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American Journal of Hypertension logoLink to American Journal of Hypertension
. 2014 May 2;28(1):58–72. doi: 10.1093/ajh/hpu077

Acute Air Pollution Exposure and Blood Pressure at Delivery Among Women With and Without Hypertension

Tuija Männistö 1, Pauline Mendola 1,✉, Danping Liu 2, Kira Leishear 3, Seth Sherman 4, S Katherine Laughon 1
PMCID: PMC4288121  PMID: 24795401

Abstract

BACKGROUND

Chronic air pollution exposure increases risk for hypertensive disorders of pregnancy, but the effect of acute air pollution exposure on blood pressure during pregnancy is less well known.

METHODS

We studied 151,276 singleton term deliveries from the Consortium on Safe Labor (2002–2008) with clinical blood pressure measured at admission to labor/delivery and diagnoses of hypertensive disorders collected from electronic medical records and hospital discharge summaries. Air pollution exposures were estimated for the admission hour and the 4 hours preceding admission using a modified version of the Community Multiscale Air Quality models and observed air monitoring data. Blood pressure was categorized as normal; high normal; and mild, moderate, or severe hypertension based on pregnancy cut points. Adjusted ordinal logistic regression estimated the odds of women having a higher admission blood pressure category as a function of air pollutant, hypertensive disorders, and their interaction effect.

RESULTS

Odds of high blood pressure at admission to labor/delivery were increased in normotensive women after exposure to nitrogen oxides (by 0.2%/5 units), sulfur dioxide (by 0.3%/1 unit), carbon monoxide and several air toxics (by 3%–4%/high exposure). The effects were often similar or stronger among women with gestational hypertension and preeclampsia. Exposure to particulate matter <10 μm increased odds of high blood pressure in women with preeclampsia by 3%/5 units.

CONCLUSIONS

Air pollution can influence admission blood pressure in term deliveries and may increase likelihood of preeclampsia screening at delivery admission.

Keywords: air pollution, blood pressure, epidemiology, hypertension, pregnancy.

Air pollution is a complex mixture of gases and particulates formed from manmade and natural processes (e.g., power plants, industry, traffic, forest fires, and volcanic eruptions) or from chemical reactions between pollutants in the atmosphere.1 Long-term ambient air pollution exposure accounts for 2% of all cardiopulmonary disease worldwide,2 and even low levels of air pollution increase cardiac event and hypertension risk in the general population.1 Several criteria of air pollutants and toxics are monitored and regulated because of their health threats, including carbon monoxide (CO), nitrogen oxides (NOx, comprising mostly nitrogen monoxide and dioxide), sulfur dioxide (SO2), particulate matter <10 μm (PM10) and <2.5 μm (PM2.5) in aerodynamic diameter, and ozone.1 Pregnant women might be vulnerable to adverse effects of air pollution because of cardiovascular changes and increased oxygen consumption occurring as part of normal pregnancy.3

Blood pressure decreases until midgestation but increases to prepregnancy levels or higher at term (≥37 weeks’ gestation).3 Women with hypertensive disorders of pregnancy have higher blood pressure throughout pregnancy with a smaller midpregnancy nadir and a greater late-pregnancy elevation.4 Most women with hypertensive disorders of pregnancy have clustering of cardiovascular risk factors, and preeclampsia (new-onset hypertension with proteinuria or other multisystem signs) is also thought to have a placental or immunological origin.5

Chronic air pollution exposure increases blood pressure and the risk of hypertensive disorders of pregnancy, preterm birth, and stillbirths.6–13 However, the reported effects of acute/recent air pollution exposure on blood pressure during pregnancy have not been consistent and appear to change by trimester.7,9,14 Because any hypertensive blood pressure measured during pregnancy increases the risk of later cardiac diseases,15 studying triggers for blood pressure elevations is important.

We studied the effects of acute air pollution exposure on blood pressure measured at admission to labor/delivery in a large population with and without hypertensive disorders of pregnancy, hypothesizing that women with hypertensive disorders have different effects than normotensive women because of their underlying differences in cardiovascular risk factors.

