What kinds of substances are measured by biomonitoring?

Biomonitoring may be used to assess the levels of any chemicals, natural or synthetic, that are present or that have been present in the environment. Natural chemicals that have been the subject of biomonitoring include those found in the earth's crust, such as the elements lead and arsenic, as well as a large variety of more complex compounds that are part of the food that we eat, the water that we drink and the air that we breathe. These complex chemicals include substances that are a part of a number of plants and animals and that have biological activity, including natural pesticides, carcinogens, and hormonally active chemicals.

Synthetic (i.e., manufactured) chemicals, often the focus of public concern, include a large range of compounds that have been or are being manufactured for a variety of applications. Also included are the substances that are produced as by-products of the synthesis and use of these compounds. In the modern world, synthetic chemicals are a part of every aspect of human life; they are critical to preventing and treating disease, to transportation, to agricultural production and to the many consumer products used for supporting the standard of living we enjoy. Therefore, it is not surprising that many of these find their way into the soil, air, water and food and thus ultimately into the fluids and tissues of individuals.

Biomonitoring is best at detecting chemicals that persist in the body, i.e., those that are not rapidly broken down and/or excreted. Examples of such persistent substances include the metals lead and arsenic and a number of synthetic organic chemicals, such as PCBs and DDT. These chemicals are often found through biomonitoring not only because they persist in the human body, but also because they remain in the environment for long periods of time after they are released. Thus, exposures to these substances can occur continuously over long periods of time. Because of the combination of environmental and biological persistence, they are often found in humans at higher levels than other chemicals, even those that may be produced or used in much higher amounts. However, even for persistent chemicals, both natural and synthetic, body levels measured by biomonitoring are generally very low, typically in the parts per million (ppm), parts per billion (ppb) or parts per trillion (ppt) range. If one ppm was expressed as time, instead of as concentration, it would be the equivalent of 1 second in 11.5 days; similarly, 1 ppb would be equivalent to 1 second in 31.7 years, and 1 ppt would be equivalent to 1 second in almost 32,000 years.

One situation in which high levels of exposure may occur is from the ingestion of foods containing significant amounts of environmental chemicals. This may happen if there is bioaccumulation and biomagnification of the chemical as it is passed up the food chain from organism to organism, e.g., from very small creatures up to large fish. Higher levels of chemicals in food may also occur as a result of unusual environmental conditions, e.g., extreme climatic conditions. For example, certain fungi that make mycotoxins may be produced on grain in greatly increased amounts when the weather is unusually hot and damp for a long period of time.

In sum, biomonitoring is a way of measuring which substances humans have been exposed to and the relative amounts of exposure to those compounds. This technique detects both naturally occurring and synthetic substances. Generally, the ones that are detected at the highest levels are environmentally and biologically persistent or are present in the environment in unusual amounts because of special conditions.