varies widely from less than 1 u to over 200u, so a large spore contains potentially more harmful material than a small spore of the same "toxicity," making a "count" that does not identify the particle and particle size ambiguous
5. Mold spore toxicity vary, even within an individual species, depending on what substrate the species is growing on in a particular instance, e.g. wood vs. drywall.
6. Viable vs. non-viable "spore counts" vary in significance: procedures that use cultures to identify "viable" spores may omit high levels of non-viable spores which nonetheless remain highly toxic. Methods that rely on culturing have a high risk of identifying a mold which is present but is not the dominant or even the most problematic mold in the building, while completely failing to disclose a problem mold which is present but which simply does not grow in the culture media used, or is overgrown by another species in that media. For more details see Mold Spore Counts: are indoor fungal spore counts valid? for further explanation.

That said, here is a sampling of some indoor air mold spore exposure level criteria and comments

Contamination levels of non-specific allergenic, pathogenic, or toxic fungal spores

1. Baxter et als: Mold contamination is considered present in a building when the total mold spore concentration per cubic meter is above 10,000. My own field and lab experience confirms this view. However in special cases, even low quantitative levels of certain particles or particle types (such as Pen/Asp spore chains in an un-treated building) may be diagnostic and may indicate a hidden mold reservoir that at least merits further investigation.

2. The National Allergy Bureau considers mold counts in air of 0-900 as low, to 2500 as moderate, to 25,000 as high, and above 25,000 as very high. At "high" levels most individuals with any sensitivity will experience symptoms. 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.

3. The University of Minnesota data (www.dehs.umn.edu/iaq/fungus/mycoglos.html) presents this table for mold levels expressed in colony forming units per gram. WARNING: mold spores may be not viable (dead), wrong culture media may be used, or one species can overgrow another, etc. - so don't produce any colonies, but may be toxic if inhaled (such as some species of Penicillium). So I would not rely on culture data. But here it is:

Concentration Qualitative Assessment of Contamination

Colony Forming Units/gram - cultured mold samples

• less than 10,000 - low
• 10,000 to 100,000 - medium
• 100,000 to 1,000,000 - medium to heavy
• > 1,000,000 - heavy

4.The American Conference of Government Industrial Hygienists (ACGIH) stated (Harriet Burge et. al) stated in 1987 that indoor mold levels are generally less than 1/3 the outdoor level and that when indoor mold is at more than this level remedial action should betaken to find the source of the elevated counts and to clean it up.

5.Other proposals have included a limit of 500 CFU/cubic meter in winter indoors in sub arctic climates (Reponen et al.1990). I suspect that focusing only on "colony-forming-units" (that is, viable mold that will grow) is a reliable way to check out a building. Elsewhere I have cited the errors and limitations of using cultures to check for viable mold, and I suggest that even non-viable mold may be allergenic or toxic.

Mold Exposure Levels - Sources of Variation in Mold Toxicity - a warning about single numbers

Single number "mold levels" are very unreliable as a measure of health risk in buildings for several reasons including at least the following:

Particle levels vary widely over short time periods: The actual level of indoor particles in air varies by several orders of magnitude over very short few-minute intervals, making reliance on any single measurement questionable, particularly if the measurement does not show evidence of a problem. The absolute level of airborne particles in buildings varies enormously, possibly by a factor of hundreds to thousands, over intervals as short as a few minutes. Simple mechanical disturbance like turning on or off a fan can completely change individual measurement results. Short term measurements are therefore inaccurate and even longer term measurements are inaccurate if they do not consider the in-use variations in dust disturbance in a building.

Human sensitivity to mold varies: Individual susceptibility to mold/mycotoxin/aflatoxin-related or allergy-related illness varies widely and human exposure is complex because people move among a variety of environments Finally, because of wide variation in individual human reaction and risk to mold, variation in allergenicity, pathogenicity, and toxicity among mold genera and individual species, and even additional variation in these levels depending on specific environmental conditions such as the substrate upon which a mold is growing (its food), quantitative risk-levels are better used as an overall indication of building cleanliness before and after a mold cleanup, and not as an absolute level of risk to building occupants.