METHODS

The Consortium on Safe Labor (2002–2008) included 228,562 deliveries (233,736 newborns) at ≥23 weeks’ gestation from 19 hospitals across the United States.16 Data on maternal demographics, medical history, labor and delivery, and discharge diagnoses were extracted from electronic medical records. The study was approved by the institutional review boards of all participating institutions. Multifetal pregnancies (n = 5,050), singleton deliveries <37 weeks’ gestation (n = 26,144), women with eclampsia (n = 93), and women missing data on blood pressure (n = 45,625), blood pressure measurement timing (n = 182), or age (n = 192) were excluded, rendering a final sample of 138,802 women with 151,276 singleton term deliveries. Most women (126,829; 91.4%) contributed only one pregnancy.

Clinical blood pressure was measured upon admission to labor/delivery by hospital staff using standard equipment. The first systolic and diastolic blood pressures and the exact date/time of admission were extracted from the electronic medical records. The following categories were created using relevant cutoffs for adult and pregnant populations:5,17,18

  • Normal blood pressure: systolic blood pressure <120mm Hg and diastolic blood pressure <80mm Hg.

  • High-normal blood pressure: systolic blood pressure 120–139mm Hg and diastolic blood pressure <90mm Hg or diastolic blood pressure 80–89 mmHg and systolic blood pressure <140mm Hg.

  • Mild hypertension: systolic blood pressure 140–149mm Hg and diastolic blood pressure <100mm Hg or diastolic blood pressure 90–99mm Hg and systolic blood pressure <150mm Hg.

  • Moderate hypertension: systolic blood pressure 150–159mm Hg and diastolic blood pressure <110mm Hg or diastolic blood pressure 100–109mm Hg and systolic blood pressure <160mm Hg.

  • Severe hypertension: systolic blood pressure ≥160mm Hg with any diastolic blood pressure or diastolic blood pressure ≥110mm Hg with any systolic blood pressure.

Pregnancies with gestational hypertension (n = 3,935), preeclampsia (n = 5,744), chronic hypertension (n = 2,490), and superimposed preeclampsia (n = 637) were identified through electronic medical records and/or maternal discharge summaries using International Classification of Diseases, Version 9 codes. Women were deemed normotensive (n = 138,470) if they had no indication of hypertensive disorders during pregnancy.

Air pollutant exposures were estimated with a modified version of the three-dimensional multipollutant regional air quality model developed by the US Environmental Protection Agency, the Community Multiscale Air Quality model (version 4.7.1).19 The model used data on pollutant emission (from the National Emission Inventories), meteorology (from the Weather Research and Forecasting model), and chemical reactions between pollutants for the air quality simulations. A 36-km resolution domain covered the entire continental United States for years 2001–2009 in the simulations. Exposure was based on the predicted hourly air pollutant concentrations within 15 nonoverlapping delivery hospital referral regions, weighted to reflect the populated areas, discounting places where women were unlikely to live and work.

The modeled estimates of ozone, CO, NOx, SO2, PM10, and PM2.5 were fused with observed data from the US Environmental Protection Agency Air Quality System using inverse distance weighting to correct for measurement error between modeled and observed exposures (G. Chen et al., unpublished data). Only modeled estimates were obtained for major PM2.5 constituents (elemental carbon, organic compounds, ammonium, sulfate, and nitrate) and the unspeciated portion of PM2.5 (dust particles) and for ambient air toxics because they were rarely measured by air quality monitors. Hourly averages of air pollutants were obtained for admission hour (time 0) and for the 4 hours preceding it (lags 1–4 hours). These times were chosen based on our previous analysis (T. Männistö et al., unpublished data).

