Frequently Asked Questions

What is the National Exposure Report?

The National Report on Human Exposure to Environmental Chemicals (also known as the Report) is a series of ongoing assessments of the U.S. population’s exposure to environmental chemicals using biomonitoring. The report provides information using individual and pooled blood or urine samples tested by Centers for Disease Control and Prevention (CDC) scientists in the Division of Laboratory Sciences. The samples are from people who took part in CDC’s National Health and Nutrition Examination Survey (NHANES). Because of NHANES’ study design, the measurements of chemicals in participants are representative of exposures in the U.S. civilian population.

Knowing the levels of environmental chemicals in the U.S. population helps physicians and public health officials determine whether people have been exposed to higher than expected levels of those chemicals. Exposure information can help scientists plan and conduct research about environmental chemicals and their health effects.

The Report does not provide health or toxicity information, state- or community-specific data, specific product or environmentally related information, or regulatory guidelines or recommendations.

The Report is periodically updated when blood or urine measurements from the NHANES survey are available and published on CDC’s National Center for Health Statistics (NHANES) website. The frequency of issuing new updates varies. Part of the reason for this is that many CDC environmental health laboratories make the chemical measurements, and some of the most complex measurement methods require more time to complete than other methods.

What is NHANES?

The National Health and Nutrition Examination Survey (NHANES) is a program of studies designed to assess the health and nutritional status of adults and children in the United States. The survey is unique in that it combines interviews, physical examinations, and blood and urine collection. NHANES is a major program of the National Center for Health Statistics (NCHS) at the Centers for Disease Control and Prevention (CDC). NCHS produces vital and health statistics for the nation.

How does CDC decide which environmental chemicals to measure?

Chemicals included in the Report are selected based on the following:

  • Scientific data that suggest exposure in the U.S. population
  • The seriousness of health effects known or thought to result from some levels of exposure
  • The need to assess the effectiveness of public health actions to reduce exposure to a chemical
  • The availability of an analytical method that is accurate, precise, sensitive, specific, and rapid
  • The availability of adequate blood or urine samples from the NHANES survey
  • The analytical cost to perform the analysis

See the National Report on Human Exposure to Environmental Chemicals site for more information.

Why are some chemicals measured for only a few years?

If a chemical or an entire group of chemicals is not detected for several years, then measurements are discontinued. A chemical also might be replaced if a new chemical or alternative offers a better measure of exposure. For more information about when chemicals may be removed from the list of chemicals measured, see the March, 2008 Federal Register notice.

How else does CDC update information on U.S. population exposures?

CDC publishes biomonitoring exposure information for the U.S. population in peer-reviewed publications, without waiting for an update to the Report.

What are the public health uses of the Report?

The Report offers unique exposure information to scientists, public health officials, and physicians. Specific uses of the Report include the following:

Providing data on levels of environmental chemicals found in the U.S. population so appropriate studies can be conducted to determine whether these levels pose a health risk

Identifying the proportion of the U.S. population with levels of chemicals that could put them at higher risk for toxicity

Establishing reference values that physicians and public health officials can use to determine whether a person has an unusual exposure

Determining the effectiveness of public health efforts to reduce the exposure of the U.S. population to specific environmental chemicals (e.g., pesticides, lead, tobacco smoke)

Determining whether levels of environmental chemicals are higher among specific groups, such as minorities, children, and women of child-bearing age

Tracking trends in the levels of exposure of the U.S. population to environmental chemicals

How will the Report inform future research?

Researchers can use data from the Report to study whether health effects occur at the levels of chemicals reported in the U.S. population. For chemicals that have measurable levels for much of the population, studying exposure levels helps researchers learn which groups might be more vulnerable to exposure or at risk for greater exposure.

CDC’s Environmental Health Laboratory also collaborates with other federal agencies, academic institutions, governments, and other organizations on about 50 to 70 studies of exposure to environmental chemicals each year. These studies address

  • health effects after exposure,
  • sources of exposure,
  • and how chemicals are processed in the body.

What does finding these levels in people’s bodies mean? Are the levels dangerous?

Detecting levels of an environmental chemical in a person’s blood or urine does not necessarily mean the chemical will cause health effects or disease. Advances in analytical methods enable CDC to measure very low levels of environmental chemicals in people, but research studies of varying levels of exposure are needed to determine if specific levels cause health effects.

Health threshold levels are better defined for some chemicals than for others. For example, CDC has published guidelines on preventing lead poisoning in children. These guidelines for lead levels in blood are based on extensive research on the human health effects of exposure to lead.

