An official website of the United States government
Exposure to carcinogens that exist as chemical pollutants or radioactive gas in our air, food, water, and soil also influences the incidence of cancer. Most exposure to toxic chemical substances and hazardous wastes results from human activities, particularly through agricultural and industrial production. Chemicals were selected for inclusion in this report based on the following set of criteria: (1) likely or probable carcinogen as classified by the International Agency for Research on Cancer (IARC) classification (Group 1 or 2A), (2) available biomarker data from the National Health and Nutrition Examination Survey (NHANES) since 2004, and (3) ubiquitous (i.e. >50% with detectable levels) in the U.S. general population (based on NHANES data).
Methodology for Chemical Exposures
This report includes the R function “svyquantile” from the R Package “survey” to estimate the percentiles and their confidence limits. Based on the Confidence Intervals for Medians and Other Position Measures article, published in the Journal of the American Statistical Association, and the Confidence Intervals for Proportions with Small Expected Number of Positive Counts Estimates from Survey Data article, published in the journal Survey Methodology, the researchers chose the “betaWald” interval option. To test whether there is statistically significant difference between the estimated percentiles obtained from different survey years, they used the “svyranktest” R function from the same package. For more details on the applicable R functions, see the Analysis of Complex Survey Samples by Thomas Lumley.