Statistics

Differences in demographics were tested by linear, logistic, or multinomial regression with generalized estimating equations for continuous, binary, and categorical variables, respectively. The effect of each air pollutant at the specified lag time (0, 1, 2, 3, and 4 hours preceding admission) on categorized blood pressure measured at labor/delivery admission was modeled as a function of the air pollutant, hypertensive disorder diagnosis, the interaction between air pollutant and hypertensive disorder diagnosis, and covariables. The coefficients were respectively interpreted as the pollutant effect among normotensive women, hypertensive disorder effect (pooled to a single estimate), and the pollutant effect among women with hypertensive disorders on blood pressure category. Normotensive women acted as the comparison group in the analyses. Air pollutants were fitted in the model independently because the air quality model accounted for the biochemical reactions between species and the effects of weather, other natural phenomena, and long-term pollutant sources.19 We used ordinal logistic regression to estimate the effect of air pollutants on blood pressure categories, where the single obtained estimate (odds ratio (OR) with 95% confidence intervals (CIs)) was interpreted as the odds of being in a higher blood pressure category, assuming a similar effect between all pairs of outcome groups. Robust standard errors with clustering within a woman were calculated to account for multiple pregnancies of the same woman. We evaluated pollutant concentrations (Supplementary Table S1) and estimated the effect of 1 μg/m3 increase in PM2.5, 1 parts per billion (ppb) increase in SO2, 0.1 μg/m3 increase in PM2.5 constituents, 5 μg/m3 increase in PM10, 5 ppb increase in NOx and ozone, and 10 ppb increase in CO to assess linear relationships. Air toxics were dichotomized at the 75th percentile because modeled concentrations were quite small and we determined that the most likely risk signal would be at the upper end of the distribution. All models were adjusted for site, maternal age, race/ethnicity, insurance status, smoking during pregnancy, and time of day of admission. The analyses of PM2.5 constituents were also adjusted for total PM2.5 mass to estimate the independent effect of constituents. Because the unadjusted and adjusted estimates were similar, only the adjusted models are presented. We also estimated the effect of air pollutants on systolic and diastolic blood pressure using linear regression with generalized estimating equations and robust standard errors (Supplementary Tables S2–S7).

Two sensitivity analyses were performed: one excluding all women taking antihypertensive medications at any time during pregnancy and one restricted to women in spontaneous labor.

All analyses were conducted using SAS version 9.3 (SAS Institute, Cary, NC).

RESULTS

Women with chronic hypertension and superimposed preeclampsia were older, less often non-Hispanic white, and less likely privately insured than normotensive women (Table 1). All hypertensive women were less likely to be normal weight, have spontaneous labor, and be admitted to labor/delivery at night, but they used antihypertensive medications more often and had higher blood pressure at admission than normotensive women. Overall, 12% of normotensive women, 59% of women with gestational hypertension or preeclampsia, 50% of women with chronic hypertension, and 72% of women with superimposed preeclampsia had hypertensive admission blood pressure. Increased ORs of having a higher admission blood pressure category were associated with all hypertension diagnoses: gestational hypertension (OR = 9.69; 95% CI = 9.00–10.43), preeclampsia (OR = 12.86; 95% CI = 12.02–13.76), chronic hypertension (OR = 7.89; 95% CI = 7.12–8.75), and superimposed preeclampsia (OR = 25.49; 95% CI = 20.83 to 31.25).

Table 1.

Characteristics of women with term singleton pregnancies and measured blood pressure