Another example is CDC’s National Tobacco Control Program which is working to reduce the exposure of nonsmokers to environmental tobacco smoke. Measurements of serum cotinine can be used to track exposure to environmental tobacco smoke in nonsmokers.

For many environmental chemicals, however, threshold levels are not known. More research is needed to assess health risks associated with the blood or urine levels of various chemicals documented in the Report.

What are the common sources of environmental exposure to the chemicals listed in the Report?

For most chemicals, people are exposed to low levels in foods they eat, water they drink, and the air they breathe. People can also be exposed to chemicals from products and containers they use.

How can people find out what their levels are?

Most of the data on chemicals in the Report are research measurements obtained by specially trained laboratory chemistry staff using highly specialized analytical instruments and techniques. These analyses are not readily available in hospitals or most commercial clinical laboratories.

Some measurements, including those for lead and mercury in blood or urine, can be ordered by a physician. If necessary, a physician can refer a person for further evaluation to a medical specialist, such as a medical toxicologist or physician specialized in occupational and environmental medicine.

CDC does not perform laboratory tests for individual patients or their physicians. CDC does perform chemical measurements in

  • population groups being investigated by public health authorities for unusual chemical exposure or possible adverse health effects as a result of the exposure,
  • populations in public health surveys such as NHANES,
  • and research studies of environmental chemical exposures.

Why does CDC measure some chemicals in “pooled samples” instead of individual samples?

Pooled samples combine measured amounts of serum from several persons to get enough serum to measure very small amounts of a chemical. In recent years, several persistent chemicals have been detected at very low concentrations in U.S. residents. Examples include polychlorinated biphenyls (PCBs), dioxins, and chlorinated pesticides (e.g., chlordane, DDT, heptachlor). Measuring such chemicals in one person would require large amounts of serum. These analyses are also costly. Pooling serum can increase the likelihood of obtaining detectable results. These considerations led CDC to begin measuring many of the persistent chemicals in pooled serum samples, starting in 2005.

What methods did CDC use to analyze these chemicals? Where can I find out more information about these chemicals?

To measure chemicals or their metabolites in blood and urine, CDC’s Environmental Health Laboratory uses advanced analytical methods, such as isotope dilution mass spectrometry and inductively coupled plasma mass spectrometry. More information about the analytical methods used for specific chemicals is available by following the “References for Biomonitoring Analytical Methods” link found at:

What statistical information is given for each chemical?

For each chemical, the Report presents descriptive statistics about the distribution of blood or urine levels of that chemical in the U.S. population. Statistics include geometric means and selected percentiles, with 95 percent confidence intervals around each estimated statistic. Percentiles describe the shape of the distribution. For all chemicals, the 50th, 75th, 90th, and 95th percentiles are presented. Higher percentiles (e.g., 97.5th, 99th) are not shown because the number of results in these higher percentiles are small and make these percentiles unreliable. Data are presented for the total population and for population groups defined by ages, sex, and race/ethnicity.

How are population categories selected?

The NHANES sample is selected to represent the U.S. population of all ages. However, sample sizes for some groups are too small for reliable estimates to be calculated. To produce reliable statistics for Mexican Americans and non-Hispanic blacks, the survey selects larger numbers (i.e., “over samples”) for these groups for each survey period. Beginning in 2011, NHANES also began selecting larger numbers of non-Hispanic Asians. Currently, the sample sizes for a 2-year NHANES period are sufficient to provide data on five racial/ethnic groups: Mexican Americans, all Hispanics, non-Hispanic Blacks, non-Hispanic whites, and non-Hispanic Asians.

Why doesn’t the Report include data on all age groups?

Although NHANES examines people of all ages, the amount of blood or urine that can be collected in some groups is limited. If samples from an age group are inadequate to obtain reliable test results for a chemical, the data are not included.

Why doesn’t the Report include data on the states or local communities that are included in the survey?

NHANES is a national survey, not a regional or state-based survey. Its findings are designed to reflect exposures in the nation overall, rather than individual counties or states. Producing accurate estimates at a state or local level would require a much larger and very different sample design. Consequently, results might not be comparable to other states or localities or allow examination of trends in chemical exposures. CDC launched the State Biomonitoring Cooperative Agreement to increase the capability and capacity of states to conduct biomonitoring and state population-based biomonitoring surveillance to assess human exposure to environmental chemicals.

Page last reviewed: March 24, 2022