2.1. Study population
Our study included 20146 participants from the 2013–2016 NHANES conducted by the Centers for Disease Control and Prevention (CDC) National Center for Health Statistics (NCHS). We retrieved all data from the website of the NCHS (https://www.cdc.gov/nchs/nhanes/index.htm). To analyze the effects of ethylene oxide on hypertension, we excluded subjects with missing ethylene oxide (n=14700), systolic blood pressure (SBP) and diastolic blood pressure (DBP) data (n=404). We also excluded pregnant women (n=37). Finally, a total of 5005 participants were included in our analysis (Fig. 1). All participants provided written informed consent, and the study protocol was approved by the Research Ethics Review Board of the NCHS.
2.2. Assessment of ethylene oxide levels
Human whole blood samples were collected in a mobile examination center. Blood ethylene oxide levels were measured by a modified Edman reaction method. The detailed measurement of blood ethylene oxide concentrations is described in detail elsewhere (wwwn.cdc.gov/nchs/data/nhanes/2015-2016/labmethods/AMDGYD_ETHOX_I_MET.pdf). The limit of detection (LOD) for blood ethylene oxide concentrations was 12.90 pmol/g Hb, while the analytic results below the LOD were recorded as the LOD values divided by the square root of 2.
2.3 Blood pressure measurements
Trained physicians used a calibrated mercury sphygmomanometer using a standardized protocol to measure arterial blood pressure, and 3 consecutive blood pressure readings were taken for each participant on the same arm. Hypertension was defined as a mean SBP of at least 140 mmHg, a mean DBP of at least 90 mmHg, or both, and/or the self-reported use of prescription drugs for diagnosed hypertension (Muntner et al., 2020).
2.4 Additional Data
We selected data consistent with previous studies (Liao et al., 2020; Liu et al., 2021) and current NHANES guidelines. Data on demographic characteristics, body examinations, medication use, and socioeconomic background were selected as appropriate. Data on age, sex (male/female), race/ethnicity (Mexican American, other Hispanic, non-Hispanic White, non-Hispanic Black, other race), education (<9th grade, 9–11th grade, high school,
college, and graduate), smoking status (smoking more than 100 cigarettes during their lifetime or not), diabetes mellitus (yes/no), alcohol use (consumed at least 12 drinks in the last 12 months or not), poverty-to-income ratio, and physical activity level (none, moderate and vigorous) was based on self-reports. Body mass index (BMI, kg/m2) was calculated as body weight divided by height squared. Information on dietary intake of calories, sodium, and potassium was also considered as covariates in our analysis models.
2.5. Statistical analysis
All statistical analyses were performed by R software (v3.6.0; R Foundation for Statistical Computing). Baseline characteristics were compared across the quantiles of blood ethylene oxide levels using 1-way ANOVA for continuous variables and χ2 tests for categorical variables. Since the ethylene oxide levels were skewed, we log2 transformed the data. Simple and multiple generalized linear regression models were performed to explore the associations of ethylene oxide levels with hypertension and blood pressure. Restricted cubic spline plots with 3 knots located at the 10th, 50th, and 90th percentiles of the distribution were generated to explore the potential nonlinear relationship between ethylene oxide levels and hypertension and blood pressure. Participants who reported taking antihypertensive medications were omitted from further separate statistical models, and adjusted regression coefficients (β) were obtained to estimate the association of ethylene oxide levels with SBP and DBP (n=3994). Generalized linear regression models and restricted cubic spline models were all adjusted for age, sex, education level, race, diabetes mellitus, smoking status, alcohol use, body mass index, poverty-income ratio, energy intake, sodium intake, potassium intake, and physical activity level. Subgroup analysis stratified by age, sex, diabetes, obesity, and physical activity level was performed because they were important cofounding factors of hypertension. A P value < 0.05 was regarded as statistically significant in our study.