Characteristic Normotensive Gestational hypertension Preeclampsia Chronic hypertension Superimposed preeclampsia
138,470 (91.5) 3,935 (2.6) 5,744 (3.8) 2,490 (1.7) 637 (0.4)
No. pregnancies in the cohort
 1 124,992 (90) 3,730 (95) <0.0001 5,403 (94) <0.0001 2,208 (89) 0.006 606 (95) <0.0001
 2 12,825 (9) 197 (5) 319 (6) 260 (10) 31 (5)
 ≥3 653 (1) 8 (0) 22 (0) 22 (1) 0 (0)
Mean age (SD), y 27.2 (5.9) 26.7 (6.1) <0.0001 26.9 (6.6) <0.0001 30.7 (6.5) <0.0001 29.6 (6.8) 0.0003
BP category on labor/delivery admission
 Normal 55,292 (40) 230 (6) <0.0001 379 (7) <0.0001 250 (10) <0.0001 28 (4) <0.0001
 High-normal 66,814 (48) 1,402 (36) 1,986 (35) 999 (40) 152 (24)
 Mild hypertension 13,186 (10) 1,464 (37) 1,925 (34) 746 (30) 189 (30)
 Moderate hypertension 2,508 (2) 619 (16) 978 (17) 301 (12) 144 (23)
 Severe hypertension 670 (0) 220 (6) 476 (8) 194 (8) 124 (20)
Nulliparous 52194 (38) 2270 (58) <0.0001 3334 (58) <0.0001 738 (30) <0.0001 246 (39) 0.03
Race/ethnicity
 Non-Hispanic white 70,621 (51) 2,230 (57) <0.0001 2,665 (46) 0.002 735 (30) <0.0001 149 (23) <0.0001
 Non-Hispanic black 26,910 (19) 951 (24) 1,512 (26) 1,105 (44) 269 (42)
 Hispanic 25,030 (18) 508 (13) 918 (16) 449 (18) 164 (26)
 Asian/Pacific Islander 5,336 (4) 66 (2) 234 (4) 39 (2) 9 (1)
 Other 3557 (3) 62 (2) 172 (3) 110 (4) 32 (5)
 Unknown 7,016 (5) 118 (3) 243 (4) 52 (2) 14 (0)
Insurance
 Private 83,868 (61) 2,279 (58) 0.0317 3,557 (62) 0.0043 1,100 (44) <0.0001 212 (33) <0.0001
 Public/self-pay 44,921 (32) 1,452 (37) 1,883 (33) 1,029 (41) 309 (49)
 Other 121 (0) 10 (0) 3 (0) 3 (0) 0 (0)
 Unknown 9,560 (7) 194 (5) 301 (5) 358 (14) 116 (18)
Pre-pregnancy BMIa
 Underweight 4,982 (4) 62 (2) <0.0001 114 (2) <0.0001 25 (1) <0.0001 10 (2) <0.0001
 Normal weight 51,164 (37) 1,080 (28) 1,358 (24) 380 (15) 124 (20)
 Overweight 21,430 (16) 744 (19) 877 (15) 394 (16) 138 (22)
 Obese 9,627 (7) 465 (12) 551 (10) 365 (15) 118 (19)
 Severely obese 6,725 (5) 468 (12) 524 (9) 570 (23) 128 (20)
 Unknown 44,542 (32) 1,116 (28) 2,320 (40) 756 (30) 119 (19)
Time of day at admission
 Night 29,507 (21) 491 (13) <0.0001 813 (14) <0.0001 357 (14) <0.0001 99 (16) <0.0001
 Morning 53,764 (39) 1,369 (35) 1,535 (27) 995 (40) 150 (24)
 Day 27,022 (20) 1,079 (27) 1,813 (32) 582 (23) 236 (37)
 Evening 28,177 (20) 996 (25) 1,583 (28) 556 (22) 152 (24)
Smoking 7,761 (6) 197 (5) 0.7003 273 (5) 0.9486 163 (7) 0.1838 48 (8) 0.7689
Antihypertension medication use 203 (0) 152 (4) <0.0001 450 (8) <0.0001 334 (13) <0.0001 99 (16) <0.0001
Spontaneous labor 82,878 (60) 1,318 (34) <0.0001 2,192 (38) <0.0001 759 (31) <0.0001 127 (20) <0.0001
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All figures are No. (%) unless otherwise specified. P values are obtained from linear, logistic, or multinomial logistic regression with generalized estimating equations for continuous, binary, and categorical variables, respectively.

Abbreviations: BMI, body mass index; BP, blood pressure; SD, standard deviation.

aCalculated as weight/height2 (in kg/m2) and categorized as the following: underweight, BMI <18.5kg/m2; normal weight, BMI 18.5–24.9kg/m2; overweight, BMI 25.0–29.9kg/m2; obese, BMI 30.0–34.9kg/m2; and severely obese, BMI ≥35.0kg/m2.

Normotensive women had increased odds of higher blood pressure category with exposure to nitrate and dust particles (by 0.1%–0.2%/0.1 units), CO (by 0.04%/10 units), NOx (by 0.2%–0.3%/5 units), SO2 (by 0.3%–0.4%/1 unit) (Table 2), acenaphthene, acenaphthylene, benz[a]pyrene, fluoranthene, fluorene, phenanthrene, pyrene (by 4%–7%/high exposure) (Table 3), 1,3-butadiene, propene, and styrene (by 3%–6%/high exposure) (Table 4). Exposure to sulfate particles and sesquiterpene were associated with opposing effects (Tables 2 and 4). Similar effects were seen in the linear models, where additional associations were seen between PM10 and lower diastolic blood pressure and between ozone, chrysene, cyclohexane, and n-hexane and higher systolic blood pressure (Supplementary Tables S2–S7).

