What Is the Difference Between Exposure and Dose?
Q. What is the difference between exposure and dose?
A. This is an important question. A lack of understanding of the difference between these terms has resulted in misinterpretation of dangers associated with fungal growth.
So, what do the terms mean? My definition for exposure is being in the presence of an agent long enough that it enters the body. Dose is specifically how much of an agent to which one is exposed reaches a site in the body where it can have an effect.
Generally, all of the steps in an environmental investigation are designed to evaluate exposure. That some dose of an agent actually is achieved is assumed (often inaccurately). Let’s go through an investigation and see where dose comes into the picture (if it does).
First is the visual observation. You see mold growing on a wall, and you assume that the mold is producing something (e.g., spores, VOCs, fragments, etc.) that could get into the air. You extrapolate from that to exposure by estimating how much time occupants spend in the space. If they are casual visitors, then you consider that exposure is low; if they occupy the space for eight to 10 hours a day, then you assume that more exposure could occur. In your mind, you relate this to the possibility of a dose of some mold agent sufficient to cause symptoms. You have, as yet, not actually documented that any exposure has occurred. If you have been hired to find the mold and remove it, this is probably enough information.
Second, you collect bulk samples. If you use microscopy or culture to analyze these samples, you can document that the mold is producing spores, and you can name the mold. This will give you some information as to the agents that could be present. It does not increase your knowledge of whether or not exposure is or is not occurring. If you analyze the bulk sample chemically, you may be able to pinpoint an actual agent (e.g., a specific mycotoxin or a specific allergen). This still does not provide any more information about whether or not exposure is occurring or how much.
Third, you collect air samples. If you use microscopy or PCR to analyze these samples, then you can estimate airborne concentrations of total spores (not particularly useful information), or you can calculate concentrations of specific spore types such as Stachybotrys or Alternaria. Combined with estimates of time spent in the space, breathing rate of the occupants, and spore size, you can estimate exposure to these spores. If the spore is produced by a potentially toxigenic fungus and you can find literature that tells you how much toxin is in each spore, then you can estimate how much toxin exposure the occupants experienced. You still don’t know the dose they received unless you know how soluble the toxin is, how fast it is metabolized and how fast it may be removed from the system.
If you use culture to analyze air samples, then you will usually have more accurate information on the species of fungi present but less information on actual concentrations. As with microscopy, the process of extrapolating to dose is a multi-step procedure that results in only a rough estimate of potential dose. If you analyze cultured samples for toxin, then you are documenting only that the captured fungus is capable of producing the toxin, not
that it actually was producing toxin in the occupied space.
Finally, if you analyze air samples for toxin, then you come closest to being able to estimate dose. In this case, you know how much of a specific toxin is present and how long each occupant spends in the space. You don’t know the size of the particle on which the toxin was borne, and you still have to estimate dose in the ways discussed above.
Too often, the presence of a fungus such as Stachybotrys is assumed to mean that occupants were receiving a sufficient dose to cause symptoms. Unless you have thought through all of the above steps, this is somewhat like assuming a shark will eat you if you swim in your swimming pool. In order to prove such an assumption, you would have to document that there actually were fish in your pool, that the fish were sharks, that the sharks were man eating sharks, that they were big enough to eat you, that they were hungry, and that you couldn’t fight them off or get out of the pool in time.
Dr. Harriet Burge is director of aerobiology at Environmental Microbiology Laboratory Inc. and associate professor and director of the microbiology laboratory at the Harvard School of Public Health. Widely considered the leading expert in IAQ, 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.
To submit a question to Dr. Burge, write to her by e-mail at askdrburge@emlab.com. All questions posed to Burge will receive a reply, although space limitations prevent us from publishing them all. By submitting a question, you agree to have your question and its answer published in a future edition of IE Connections.
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