CDC's Environmental Health Laboratory conducts the National Biomonitoring Program (NBP). The Program specializes in biomonitoring, which is the direct measurement of people's exposure to toxic substances in the environment by measuring the substances or their metabolites in human specimens, such as blood or urine. Biomonitoring measurements are the most health-relevant assessments of exposure because they indicate the amount of the chemical that actually gets into people from all environmental sources (e.g., air, soil, water, dust, food) combined, rather than the amount that may get into them.