Table 2.

Odds ratios (95% confidence intervals) of a higher blood pressure category on admission to labor/delivery after exposure to criteria air pollutants, by hypertension statusa

Exposure by hypertensive status Time (hours)
0 1 2 3 4
Normotensive women
 PM10 0.997 (0.992–1.001) 0.997 (0.992– 1.001) 0.997 (0.993–1.002) 0.997 (0.993–1.002) 0.998 (0.993–1.002)
 PM2.5 1.000 (0.999–1.001) 1.000 (0.999–1.002) 1.000 (0.999–1.002) 1.000 (0.999–1.002) 1.000 (0.999–1.002)
 Elemental carbonb 1.000 (0.996–1.004) 0.999 (0.995–1.003) 0.999 (0.995–1.003) 1.000 (0.996–1.004) 1.000 (0.996–1.004)
 Organic compoundsb 1.000 (0.997–1.002) 1.000 (0.997–1.002) 1.000 (0.997–1.002) 1.000 (0.997–1.002) 1.000 (0.997–1.002)
 Ammonium particlesb 0.998 (0.995–1.001) 0.998 (0.995–1.001) 0.998 (0.995–1.002) 0.998 (0.995–1.001) 0.997 (0.994–1.001)
 Nitrate particlesb 1.002 (1.001–1.003) 1.002 (1.001–1.003) 1.002 (1.001–1.003) 1.002 (1.001–1.003) 1.002 (1.001–1.003)
 Sulfate particlesb 0.998 (0.998–0.999) 0.998 (0.998–0.999) 0.998 (0.998–0.999) 0.998 (0.998–0.999) 0.998 (0.998–0.999)
 Dust particlesb 1.001 (1.000–1.001) 1.001 (1.000–1.001) 1.001 (1.000–1.001) 1.001 (1.000–1.002) 1.001 (1.000–1.002)
 CO 1.000 (1.000–1.000) 1.000 (1.000–1.000) 1.000 (1.000–1.000) 1.000 (1.000–1.001) 1.000 (1.000–1.001)
 Ozone 0.997 (0.993–1.001) 0.998 (0.995–1.002) 0.999 (0.995–1.003) 0.998 (0.994–1.001) 0.997 (0.993–1.000)
 NOx 1.001 (0.999–1.002) 1.001 (0.999–1.002) 1.002 (1.000–1.003) 1.002 (1.001–1.004) 1.003 (1.001–1.005)
 SO2 1.003 (1.001–1.006) 1.004 (1.002–1.007) 1.003 (1.001–1.006) 1.003 (1.000–1.005) 1.002 (1.000–1.005)
Gestational hypertension
 PM10 1.013 (0.987–1.039) 1.014 (0.988–1.040) 1.016 (0.990–1.043) 1.016 (0.990–1.042) 1.015 (0.999–1.041)
 PM2.5 1.006 (0.998–1.014) 1.006 (0.999–1.014) 1.007 (0.999–1.015) 1.006 (0.998–1.014) 1.005 (0.997–1.014)
 Elemental carbonb 1.016 (1.000–1.033)* 1.016 (1.000–1.032)* 1.016 (0.999–1.032)* 1.015 (0.998–1.032) 1.013 (0.996–1.030)
 Organic compoundsb 1.006 (0.999–1.014) 1.006 (0.999–1.014) 1.006 (0.999–1.013) 1.005 (0.998–1.013) 1.005 (0.997–1.012)
 Ammonium particlesb 1.001 (0.995–1.008) 1.002 (0.995–1.008) 1.002 (0.995–1.008) 1.001 (0.994–1.007) 0.999 (0.993–1.006)
 Nitrate particlesb 1.001 (0.997–1.006) 1.002 (0.998–1.006) 1.003 (0.999–1.007) 1.002 (0.998–1.006) 1.002 (0.998–1.006)
 Sulfate particlesb 1.000 (0.998–1.003) 1.000 (0.998–1.002) 1.000 (0.998–1.002) 1.000 (0.997–1.002) 0.999 (0.997–1.002)
 Dust particlesb 1.001 (0.998–1.004) 1.002 (0.999–1.005) 1.003 (1.000–1.005) 1.003 (1.000–1.006) 1.003 (1.000–1.006)
