Further Thoughts About Surface Sampling
Qualitative surface sampling is used to discover whether or not discoloration represents fungal (or bacterial) growth, to identify visible fungi, and to evaluate the types of fungi and bacteria on surfaces and in settled dust that has not been disturbed. For these qualitative applications, surface sampling is entirely appropriate. The samples are easily obtained and the data is readily interpreted. This kind of data is useful for tracking specific fungi from source to aerosol, and to discover whether or not an unusual population of organisms is present in settled dust.
Quantitative surface sampling is used to evaluate the presence or absence of specific organisms, and to evaluate how many organisms are present on a particular surface. Quantitative surface sampling is much more difficult to perform appropriately, and interpretation of the data has many pitfalls.
Quantitative surface sampling involves sampling a measured area and is done by swabbing, wiping, pressing contact plates with culture medium against a surface, pressing sticky tape against a surface, collecting bulk samples of a material, and vacuuming particles from a surface. Swabbing, wiping, and contact plates are frequently used in hospitals and the pharmaceutical industry to evaluate surfaces for the presence of specific organisms, and to match total cells/unit area to established guidelines or control limits. Tape sampling has generally been restricted to fungi because they do not necessarily have to be cultured to be identified. Relatively recently, a new tape sampling method has been described that uses epifluorescence microscopy to count living and dead bacterial cells (Yamaguchi et al., 2003).
I have often said that surface sampling is not quantitative, and I have not changed my mind, in spite of the fact that it is commonly used, as mentioned above, in health care related environments, and standards have been set for allowable concentrations and types of organisms on surfaces at these sites.
During a literature search on surface sampling I discovered that the collection efficiency of swabs of various types ranges from about 25% to 89% depending on the type of swab, the number of organisms on the swabbed surface, and the way the organisms got there (i.e., sprayed on in liquid form as from a cough or a sneeze, or deposited dry from aerosol). Contact culture plates have the same problem, with efficiencies ranging from about 25-60%. Yamaguchi's adhesive film sampling recovered 54% of deposited cells compared to 27% collected by swab sampling. It is probably safe to estimate that you are collecting at most half of the spores or cells on the surface being sampled using these methods.
How about vacuuming dust? In studies with bacteria between 22 and 35% of seeded cells were recovered from wallboard, concrete and residential carpeting. If all of these percentage recoveries were the same, one could just apply a correction factor. However, they are not the same, so you really don't know just how many cells are present. On the other hand, if you sample the same surfaces over and over (as is done in hospitals and pharmacies) and use exactly the same method every time, and if the standards were developed using your method, then you can successfully determine whether or not any change has occurred.
The second big problem with quantitative surface sampling is the fact that surfaces are usually much larger than the area sampled. With residential tape sampling, a 1cm2 sample is collected from a wall that may be 8ft x 12ft (96ft2) or 89,187 cm2. This means you are sampling 1x10-3% of the wall surface. Even in a laminar flow hood the percentages sampled are small. A Rodak surface sampling culture plate is 55mm in diameter, or 5.5cm. A hood might have a work surface area of about 3ft x 4 ft (12ft2). If you collect three samples (a commonly used number) you have collected particles from 52 cm2 or 0.5% of the surface. Is this sufficient to decide whether or not you have no Staphylococcus or less than 1 organism/cm2?
Statistically, these are ridiculous numbers. The saving grace of the method is the long experience these people have in this kind of sampling, and the fact that epidemics of disease or of contamination of pharmaceuticals don't appear to happen when the guidelines are followed.
In the indoor air industry, we don't have years of experience sampling exactly the same surfaces over and over, and we don't have a measurable endpoint that tells us that our interpretations are correct. Given all the errors involved, it is far better to use surface sampling as a qualitative estimate of microbial populations, and forget about trying to obtain quantitative data in this way.
This article originally appeared in the February 2010 issue of Indoor Environment Connections. Reprinted by permission.
Dr. Harriet Burge is EMLab P&K's Director of Aerobiology and Chair of EMLab P&K's Scientific Advisory Board. Widely considered the leading expert in indoor air quality (IAQ), Dr. Burge pioneered the field more than 30 years ago. She has served as a member of three National Academy of Sciences committees for IAQ, including as Vice-Chair of the Committee on the Health Effects of Indoor Allergens.
View Dr. Burge's Curriculum Vitae.