They say “the dose makes the poison”. But what’s the dose for indoor mould? Currently there is no answer to this question. Determining this dose would be a difficult task for a number of reasons:
- There are several species of moulds with differing health effects.
- People are not just exposed to mould but also to other environmental pollutants including bacteria, protozoa, dust mites and non-biological compounds.
- Some people are highly sensitive to mould even at concentration levels that may be considered ‘normal’.
It is generally accepted that no amount of mould should be allowed in occupied dwellings. However, it is impractical to get rid of all the mould in a building. A number of organisations have provided some guidelines in the past on what levels of mould would be considered a problem in occupied dwellings. But none of these guidelines have been accepted widely and some have even been dropped as more information concerning indoor mould became available. Below is a summary of some of these guidelines.
In 1989, the American Conference of Governmental Industrial Hygienists (ACGIH®) had considered fewer than 100 colony forming units per cubic meter of air (CFU/m3) as of no concern. For duct insulation, the Department of Environmental Health and Safety (DEHS) of the University of Minnesota, considers 10,000 CFU/gram as low, 10,000 to 100,000 CFU/gram as medium, 100,000 to 1,000,000 CFU/gram as medium to heavy and > 1,000,000 as heavy contamination. The Health Canada Technical Guide, 1993, states that if more than 50 CFU/m3, of the same species other than Cladosporium or Alternaria were detected in indoor air, there may be reason for concern. It also states that up to 150 CFU/m3 would be acceptable if there was a mixture of species reflective of outdoor air. Also, up to 500 CFU/m3 would be acceptable in the summer, if the species present were primarily Cladosporium.
There are other guidelines. The German Federal Environmental Agency (Umweltbundesamt, 2002) has probably the most comprehensive guidelines. Unfortunately the guide is only available in German language.
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