Shelton BG, Kirkland KH, Flanders WD, Morris GK. . Applied and Environmental Microbiology. April, 2002; 68(4):1743-1753.


Purpose: This comprehensive analysis compared the relative frequencies of indoor and outdoor airborne fungi in samples taken at different seasons and regional locations around the United States.

Design: Standardized survey quantifying fungal concentrations from air samples and interviews with a subgroup of building occupants. Over twelve thousand fungal air samples (9,619 indoor samples and 2,407 outdoor samples) from 1,717 buildings across the US were collected using Anderson N6 samplers. Actual fungal counts were measured as colony forming units per cubic meter of air. A follow-up telephone questionnaire of 100 random building owners determined the type of building, reason for performing air quality investigation, and whether there were any health complaints among the building occupants.

Outcome: Both the indoor and outdoor fungal concentrations were highest in the fall and summer and lowest in the spring and winter. Overall, the ratio of indoor and outdoor concentrations was one to one or lower for the majority of the buildings throughout the year, indicating that the amount of mold indoors was equal to or less than the amount of mold detected outdoors. The ratios indicated that the indoor concentrations were greater than the outdoor concentrations for 15% of the buildings studied. The indoor to outdoor fungal concentration ratios did not vary significantly by season, whereas the regional variation was substantial with concentrations highest in the Southwest, Far West, and Southeast. The outdoor levels were lowest in the Northwest and the indoor levels were lowest in the Northeast. Cladosporium, Penicillium, Aspergillus, and nonsporulating fungi were the most common fungi found indoors and outdoors in each season and in each region. The results suggested that the presence of Stachybotrys is not unusual and occurred at similar frequencies across the country. There was no association between fungal counts and any reported respiratory, dermatologic or eye problems amongst the building occupants that were interviewed. There was a relationship between elevated indoor fungal counts and complaints of hypersensitivity pneumonitis.

Significant Quotes: "This is the largest study of airborne indoor and outdoor fungal species and concentrations conducted with a standardized protocol to date." (p. 1743).

"[R]egion may be more important than season in determining ratios of indoor [fungal] population size to outdoor [fungal] population size." (p. 1752).

"Although the purpose of this study was not to measure any potential health association and the number of questionnaires was small, we found no association between reported health complains and the presence of any common fungal type or potentially toxigenic fungus." (p.1752).

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