Exposure and Dose Reconstruction for Chemicals: A Complex Activity
Defining the cause of an illness – genetics? Lifestyle? Occupational?
When someone becomes ill and it’s not something that can be easily traced to a source, the question always arises: where did this come from? For example, a cold or flu can generally be traced to an event or source—kids, work, airline travel. But, what if it’s cancer or emphysema? These illnesses are known to be due to some event or series of events over the distant past. They possibly can be caused by genetics with no known environmental reason.
Linking A Disease To A Possible Cause
Attempting to sort out possible causes is a complex task. First, a physician needs to define the condition and suggest possible causes such as genetics, lifestyle, living environment and workplace. Next, the individual must consider the past to see if anything that could be a cause actually occurred. If there is a possibility that the environment or the workplace could have been a source, then an industrial hygienist, a toxicologist or an epidemiologist must try to determine if there is a link between the disease or condition—and possible sources of exposure.
The next step would be dose reconstruction. That is, was the past event or exposure a possible cause? As a practical matter, this may be difficult. The exposure may have changed or the conditions of original exposure may no longer exist. Hopefully, past sampling may exist so that discussion can be made on the extent of the exposure. If not available, then a literature search may reveal test results performed in similar operations so that exposure comparison can be made.
Do the symptoms match the exposure?
If the condition or illness could have been caused by the agent in question, then it must be asked if the amount of agent present at the time of exposure was sufficient to have caused the symptoms. Almost invariably, no measurements of the agent’s concentration in the breathing zone air had been made, and no data are available for evaluation. Sometimes, the exact conditions of the exposure can be reproduced and air samples collected to be measured.
Often, the literature reports levels at which certain symptoms can be expected to appear in an exposed group. If the lab data show much lower levels of the agent in the breathing zone air than the levels that typically are associated with the complaint symptom, then it’s a fairly safe bet that the symptoms were not caused by the agent in question.
Calculating The Cause Using Mathematics Probability
What if the exposure conditions can’t be reproduced, sampled and evaluated? Then, we may need to rely on some mathematics to help us estimate what the exposure may have been. Here is a hypothetical example. In a room of known dimensions, some solvent vapors are released into the air. An employee is in the room for a known period of time, breathing the solvent containing air. We have a good estimate of the amount of solvent that had been spilled and some of its physical and chemical properties. We also know that the room was ventilated mechanically, and the rate and volume of dilution air that was introduced. From this information, we can begin to estimate the concentration of the solvent in the room air, and from that, the probable exposure dose.
Investigating Exposures Using Toxicological Data
Before launching into lengthy and involved calculations, it is almost always profitable to make some simple calculations to define the “worst-case” scenario. Let’s assume (by way of illustration) that we spilled a 1.5 fluid ounce bottle of nail polish remover (acetone) onto the floor of a room whose dimensions are 14 x 18 x 8 feet. The volume is therefore 2,016 cubic feet (57.1 cubic Meters, M3). One-and-one-half fluid ounces is equal to 44 milliliters (mL) of acetone. The worst-case condition is that all 44 mL of acetone is instantaneously evaporated into the 57.1 M3 room, producing a uniform concentration of acetone throughout the room. Acetone weighs 790 milligrams Lmg) per mL of liquid. So, 44 mL of liquid acetone weighs 34,760 mg, which is instantly converted to a vapor, producing a concentration of 34,760/57.1M3 (609 mg acetone per M3 of air). This can also be expressed as 256 parts per million (ppm). The current OSHA Permissible Exposure Limit (PEL) averaged over any 8-hour period of exposure, is 1,000 ppm (2,400 mg/M3). Since the maximum concentration of acetone that could have been produced was 256 ppm (609 mg/M3), which is about ¼ of the OSHA PEL, it’s probably unlikely that our employee, even if in the room for a full 8 hours, had enough of an exposure to cause a headache. If we take into consideration the diluting effect of the ventilation system, then these levels would become even lower.