Environmental tobacco smoke (ETS) contains a mixture of toxic chemicals, including known human carcinogens. The U.S. Surgeon General has concluded that ETS causes a range of adverse health outcomes in adults, ranging from nasal irritation to increased risk of coronary heart disease to lung cancer. In children, the Surgeon General has concluded that ETS causes lower respiratory illnesses, adverse effects on lung function, onset of wheezing, asthma, middle ear disease, and Sudden Infant Death Syndrome (HHS, 2006). Household ETS exposure is an important issue because many people, especially young children, spend much time inside their homes. Infants and younger children are more susceptible and vulnerable to the effects of ETS than are older children because they are still developing physically, have higher breathing rates than adults, and have little control over their indoor environments (HHS, 2006; U.S. EPA, 2018).

Exposure to ETS leaves traces of specific chemicals in people's serum, urine, saliva, and hair. Cotinine is a chemical that forms inside the body following exposure to nicotine, an ingredient in all tobacco products and a component of ETS. Following nicotine exposures, cotinine can usually be detected in serum for at least 1 or 2 days (Pirkle et al., 1996). Active smokers almost always have serum cotinine levels higher than 10 nanograms per milliliter (ng/mL), while non-smokers exposed to typical levels of ETS have serum concentrations less than 1 ng/mL. Following heavy exposure to ETS, non-smokers can have serum cotinine levels between 1 and 10 ng/mL (CDC, 2009, 2015).

The purpose of this indicator is to track exposure to ETS, or secondhand smoke, among the non-smoking U.S. population. Cotinine is considered the best biomarker for tracking exposure among non-smokers to ETS. Accordingly, this indicator reflects serum cotinine concentrations in ng/mL among non-smokers for a representative sample of the U.S. population, age 3 years and older, as measured in the 1988-1994, 1999-2000, 2001-2002, 2003-2004, 2005-2006, 2007-2008, 2009-2010, 2011-2012, 2013-2014, and 2015-2016 National Health and Nutrition Examination Survey (NHANES). NHANES is a series of surveys conducted by the Centers for Disease Control and Prevention's (CDC's) National Center for Health Statistics, designed to collect data on the health and nutritional status of the civilian, non-institutionalized U.S. population using a complex, stratified, multistage, probability-cluster design. Serum cotinine also was monitored in individuals age 4 years and older as part of NHANES III, between 1988 and 1994. CDC's National Center for Environmental Health conducted the laboratory analyses for the biomonitoring samples. Beginning in 1999, NHANES became a continuous and annual national survey. The continuous NHANES does not include cotinine data for children from birth to 3 years of age, the group reported to be the most vulnerable to the effects of ETS.

The results of NHANES III (1988-1994) and continuous NHANES (1999-2016) are presented here for the different survey periods by sex (Exhibit 1), race and ethnicity (Exhibit 2), and age (Exhibit 3). During the 1988-1994 survey period, the median (50^th percentile) serum cotinine level among non-smokers in the general U.S. population was 0.176 ng/mL. In 1999-2000, the estimated median serum cotinine level among non-smokers nationwide had decreased to 0.060 ng/mL. During the most recent survey period (2015-2016), the estimated median serum cotinine level for the U.S. population was 0.017 ng/mL (see Exhibit 1). This marks a greater than 70 percent decrease from levels measured in 1999-2000 and a 90 percent decline since NHANES III (1988-1994)—a consistent reduction over time that suggests a marked decrease in exposure to ETS. Similar decreasing trends in serum cotinine levels are observed between NHANES III (1988-1994) and the most recent 2015-2016 survey across sex, race/ethnicity, and age groups. However, 38.0 percent of children age 3 to 11 years still had "detectable" cotinine in their serum (defined as serum cotinine levels of 0.05 to 10 ng/mL) in 2015-2016 (CDC, 2019; data not shown).

Other observations include: (1) non-smoking males have slightly higher cotinine levels than females across survey periods (Exhibit 1); (2) of the race/ethnicity groups presented, non-Hispanic blacks generally have the highest cotinine levels (Exhibit 2); and (3) in general, people below the age of 20 have higher serum cotinine levels than people age 20 years and older (Exhibit 3).

Exhibit 4 shows the percentage of non-smokers aged 3 to 17 years with serum cotinine levels more than 0.05 ng/mL and less than or equal to 10 ng/mL (the standard range for classifying ETS exposure) for the total age group and by selected race and ethnicity breakdowns. Among the three subgroup populations presented, Mexican American children had the lowest percentage of serum cotinine levels greater than 1.0 ng/mL and less than or equal to 10 ng/mL for all time periods displayed. Between 1988-1994 and 1999-2000, non-Hispanic black children had the largest absolute decline in the percentage of serum cotinine levels greater than 1.0 ng/mL and less than or equal to 10 ng/mL, but that population also started off with the highest percentage in this range (33.7 percent). In 2015-2016, among the percent of children with serum cotinine levels greater than 1.0 ng/mL and less than or equal to 10 ng/mL, there was an absolute decrease of 1.1, 0.9, and 14.5 percent for non-Hispanic blacks, Mexican Americans, and non-Hispanic whites, respectively, compared to 1999-2000.

• The relatively small number of samples collected in a 2-year cycle (e.g., 1999-2000 or 2001-2002) may, in some cases, result in measures of central tendency that are unstable from one survey period to the next.

Continuous NHANES data from 1999-2014 in Exhibits 1 through 3 of this indicator were obtained directly from CDC's Fourth National Report on Human Exposure to Environmental Chemicals, Updated Tables, March 2018, Volume One (CDC, 2018). NHANES III (1988-1994) and continuous NHANES 2015-2016 data for Exhibits 1 through 3, as well as all data used for Exhibit 4, were generated with Stata statistical software using NHANES laboratory data files available online in SAS^® transport file format (CDC, 2019).