NES’s Informal Spore Guidelines

 Interpreting Airborne Spore Data – Guidelines:

With the absence of numerical governmental standards in the United States, assessors are often unsure how to interpret data they have collected from airborne samples.  Historically the initial interest in airborne mold sampling built on the Industrial Hygienists’ concern for the presence of infectious organisms in the air, primarily bacteria.  These biological agents had their impact by growing or reproducing in mammals and disrupting an individual’s normal physiology, perhaps having such a significant impact that death was the consequence.  Hence initial sampling was for viable organisms found by culturing samples.

With the increasing awareness that a major impact from exposure to mold spores was allergic reactions, etc., not infection, there has been a transition in recent years to total spore sampling using any of various spore traps.  Spore traps are now the dominant type of airborne mold sampling.  While there are many technical discussions on the trade-offs between short- term and long-term sampling, number of samples for a proper characterization of a space, and whether the space is “moldy” or not, the assessor often is overwhelmed by the technical issues when he just wants to have some idea about whether his sample data indicate a problem condition or not.

There have been recent papers, presentations, and scientific articles that address this general topic.  What emerges from these evaluations is that the most common indicator of mold problems in a damp environment is the elevated presence of Aspergillus/Penicillium like spores, not unexpected since they are primary colonizers according to the World Health Organization (WHO – 2009 Table 1 below) and often amplify in the indoor environment in response to increased moisture.

Michael Meyer:

Among the assessors who have evaluated at the data patterns from samples collected over the years is Michael Meyer who presented a paper on his evaluations at the 16th Annual Meeting of the Indoor Air Quality Association, Orlando, FL (“An Argument for Developing an Airborne Total (non-viable) Mold Guideline for Indoor Exposure”).  Based on thousands of airborne tests at some 394 sites, Mr. Meyer found there is often a large difference between the types and levels of mold in non-problem versus problem rooms (even in the same building) (see: Meyer 2009 -, “What are normal and problem indoor total (non-viable) airborne mold levels? An IAQ consultant’s perspective based on 10 years’ experience at hundreds of houses and offices in the Midwest, Healthy Buildings 2009, Syracuse, NY, paper 203).  In that 2009 paper, Mr. Meyer “compiled statistics from 394 buildings for total mold, Cladosporium, Aspergillus/Penicillium and Chaetomium-Stachybotrys (Meyer, 2009)”. Mr. Meyer “found that the upper range of normal can be a useful guide – being below levels where health symptoms are usually noticed but still high enough to allow some flexibility in interpreting results (avoiding excessively strict interpretations)”.

“For non-problem buildings and rooms (without noticeable complaints)”, Mr. Meyer found “total mold spores often <2000-4000 total spores/m3, Cladosporium often <2000-3000 spores/m3, Aspergillus-Penicillium often <500 spores/m3 but sometimes present at <1000-2000 spores/m3. For Chaetomium-Stachybotrys, we typically found no detectable spores but in some cases a few spores were present at <250-500 spores/m3.”  “For complaint buildings, there was an often large gap between non-problem and problem levels. The key problem indicator was usually Aspergillus-Penicillium.  When the Asp/Pen level exceeded about 5,000 to 10,000 spores/m3, health complaints became more frequent (sometimes only affecting one or two persons in a house or office). Increasing health symptoms (affecting several persons) often occurred when (Asp/Pen) levels >10,000 to 100,000 spores/m3.  Elevated levels of Chaetomium-Stachybotrys above about >500-1000 spores/m3 were also often associated with health symptoms. We can exploit this gap to avoid problems with too strict levels (relying on “normal”) while still meeting levels below which the great majority of persons will not notice symptoms.”  “Of course, for some special situations, such as the operating rooms of hospitals, much lower levels may be appropriate.”

“By using a range of values and more than one type of mold, this provides some flexibility and is less prone to rigid misinterpretation. These are realistic goals that cleaning firms can normally meet without great difficulty (following appropriate IICRC guidelines).”


