Association between blood ethylene oxide levels and the prevalence of hypertension

The relationship of blood ethylene oxide levels with hypertension and blood pressure has not been addressed. A total of 5005 participants from the National Health and Nutrition Examination Survey (NHANES) 2013–2016 were enrolled. Hypertension was defined as a mean systolic blood pressure (SBP) of at least 140 mmHg, a mean diastolic blood pressure (DBP) of at least 90 mmHg, or both, and/or the self-reported use of prescription drugs for diagnosed hypertension. Generalized linear regression models and restricted cubic spline plots were performed to explore the associations of ethylene oxide levels with hypertension and blood pressure. The prevalence of hypertension in the study sample was 27.6%. After adjusting for confounding factors, compared with the lowest quantile, the odds ratios (ORs) with 95% confidence intervals (CIs) of hypertension across the quantiles of ethylene oxide levels were 0.80 (0.63, 1.03), 0.91 (0.71, 1.16), and 1.39 (1.06, 1.82), respectively (P-value for trend = 0.001). Compared with the lowest quantile, the highest quantile of blood ethylene oxide levels was significantly associated with the worst DBP profile by approximately 2.67 mmHg. Blood ethylene oxide levels showed a strong nonlinear and positive association with DBP, while no significant association was observed between blood ethylene oxide levels and SBP. These results provide epidemiological evidence of elevated blood levels of ethylene oxide in relation to a higher prevalence of hypertension and higher DBP. Further study is warranted to address these issues.


Introduction
Hypertension is a global health problem that can lead to serious consequences, including renal failure, cardiovascular disease (CVD), stroke, and other chronic diseases (Hamrahian and Falkner 2017;Lackland and Weber 2015;Lamprea-Montealegre et al. 2018). Hypertension was also identified as a vital influential risk factor contributing to disability-adjusted life-years worldwide (Virani et al. 2021). Recent studies have demonstrated that environmental pollutants such as acrylamide (Liang et al. 2022), perfluoroalkyl substances (Ma et al. 2019), and metals (Rahman et al. 2022) may be related to increased odds of hypertension. Therefore, the discovery and reduction of hypertension exposure factors may have significant public health benefits.
Ethylene oxide is an important industrial and environmental chemical and has been classified as a group 1 human carcinogen by the International Agency for Research on Cancer (Yang et al. 2018). There are two pathways of ethylene oxide exposure, endogenous and exogenous pathways, and the endogenous pathway is a major source of exposure among the general population (Rasool and Malik 2021). Environmental exposure to ethylene oxide occurs during its production and use, such as gaseous sterilant for medical devices and as a fumigant to disinfect food products such as spices. Ethylene oxide is a gas at room temperature, and inhalation is the primary method of human exposure. After inhalation, ethylene oxide is easily absorbed into the blood and rapidly distributed throughout the human body (Jinot and Fritz 2018). As described in the previous literature, ethylene oxide is directly responsible for increased oxidative stress and related disorders (Rasool and Malik 2021), while increased oxidative stress is a central mechanism contributing to elevated blood pressure (Prado et al. 2021). In view of the former considerations, we hypothesize that ethylene oxide is positively correlated with hypertension.
Thus, in the present study, we aimed to explore whether elevated blood ethylene oxide levels were independently associated with an increased prevalence of hypertension and blood pressure using nationally representative data from the National Health and Nutrition Examination Survey (NHANES) series.

Study population
Our study included 20,146 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 = 14,700), 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.

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 (https:// wwwn. cdc. gov/ nchs/ data/ nhanes/ 2015-2016/ labme thods/ 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.

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 selfreported use of prescription drugs for diagnosed hypertension (Muntner et al. 2020).

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/m 2 ) 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.

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 χ 2 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 confounding factors of hypertension. A P-value less than 0.05 was regarded as statistically significant in our study.

Baseline characteristics
The baseline characteristics of the study subjects across the quantiles of blood ethylene oxide levels are shown in Table 1. Differential significance was found for age, sex, education level, race/ethnicity, smoking status, alcohol use, body mass index (BMI), dietary energy intake, dietary sodium intake, dietary potassium intake, poverty-to-income ratio, and physical activity level. The prevalence of hypertension in the study sample was 27.6%, and the prevalence of hypertension across the quantiles of blood ethylene oxide levels was 28.5%, 23.2%, 24.8%, and 33.7%, respectively.

Associations between ethylene oxide levels and hypertension
The fully multivariable adjusted odds ratios (ORs) with 95% confidence intervals (CIs) for hypertension across the quantiles of blood ethylene oxide levels compared with the lowest quartile were 0.80 (0.63, 1.03), 0.91 (0.71, 1.16), and 1.39 (1.06, 1.82), respectively (P for trend 0.001, Table 2). Cubic spline plots of the association between blood ethylene oxide levels and the prevalence of hypertension were approximately nonlinear (Fig. 2); however, the tests for nonlinearity were not significant.

