Indoor Air Quality
IAQ Guidelines & Reference Tables
Interpretive guidelines and reference tables for indoor air quality mold spore analysis. Reference data drawn from Baxter, D.M. (1995).
Normal Spore Levels
Indoor spore levels usually average 30% to 80% of the outdoor spore levels at the time of sampling, with the approximate same distribution of spore types. Filtered air, air-conditioned air or air that is not in the proximity of outdoor sources may drop to 5% to 15% of the outdoor spore levels at the time of sampling. As these are general guidelines, a major factor is the accessibility of outdoor air. A residence with heavy foot traffic, open door and windows, etc., may average 95% of the outdoor levels. An office building with limited air exchange may average as low as 2% of the outdoor levels. Dusty interiors may exceed 100% of the outdoor spore levels but will mirror the outdoor distribution of spore types.
Problem Interiors
A substantial increase of one or two spore types, which are inconsistent and not reflective of the outside spore distribution. This is usually indicative of mold growth.
Data Interpretation & Air Quality Standards
There are no specific regulations governing surface microbiological contamination or airborne microbiological contaminants in indoor air (bioaerosols). This is in part due to the many variables involved with sampling for microorganisms, dramatic fluctuations in background levels of microorganisms, lack of agreement between researchers about what constitutes a "problem situation" and an overall lack of industry experience in interpreting microbiological laboratory data. Therefore, it is critical that the indoor air consultant be able to combine experience and knowledge of microbiology to evaluate laboratory results of samples collected from interior locations.
Suggested Guidelines for Mold Spore and Skin Cell Fragment Concentrations
Residential Buildings · Counts per Cubic Meter (m³)
| Suggested Guideline | Total | Penicillium / Aspergillus | Ascospores / Basidiospores | Cladosporium | Zygomycetes | Skin Cell Fragments |
|---|---|---|---|---|---|---|
| "Average" Clean Residence | <1,800 | <600 | <200 | <100 | <100 | <9,000 |
| "Clean" Residence (Maximum) | <3,000 | <1,400 | *<900 | *<800 | <600 | <16,000 |
| Indoor Contamination Present | ***>8,000 | >4,000 | *>1,500 | *>600 | >700 | >20,000 |
| Indoor Amplification May Be Occurring | *>12,000 | >8,000 | *>1,500 | *>1,350 | >1,000 | **>30,000 |
* May depend on outside spore concentration for each species
** Based on mean plus standard deviation of contaminated residences indicating inadequate housekeeping
*** Based on median of contaminated residences
Reference: Baxter, D.M. "Airborne Mold Spore Concentrations in Commercial & Residential Buildings". Environmental Testing Associates, San Diego, CA, 1995.
Summary of Mold Spore Species Distribution
| Building Type | Penicillium / Aspergillus | Ascospores / Basidiospores | Cladosporium | Zygomycetes | Skin Cell Fragments |
|---|---|---|---|---|---|
| "Clean" Commercial Buildings | 37% | 24% | 11% | 5% | 23% |
| "Contaminated" Commercial Buildings | 66% | 6% | 4% | 10% | 14% |
| "Clean" Residential Buildings | 39% | 18% | 21% | <1% | 22% |
| "Contaminated" Residential Buildings | 20% | 76% | 1% | 1% | 2% |
| "Contaminated" Buildings Sampled During Drywall Demolition | 92% | <1% | <1% | 5% | 3% |
Reference: Baxter, D.M. "Airborne Mold Spore Concentrations in Commercial & Residential Buildings". Environmental Testing Associates, San Diego, CA, 1995.
Source & Attribution
These tables and guidelines are reproduced from public-domain sources for reference. Approved by Adam Jahnsen, Lab Director, AdamLabs, Inc.
For genus-level definitions of common molds, see the Mold Library →