"Interpretation of Results of Microbiological Testing"
By Alexander Robertson IV
Editor's Note: Alexander Robertson, IV, is the managing partner of the Los Angeles law firm Knopfler and Robertson, LLP, a civil litigation practice that specializes in construction defect litigation.
"There are no 'official' standards or guidelines for fungal or bacterial bioaerosols. Some researchers have expressed an opinion that 100 250 CFU's are acceptable, provided no opportunistic fungi are present. The same range is also used by the U.S. Public Health Service, Federal Employee Occupational Service (Region III). A range of concentrations proposed by the World Health Organization and Health Canada suggests that microbial concentrations below 50 CFU for a single species (other than outdoor common fungi), 150 CFU for a mixture of species reflective of the outdoor air spores, or 500 CFU during the summer for common outdoor fungi (such as Cladosporium) are acceptable. There are other ranges (called background numbers or guidelines) used by organizations such as the American Conference of Governmental Industrial Hygienists (ACGIH) and OSHA.
Since there are no governmental established guidelines to follow regarding airborne fungi, indoor results must be interpreted with respect to the control samples. In general, mechanically ventilated buildings should have indoor fungal counts that are lower than those found outside. In addition, the species found inside should be similar to those identified outside the building. A situation should be considered unusual when the fungal levels inside are an order of magnitude or greater than those found in the outdoor control sample. Further, the presence of any slimy spored toxigenic fungi, such as Stachybotrys chartarum and Fusarium moniliforme, should be considered unusual, and may suggest an indoor contamination source. The consistent detection of some fungi, such as Aspergillus or various species of Penicillium, could indicate water damage and subsequent fungal amplification."