Mold Spore - Particle toxicity varies for many reasons

There is very wide variance among the toxicity of individual mold species, ranging from none to probably highly toxic, pathogenic, or even carcinogenic

Mold Spore size varies widely from less than 1 u to over 200u, so a large spore contains potentially more harmful material than a small spore of the same "toxicity," making a "count" that does not identify the particle and particle size ambiguous

Mold spore toxicity may vary, even within an individual species, depending on what substrate the species is growing on in a particular instance, e.g. wood vs. drywall.

Viable vs. non-viable toxic mold "spore counts"

These mold spores vary in significance: procedures that use cultures to identify "viable" spores may omit high levels of non-viable spores which nonetheless remain highly toxic. Methods that rely on culturing have a high risk of identifying a mold which is present but is not the dominant or even the most problematic mold in the building. For more details see Mold Spore Counts: are indoor fungal spore counts valid? for further explanation.

Other Government Mold Exposure Standards

• Brazil < 750
• CMHC >>50 >200 >500
• CEC <2000, <500, >500

• Czech >2000 complaints
• Finland >100-500 = investigate
• Netherlands >10<2000, <500, >500
• Czech >2000 complaints
• Finland >100-500 = investigate
• Netherlands >10⁴ >500 pathogen
• New York City Department of Health Mold Severity Levels: > I/O >1, >1000 Evacuate
• Nordic Council 1000-10,000 Sick
• Poland >5000
• US OSHA >1000 (Anderson)
• World Health Organization - WHO mold standard: <500, <150, >50

Worldwide Mold Exposure Standards

An extensive collection of mold exposure standards from around the world has been published as "Worldwide Exposure Standards for Mold and Bacteria," Robert C. Brandys, Gail M. Brandys, available atwww.oehcs.com

The Toxic Effects of Mycotoxins

Mycotoxins are produced by some common molds found in buildings, including Aspergillus, Penicillium, Fusarium, Stachybotrys, and even Alternaria. Common classification groups of mycotoxins include aflatoxins, fumonisins, trichothecens, and ergot alkaloids. Arora has pointed out that the following conditions are necessary for mycotoxins to affect humans:

1. The right environment must be present for mycotoxin-producing molds to actually produce mycotoxins. [I have pointed out elsewhere that a given mold may produce or not product mycotoxins as a function of varying environmental conditions including the presence or absence of particular nutrients available to the mold -- i.e. it may depend on just what material is supporting the mold growth, not simply on the presence of mold and moisture.]
2. The mycotoxin(s) must have a pathway from the source to a susceptible person.
   The pathway may be direct contact with moldy material, inhalation of airborne spores or fungal fragments or of contaminated building materials. Important in understanding this pathway and the significance of "moldy odors" which I ascribe to volatile organic compounds given off by some molds under some conditions, mycotoxins are not volatile. Therefore, as Arora explains, airborne mycotoxin transmission has to be by inhalation of aerosolized mold or mold-contaminated materials. I point out elsewhere that the particle levels in air in a building vary widely over time for many reasons. However it should be no surprise to find airborne mold or mold-contaminated debris in a moldy building, particularly if demolition or improper "mold remediation" is going on.
3. The person has to absorb a toxic dose of the mycotoxin(s) over a sufficiently short time period. "Very little is known about what constitutes a toxic dose for humans -- see Stachybotrys chartarum mycotoxin discussion below.

Contamination levels of specific allergenic or toxic mold spores

Some species specific acceptable levels have been addressed and vary considerably by species, environmental conditions, and proponent.

REFERENCE:A number of resources cited here are reviewed more carefully in "Comparisons of seasonal fungal prevalence in indoor and outdoor air and in house dusts of dwellings in one Northeast American County," Ping Ren, Thomas M. Jaunkun and Brian P. Leaderer, Division of Environmental Health Sciences, Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven Ct. and appearing in the Journal of Exposure Analysis and Environmental Epidemiology (1999) 9, 560-568. http://www.stockton-press.co.uk
See our main website (below) for important additional information such as mold testing, cleanup and mold remediation guideline resources.

