Collecting Outdoor Mold Samples

Should outdoor air samples be collected when there is snow cover?  Should outdoor samples be collected during rain or shortly afterwards?  If I have a suspect contamination in my basement but no visible sign of mold, should a comparison sample be taken somewhere else within the home to determine if there is a significant difference between the two areas?  I’ve contacted two firms who will perform air sampling (spore traps), and they have each given me different answers for when testing should occur. 

My basement is ceramic tile in the family room and bare concrete in the laundry area.  We have wall paneling in the family room and there is no visible signs of moisture anywhere but every once in a while there is a musty odor which is noticeable only in the basement area.-Steve, New York

 

            The standard practice in total mold spore as well as culturable-viable mold sampling is to collect samples both indoors and outdoors for comparison.  Comparisons are to be made of mold types as well as concentrations. 

            Comparisons of mold types are of particular importance since an individual who has collected the sample should be able to determine (from such comparisons) whether indoor and outdoor samples are from the same population (little or no difference in types of mold observed in indoor and outdoor samples).  When there are distinct differences a “mold expert” can relatively easily conclude that mold spores collected are from distinct or, in some cases, mixed populations (both indoor and outdoor mold types are present).  In such instances one may conclude that indoor sampling results indicate that an indoor source is present. 

            In my experience outdoor samples have a much greater diversity of species than outdoor samples.  As such, species diversity is a good indicator of whether indoor and outdoor samples are from the same or different populations.  A major problem in making indoor-outdoor comparisons is that reported concentrations of Aspergillus/Penicillium and Cladosporium are not specific enough.  The former includes in addition to Aspergillus and Penicillium species a number of other mold types with round or ovoid spores/cells.  Most notable is yeast.  Here in  the Midwest  yeast-type cells often dominate outdoor samples, particularly during the winter months.  They would be counted as Aspergillus/Penicillium (when they are counted at all) and suggest higher than normal concentrations of Aspergillus and/or Penicillium concentrations.

            I have collected outdoor samples in light rain and even snow.  Those samples to my surprise still had relatively significant mold spore/cell concentrations.  They are often dominated by yeast cells. 

            When one is collecting outdoor samples on a rainy or snowy day, it is best to collect them in a somewhat protected area such as a porch.  One should also be aware that the rain and snowy conditions may result in samples having a unique mold-type spectrum. 

            I mentioned Cladosporium as a category that is not specific enough.  In many outdoor samples at certain times of the year (last summer and fall) outdoor Cladosporium is dominated by C. herbarum, a species that produces very large spores and other cellular structures.  They are easily seen and identified at 400 and 600X magnifications.  Other species such as C. cladosporioides or C. sphaerospermum are much smaller with C. cladosporioides not visible at 400 and 600X magnification.  C. sphaerospermum is reported more indicative of indoor as compared to outdoor air. 

            I always recommend taking at a minimum two samples indoors and usually take 3-4.  These can be used to determine whether there are differences in different parts of the house, for example, differences between the basement and upstairs living areas.  In some cases basement concentrations are very high with unique species present.  In other cases basement samples have low concentrations with few species present. 

            In comparing basement samples to living area samples, one should be aware of the fact that if the furnace/AC is in the basement mold spores originating in the basement are readily drawn into the blower housing and then dispersed around the house.  Basement mold thus becomes an upstairs exposure problem.

 

January 13, 2005