Joe Spurgeon, PhD:

Joe Spurgeon, PhD, presented a paper entitled “Effects of Sampling Time and Data Interpretation Methods on The Quality of Airborne Data,” at the 16th Annual Meeting of the Indoor Air Quality association (Orlando, FL) in which he reviewed various means of data analysis and extracted data from two other studies plus his own data regarding guidelines that are suggested by the data.  The following table summarizes his findings with a focus on Aspergillus/Penicillium data from spore traps, since the Asp/Pen molds are primary colonizers (WHO) in damp environments.  They are the spore type frequently reported by these and other studies as early indicators of contamination in the indoor environment.  The following table from Dr. Spurgeon’s talk is a summary of guidelines from three studies cited by Dr. Spurgeon:

Table: Asp/Pen levels considered as indicative of a “moldy” environment by three independent studies:

Baxter data: Asp/Pen ≥ 950 spores/m3

Rimkus data: Asp/Pen ≥1,000 spores/m3

Spurgeon data: Asp/Pen ≥ 1,000‐1,100 spores/m3

Baxter:  Baxter, Perkins, McGhee &Seltzer; JOEH, 2:8-18 (2005)

Rimkus:  Data gathered by the Rimkus Consulting Group and provided to J. Spurgeon for statistical analysis – See “Interpreting Airborne Fungal Spore Samples Part 1: Methods for Comparing Data (2004)” at Dr. Spurgeon’s website –

Spurgeon:  Data gathered by Dr. Spurgeon, mostly in California, and included in the above 2004 paper on Dr. Spurgeon’s website –

The data in this table are generally consistent with the perspective provided in Michael Meyer’s article and talk – “for non-problem buildings Aspergillus/Penicillium (spores were often found at levels) <500 spores/m(less than) but sometimes present at <1000-2000 spores/m3.”


The assessor may utilize the above information and other guidelines not cited here to develop his own guideline.  The assessor needs to recall that the above levels were generated by the subjective criteria for “complaint” versus “non-complaint” and “problem” versus “non-problem” areas used by the general public.  Should the assessor be aware that sensitized or allergic individuals may well be occupying these areas, it might be appropriate to tighten one’s use of these informal guidelines by applying lower number in evaluations when such occupancy is known.  Overall it is important to have some rationale behind any guideline used and then consistently apply it.  As more research is carried out and reported in the technical literature, additional perspectives on potential guidelines are likely to emerge and may be incorporated in deciding on an informal guideline to use.


Bottom line for Nauset Environmental:

Since the above cited studies have a general common convergence on 1,000 spores/m3 as the transition from “complaint” to “non-complaint” locations  or “problem” to “non-problem” locations for Aspergillus/Penicillium, NES uses 1,000 spores/m3 as its informal guideline for the general population.  Knowing that some sensitized individuals are affected at lower levels, if such individuals are present or occupy a given structure, NES utilizes 500 spores/m3 as the guideline to meet when sensitized individuals are likely to be present.


Please contact NES to answer questions you may have about this topic.


WHO guidelines for indoor air quality: dampness and mould (2009) ISBN 978 92 890 4168 3

Table 1. Moisture levels required for growth of selected microorganisms in construction, finishing and furnishing materials
Moisture level Category of microorganism
High (aw, > 0.90; ERH, > 90%) Tertiary colonizers (hydrophilic) Alternaria alternata Aspergillus fumigatus Epicoccum spp. Exophiala spp. Fusarium moniliforme Mucor plumbeus Phoma herbarum Phialophora spp. Rhizopus spp. Stachybotrys chartarum (S. atra) Trichoderma spp. Ulocladium consortiale Rhodotorula spp. Sporobolomyces spp. Actinobacteria (or Actinomycetes)
Intermediate (aw, 0.80–0.90; ERH, 80–90%) Secondary colonizers Aspergillus flavus Aspergillus versicolora Cladosporium cladosporioides Cladosporium herbarum Cladosporium sphaerospermum Mucor circinelloides Rhizopus oryzae
Low (aw, < 0.80; ERH, < 80%) Primary colonizers (xerophilic) Alternaria citri Aspergillus (Eurotium) amstelodami Aspergillus candidus Aspergillus (Eurotium) glaucus Aspergillus niger Aspergillus penicillioides Aspergillus (Eurotium) repens Aspergillus restrictus Aspergillus versicolorb Paecilomyces variotii Penicillium aurantiogriseum Penicillium brevicompactum Penicillium chrysogenum Penicillium commune Penicillium expansum Penicillium griseofulvum Wallemia sebi
Note. aw, water activity; ERH, equilibrium relative humidity Sources: Grant et al. (1989); Gravesen, Frisvad, Samson (1994); ISIAQ (1996) a at 12 °C; b at 25 °C


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