How is biomonitoring performed?

The process of biomonitoring involves three steps: (1) selecting who will be monitored as well as when and where, (2) collecting tissue and/or fluid samples, and (3) deciding which chemicals to study and analyzing for those chemicals in the samples that are collected. This is a complex and expensive process, especially if the goal is to obtain results that reflect how body levels vary by age, sex, ethnic group, geographical location, and state of health of the individual. Biomonitoring also depends on the ability of analytical chemists to detect minute amounts of chemicals, an ability that has increased significantly in the past decade. Thus, it is not surprising that biomonitoring efforts in the U.S. have been limited.

In recent years, however, the U.S. Centers for Disease Control and Prevention (CDC) has embarked on an ambitious program of monitoring. In 1999, the CDC collected samples from more than 4,000 people in about a dozen locations and analyzed these for twenty-seven chemicals, comprising metals, organophosphate pesticides, tobacco smoke and phthalates. Following this, in 1999-2000, the CDC expanded the study to include a total of 127 chemicals although the numbers of people and locations involved were about the same as previously. Even though a fairly large number of people and locations were included in these investigations, the results of the studies can only provide reliable national averages rather than detailed information about specific sectors of the population.

Because of resource limitations, all of the CDC tests were performed using blood and/or urine samples, even though sampling other fluids, such as breast milk, or certain tissues might have provided additional significant information. For example, studies of breast milk levels can provide specific data on which chemicals breast-fed children are ingesting and in what amounts. This can be especially useful information for compounds, such as DDT, for which international experts have estimated acceptable maximum levels in breast milk. For another example, studies of metals, such as mercury, in hair can provide long-term rather than instantaneous exposure information because the mercury is incorporated into the hair as it grows. Thus, the levels at the tips of the hair often represent mercury exposures a year or more prior to the time the test is taken, while levels in the hair nearer the scalp reflect more recent exposures.

The actual analyses are quite straightforward in most cases, as they are based on commonly accepted laboratory techniques. However, they do generally require sophisticated analytical instruments and techniques, because it is only the application of very sensitive test methods that provides the opportunity to detect the very small amounts of many of the environmental chemicals found in humans. These special tests cannot be performed by the medical laboratories that routinely do the blood and urine analyses ordered by doctors. Even with analytical advances, uncertainties may arise when measured levels are near the minimum levels that can be detected (limit of detection) or in situations where the analysis for a particular chemical is very difficult or has not been validated. In addition, there may be questions about which form or combination of forms is most appropriate to measure in cases where a chemical occurs in more than one form.

In sum, biomonitoring of compounds present in the general environment is a complex undertaking, requiring a great deal of skill and resources. Because of all of these limitations, even the largest current studies may not provide answers to many of the critical exposure questions, such as exactly which compounds are present in human tissues and fluids, how the levels of each chemical vary in particular segments of the population and at specific locations, and how these levels change over time.