Mold remediation clearance.  What type of air sampling and analysis provides acceptable clearance?  What is an acceptable ratio (percentage of indoor versus outdoor spore counts)?  Should indoor counts be “no more than 50% of outdoor counts or significantly lower indoors as opposed to outdoors”?  This seems to be a grey area everywhere I look.-Matthew, South Carolina 

            In response to your questions, I unfortunately may make the subject even greyer.  Clearance results, of course, are of considerable interest to building owners and occupants, mold remediators, and consultants who collect samples and responsible for interpreting analytical results.

            Clearance testing can be conducted using spore trap, culture plate, or QPCR (DNA) methods.  All three have limitations when it comes to clearance testing.

            Spore trap sampling (usually using Air-O-Cell or similar cassettes) is the most widely used method for indoor airborne mold sampling, particularly for clearance sampling.  In theory ,this should be a very good approach for clearance sampling since all mold particles including those that are both viable (alive) and nonviable mold particles are counted.  As allergenicity is independent of viability, sampling results should be a good indication of potential health-affecting mold exposure risk.

            In the real world, however, spore trap results as reported by commercial laboratories have significant reliability problems.  This is so for a variety of reasons.  Firstly, in many building environments the primary mold types associated with water-damaged materials are various species of Penicillium and Aspergillus.  Most analysts that conduct spore trap sample counts cannot distinguish between spores of Aspergillus and Penicillium and usually lump these together.  These may in many cases be identified as amerospores.  There are a variety of other mold spores that may be counted as Aspergillus/Penicillium.  Among the more commonly encountered mold spores that are likely to be counted by analysts as Aspergillus/Penicillium are Mucor, Acremonium, Aureobasidium, Paecilomyces, yeasts and a variety of ascospores.  This list is expanded when other predominantly outdoor mold types are included.

            In my opinion, the most important mold types that need to be addressed for clearance sampling are the various species of Aspergillus and Penicillium that are associated with water-damaged materials and of course Stachybotrys chartarum.  A sampling and analytical method that cannot distinguish Aspergillus and Penicillium spores from other mold types that may be present in abundant concentrations cannot be considered to be reliable for clearance purposes. Counts can be conducted by adequately trained analysts to distinguish  Aspergillus and Penicillium spores from other mold particles particularly yeasts and yeast-like spores that are very abundant outdoors and may be in significant concentrations indoors.  Such counts can only be conducted under 1000X magnification.  About half of the laboratories I have surveyed conduct counts at 600X and another half at 400X.  At such magnifications, one cannot see clearly (or even not at all) many spores/mold particles that would be classified as Aspergillus/Penicillium.  Save for Stachybotrys chartarum, conducting clearance sampling with spore trap methods that are analyzed at 400 or 600X is clearly  a waste of time, money and effort and may result in a  occupant/building owner being assured that their building is “safe” when it may not be. As such comparing indoor/outdoor concentrations when sampling results are unreliable is, of course, meaningless. 

To be continued. 

July 11, 2005