Stachybotrys chartarum in air

According to A.S. Arora: Stachybotrys chartarum strain s72 contains 1.0x10-4 nanograms of satratoxin H per spore. 10 billion spores of s72 S. chartarum must be present in a single cubic meter of air to produce one milligram of satratoxin H per meter of air. Dr. Arora continues: "... it was calculated that a no-effect dose [in rats] corresponds to a 24-hour exposure to 3 billion spores per cubic meter of air for an infant [human], 9.5 billions spores per meter for school-age children, and 22 billion spores per cubic meter for adults." Dr. Arora did not expand his discussion to address any potential cumulative toxicity of longer term exposure to lower levels of mycotoxins in buildings.

This "toxic black mold," Stachybotrys chartarum, infamous thanks to media attention, is not designed for airborne transmission, being large and sticky. I find it in air samples when a dense colony of such mold is being mechanically disturbed, but I have never seen it in air anywhere close to the levels cited as the "no-effect dose." I believe that we've been barking up the wrong mold-tree, and that one would be smarter to look in buildings for the Penicillium and Aspergillus families which spread throughout a building easily by airborne transmission.--DF

REFERENCE: Content of this section paraphrased and others quoted from a guest column, "Understanding the Health Effects of Mold," by Dr. Ajit S. Arora, MD, PhD, appearing in AIHA's magazine "The Synergist," September 2003, in the AIH Diplomate section, Pages vi-vii. Dr. Arora is a forensic medical examiner and medical toxicologist in Los Angeles. see partheniamedical.com.

Penicillium sp.

"Clean" residential buildings are typically 230/m3 +/- 630.
Buildings with evidence of flooding are typically much higher: 2200/m3 and
mold-damaged buildings are typically extremely high in comparison: 36,000/m3.

I usually find only trace levels of Penicillium/Aspergillus indoors in residential buildings and offices, so counts even close to the "clean" level may make further investigation a reasonable course of action, particularly where the building houses occupants have complaints possibly due to mold, or occupants who may be at extra risk.

Aspergillus sp.

Holmberg (1987) associated Aspergillus spore levels over 50 CFU / cubic meter with sick building syndrome. I comment elsewhere that depending on culturing mold in buildings is unreliable since toxic or allergenic particles may be present but may be of a species or genera which either does not grow on a culture medium, or which is overgrown by something else on the medium. Further, relying on settlement plates to culture mold produces quantitatively skewed results since different particles settle out of air at different rates, confounding an estimate of their numbers. Anderson(TM) multi-stage samplers which develop culturable samples attempt to overcome the particle size problem.(C)Trap DJ Friedman

Aspergillus sp. may be the most common and serious airborne mold problems in buildings: Opinion: from the author (DJ Friedman) based on extensive field investigations and laboratory work, I suspect that several species of Aspergillus are probably the most common and widespread problem mold found indoors in buildings. These spores are small, easily airborne, grow on a variety of surface materials, and move readily throughout a building almost like a gas (due to their small size) riding on building convection currents and remaining airborne for long periods of time.

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More expert information on this topic

More Information Testing and Reporting Mold Problems in Buildings & on Building Diagnostic Inspections and Repairs

• Mold Levels on Surfaces in Buildings This article discusses mold test report terms - one set of suggestions about how to report mold levels in mold test samples of surfaces in buildings
• Mold Investigation Methodology. Mold & air quality inspection and testing: which home test kits for mold are valid - tape, swabs, cultures & air testing.
• Mold Risk Levels in Buildings: Based on Visual Inspection.
• Mold Spore Counts: are indoor fungal spore counts valid?
• Mold Classes: what types of cosmetic, allergenic, or toxic mold are a problem? Can mold be cleaned-up successfully?
• Validity of Bulk Surface or Tape Sampling for toxic mold inspections
• Validity of Air Sampling for toxic mold inspections
• Validity of culture methods for toxic mold inspections - home test kits, sterile swabs, settlement plates, Petri dishes
• When is it appropriate to hire a professional to inspect a building and test for toxic or allergenic mold?
• What to look for in a toxic survey report or toxic mold test lab report

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