Associations between ethylene oxide levels and blood pressure
In the full multiple generalized linear regression, no significant association between blood ethylene oxide levels and SBP was observed (P for trend = 0.869, Table 3 and Fig. 3A). Participants in the highest quartile, in comparison with the lowest quartile of blood ethylene oxide levels, showed increased DBP by 2.67 mmHg. The restricted cubic spline plot displayed a J-shape between blood ethylene oxide levels and DBP (P for nonlinearity = 0.021, Fig. 3B).

Subgroup analysis
Stratified analysis confirmed a stronger association between blood ethylene oxide levels and the prevalence of hypertension among participants aged ≤ 60 years, female participants, nondiabetic participants, participants with BMIs > 30 kg/m 2 , and participants with poor physical activity levels (Table 4). Since the interaction of blood ethylene oxide levels with age groups was significant, age-stratified models suggested that the association of blood ethylene oxide levels with the prevalence of hypertension was significant among participants aged ≤ 60 years but not participants aged > 60 years.

Discussion
Using the national representative survey in the USA, this analysis is the first to demonstrate that the concentration of blood ethylene oxide is positively associated with the prevalence of hypertension and DBP. The significant association between elevated blood ethylene oxide levels and the prevalence of hypertension and DBP persisted even after adjustment for a wide range of established confounding factors, including age, sex, education level, race, diabetes mellitus, smoking status, alcohol use, body mass index, povertyincome ratio, energy intake, sodium intake, potassium intake, and physical activity level.
Previous studies regarding the cardiotoxicity of ethylene oxide exposure in the general population are sparse. Collectively, our findings corroborate the findings of the few previously published studies (cross-sectional study) that reported the association between ethylene oxide exposure and cardiovascular disease in the general population  (Zeng et al. 2021). Specifically, in a cross-sectional study of 3410 participants from the NHANES, the authors found that elevated blood levels of ethylene oxide were independently associated with an increased prevalence of cardiovascular disease in the general population (Zeng et al. 2021). However, there was little evidence of any ethylene oxide relationship to all-cause and cardiovascular mortality in a cohort of 18,235 ethylene oxide-exposed workers (Steenland et al. 2004). The results of our study suggest the possible cardiotoxic effects of ethylene oxide exposure on blood pressure. Our results have demonstrated that environmental exposure to ethylene oxide may be related to increased odds of hypertension. Furthermore, findings of positive association of ethylene oxide with hypertension were consistent in men and women, non-diabetes and diabetes groups, obese and non-obese groups, and no regular and regular moderate-to-vigorous physical activity groups. However, the positive association of blood ethylene oxide levels with hypertension was significant among participants aged ≤ 60 years but not participants aged > 60 years.
The biological mechanisms linking ethylene oxide levels to a higher prevalence of hypertension and higher DBP are not fully understood. Data from animal studies have suggested that the metabolism of ethylene oxide hydrolysis produced ethylene glycol and 2-chloroethanol (Koga et al. 1987). Accumulating evidence suggests that ethylene oxide and its metabolism promote increased oxidative stress and inflammation, two possible determinants of hypertension (Adedara and Farombi 2010;Kuiper et al. 2008;Rasool and Malik 2021). Ethylene glycol forms a conjugate with glutathione (GSH), resulting in its excretion through urine, forms S-carboxymethyl GSH and S-carboxymethyl cysteine, and disrupts the formation of thiol (-SH) groups (Rasool and Malik 2021). Both increases in the formation of S-carboxymethyl GSH and S-carboxymethyl cysteine and decreases in thiol (-SH) Table 2 Adjusted odds ratios for associations between ethylene oxide and the prevalence of hypertension Bolding indicates significant (P < 0.05). Model 1 was adjusted for age and sex. Model 2 was adjusted for age, sex, education level, race, diabetes mellitus, smoking status, alcohol use, and body mass index. Model 3 was 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. OR, odd ratio; CI, confidence interval  . 2 Restricted cubic spline plots of the associations between blood ethylene oxide and hypertension. Analyses were 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. The solid line and dashed line represent the odds ratios of hypertension and corresponding 95% confidence intervals There are several limitations in the present study. First, given the observational nature of this cross-sectional study, causality cannot be determined. Second, we used a single measurement of ethylene oxide levels in the blood to reflect chronic exposure to ethylene oxide, while dynamic changes in blood ethylene oxide levels may lead to exposure misclassification. Finally, although the data were tested with multiple adjustments, unmeasured confounding factors may also play a role in the relationship of blood ethylene oxide levels with hypertension and blood pressure.

Conclusion
In summary, our study demonstrates that exposure to ethylene oxide is related to an elevated prevalence of hypertension and higher diastolic blood pressure in the general Fig. 3 Restricted cubic spline plots of the associations between blood ethylene oxide and systolic (A) and diastolic blood pressure (B). Analyses were 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. The solid line and dashed line represent the predicted value of blood pressure and corresponding 95% confidence intervals Table 4 Subgroups analysis for the associations of ethylene oxide with the prevalence of hypertension Bolding indicates significant (P < 0.05). Analyses was 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 when they were not the strata variables. OR, odd ratio; CI, confidence interval population. The mechanism underlying the effect of ethylene oxide levels on blood pressure needs further exploration.