 CO 0.999 (0.998–1.001) 0.999 (0.998–1.001) 1.000 (0.998–1.001) 1.001 (0.999–1.002) 1.001 (0.999–1.003)
 Ozone 0.983 (0.966–1.000) 0.983 (0.966–1.000) 0.982 (0.965–0.999) 0.980 (0.963–0.997) 0.980 (0.963–0.998)
 NOx 0.995 (0.987–1.003) 0.997 (0.989–1.005) 1.000 (0.992–1.008) 1.003 (0.995–1.011) 1.006 (0.997–1.014)
 SO2 1.006 (0.990–1.022) 1.004 (0.988–1.019) 1.004 (0.989–1.020) 1.009 (0.993–1.026) 0.998 (0.982–1.015)
Preeclampsia
 PM10 1.031 (1.009–1.053)* 1.033 (1.011–1.055)* 1.032 (1.010–1.054)* 1.029 (1.007–1.051)* 1.025 (1.003–1.047)*
 PM2.5 1.004 (0.998–1.010) 1.004 (0.998–1.010) 1.004 (0.998–1.010) 1.003 (0.997–1.009) 1.003 (0.997–1.009)
 Elemental carbonb 1.010 (1.001–1.019) 1.011 (1.003–1.020)* 1.011 (1.003–1.020)* 1.011 (1.002–1.019)* 1.010 (1.001–1.018)*
 Organic compoundsb 1.004 (0.999–1.009) 1.005 (1.000–1.010)* 1.005 (1.000–1.010)* 1.004 (0.999–1.009) 1.002 (0.998–1.007)
 Ammonium particlesb 1.002 (0.997–1.007) 1.002 (0.997–1.007) 1.002 (0.997–1.008) 1.002 (0.997–1.007) 1.001 (0.996–1.006)
 Nitrate particlesb 1.006 (1.003–1.008) 1.006 (1.003–1.009)* 1.006 (1.003–1.009)* 1.006 (1.003–1.008)* 1.006 (1.003–1.008)*
 Sulfate particlesb 0.998 (0.997–1.000) 0.998 (0.996–1.000) 0.998 (0.996–1.000) 0.998 (0.996–1.000) 0.998 (0.996–1.000)
 Dust particlesb 1.004 (1.002–1.006)* 1.005 (1.003–1.007)* 1.005 (1.003–1.007)* 1.005 (1.003–1.007)* 1.004 (1.002–1.007)*
 CO 0.998 (0.997–1.000)* 0.999 (0.997–1.000) 1.000 (0.998–1.001) 1.001 (0.999–1.002) 1.000 (0.999–1.002)
 Ozone 0.984 (0.970–0.998) 0.984 (0.970–0.998)* 0.983 (0.969–0.997)* 0.981 (0.967–0.995)* 0.979 (0.965–0.993)*
 NOx 0.995 (0.988–1.002) 0.999 (0.992–1.006) 1.002 (0.995–1.009) 1.002 (0.996–1.009) 1.002 (0.995–1.009)
 SO2 1.002 (0.994–1.011) 1.001 (0.994–1.009) 1.003 (0.995–1.012) 1.002 (0.994–1.011) 1.001 (0.992–1.009)
Chronic hypertension
 PM10 0.968 (0.935–1.002) 0.968 (0.936–1.002) 0.970 (0.937–1.004) 0.974 (0.941–1.009) 0.986 (0.952–1.020)
 PM2.5 0.994 (0.984–1.004) 0.994 (0.984–1.004) 0.994 (0.984–1.004) 0.996 (0.985–1.006) 0.998 (0.988–1.008)
 Elemental carbonb 0.988 (0.971–1.006) 0.988 (0.971–1.007) 0.990 (0.971–1.009) 0.994 (0.975–1.014) 1.002 (0.982–1.023)
 Organic compoundsb 0.999 (0.990–1.007) 0.999 (0.990–1.007) 0.999 (0.990–1.008) 1.000 (0.991–1.009) 1.002 (0.993–1.012)
 Ammonium particlesb 0.993 (0.986–1.001) 0.994 (0.986–1.001) 0.994 (0.986–1.001) 0.994 (0.987–1.001) 0.995 (0.988–1.003)
