Sampling for Environmental Allergens
By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
"Sampling" for allergens may be as simple as looking for the sources. A large pollinating birch tree in the front yard of an allergy sufferer's home may be the cause of symptoms blamed on indoor contamination. Seeing a cat, dog, or other pet in a home is a sure indication that allergens are present as well as the presence of mouse and rat droppings. Trails of, odor of, or visible cockroaches, indicate that cockroach allergens are present. Generally, it is not necessary to sample in these situations. Remediation is focused on removing the sources from the environment. Note that people often will not give up their pets for any reason and prefer to suffer. This becomes a serious problem when a young child is the sufferer and the responsible adult is the pet lover.
Air sampling can be used for allergens and allergen sources that are routinely airborne. This includes pollen, fungal spores, cat and dog allergens, and rodent allergens. Pollen and fungal spores can be sampled using a spore trap if visual (microscopic) identification is to be used. Five-minute samples at 15 liters/minute are usual, although to be representative, several of these should be taken. Note that this type of sampling provides information on potential sources for allergens, not the allergens themselves. For pollen this works reasonably well. However, fungal spores vary significantly in allergen content depending on the strain, age, metabolic status, and many other factors. Some spores must be alive and able to germinate for allergens to be released. For these, particle sampling can introduce large errors.
Filtration air sampling is used for particles that are to be analyzed using immunoassay when concentrations are relatively high, and PCR when concentrations are low. Immunoassay analysis can detect concentrations of actual allergens, and is commonly used for arthropod and mammalian allergens. PCR detects DNA for specific organisms and therefore is revealing for potential sources of allergens. If 5 liter/minute sampling pumps are to be used, at least 100 liters of air should be collected. Note that both of these analytical methods require that you know precisely what you are looking for as unknowns will not be recognized.
Dust sampling is the most common approach for studying allergen concentrations. Samples are collected using a vacuum device. The type and power of the vacuum will determine the percentage of dust that is removed from the substrate (e.g., carpeting). The type of substrate also controls the amount of dust removed. Virtually all of the dust can be removed from smooth surfaces and thin commercial carpeting, such as might be found in offices and schools. The thicker the carpet pile, the less dust will be recovered. For these reasons, the amount of dust collected from a measured area is not quantitative with respect to total dust available for aerosolization. Thus, dust allergens are usually reported as micrograms of allergen per gram of dust. If you have sampled a measured area, you can estimate total load for that surface and (possibly) similar surfaces in the environment. However, published guidelines for dust allergens are based on the microgram/gram of dust measure.
Dust allergens are analyzed using immunoassays. The dust is first sieved to remove large particles (e.g., sand, cheerios, cockroaches etc.), then weighed. A measured portion of the weighed dust is suspended in a buffer solution. Allergens are, by nature, soluble, so over time, with stirring, the allergens dissolve into the liquid. The liquid is then assayed using a specific set of immunoassays. The results from the machine are in concentrations per milliliter of liquid. These results are then converted to allergens/gram of dust. There are published guidelines for interpreting results of some allergen assays. These guidelines are based on epidemiological data, and represent the concentration at which a relationship with a particular symptom becomes significant. This does not mean that people won't respond to a much lower concentration if they are especially sensitive. Table 1 presents commonly quoted guidelines for some important dust allergens.
Table 1. Exposure thresholds for sensitization (Chapman et al., 1995)
|Sensitization Risk||Dust Mite: Group 1 (µg/g)||Cat: Fel d 1 (µg/g)||Dog: Can f 1 (µg/g)||Cockroach: Bla g 1 (µg/g)||Cockroach: Bla g 2 (µg/g)|
|High||> 10||1-8||1-8||> 8||> 1|
|Low||< 0.3||0.5||0.5||0.6||< 0.08|
Note: These are listed as "thresholds" implying that sensitization will not occur below these levels. However the numbers represent concentrations where a relationship between sensitization and allergen concentration becomes significant. Sensitization can occur below the "low" levels. Also note that, at least for cat and dog allergens, medium levels are higher than high levels. This is because high level exposure to these allergens is considered to be protective against sensitization. Finally, concentrations for symptom development are higher than those for sensitization.
As with all sampling, it is of utmost importance to use well tested methods for sampling and analysis, and use the same method each time you sample.
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This article was originally published on June 2014.