 Nitrate particlesb 1.000 (0.996–1.004) 1.000 (0.996–1.003) 1.000 (0.996–1.003) 1.000 (0.996–1.004) 1.001 (0.997–1.005)
 Sulfate particlesb 0.997 (0.994–1.000) 0.997 (0.994–1.000) 0.997 (0.994–1.000) 0.997 (0.994–1.000) 0.997 (0.994–1.000)
 Dust particlesb 0.996 (0.993–0.999)* 0.996 (0.993–0.999)* 0.997 (0.993–1.000)* 0.998 (0.994–1.001) 0.999 (0.995–1.002)
 CO 1.002 (0.999–1.004) 1.002 (0.999–1.004) 1.002 (0.999–1.004) 1.003 (1.000–1.006)* 1.004 (1.001–1.007)*
 Ozone 1.012 (0.988–1.037) 1.016 (0.991–1.041) 1.014 (0.989–1.039) 1.014 (0.990–1.039) 1.013 (0.988–1.038)
 NOx 1.007 (0.995–1.019) 1.008 (0.997–1.020) 1.010 (0.999–1.021) 1.011 (1.001–1.022) 1.017 (1.005–1.029)*
 SO2 1.015 (0.993–1.038) 1.023 (1.003–1.043) 1.027 (1.005–1.049)* 1.009 (0.988–1.032) 1.023 (1.002–1.045)
Superimposed preeclampsia
 PM10 1.039 (0.959–1.125) 1.048 (0.966–1.136) 1.053 (0.971–1.142) 1.047 (0.967–1.134) 1.043 (0.964–1.129)
 PM2.5 1.013 (0.991–1.037) 1.016 (0.993–1.039) 1.017 (0.995–1.039) 1.015 (0.994–1.037) 1.015 (0.995–1.036)
 Elemental carbonb 0.984 (0.944–1.026) 0.990 (0.950–1.032) 0.996 (0.955–1.038) 0.997 (0.956–1.039) 1.000 (0.959–1.042)
 Organic compoundsb 1.006 (0.987–1.026) 1.008 (0.989–1.028) 1.009 (0.990–1.029) 1.007 (0.988–1.027) 1.007 (0.987–1.027)
 Ammonium particlesb 0.994 (0.978–1.010) 0.996 (0.980–1.012) 0.999 (0.984–1.015) 0.998 (0.983–1.013) 0.998 (0.983–1.013)
 Nitrate particlesb 0.992 (0.981–1.003) 0.995 (0.984–1.005) 0.996 (0.986–1.006) 0.996 (0.986–1.006) 0.996 (0.987–1.006)
 Sulfate particlesb 1.002 (0.996–1.009) 1.002 (0.996–1.008) 1.002 (0.996–1.008) 1.002 (0.996–1.007) 1.001 (0.996–1.007)
 Dust particlesb 0.998 (0.990–1.006) 0.999 (0.990–1.009) 1.000 (0.991–1.009) 0.999 (0.991–1.008) 0.999 (0.991–1.008)
 CO 1.001 (0.995–1.007) 1.004 (0.997–1.010) 1.005 (0.998–1.011) 1.007 (1.000–1.013) 1.006 (1.000–1.012)
 Ozone 1.111 (1.061–1.164)* 1.108 (1.057–1.160)* 1.104 (1.054–1.156)* 1.090 (1.040–1.142)* 1.080 (1.029–1.132)*
 NOx 0.986 (0.958–1.014) 0.991 (0.967–1.016) 0.995 (0.972–1.017) 1.003 (0.977–1.031) 1.009 (0.982–1.035)
 SO2 0.983 (0.945–1.023) 0.999 (0.971–1.029) 0.979 (0.943–1.016) 0.963 (0.930–0.997)* 0.993 (0.952–1.036)
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Abbreviations: CO, carbon monoxide; NOx, nitrogen oxides; PM2.5, particulate matter <2.5 μm; PM10, particulate matter <10 μm; ppb, parts per billion; SO2, sulfur dioxide.

aOrdinal logistic regression with a function of air pollutant, hypertensive disorder, and their interaction and results clustered within woman estimated the effect of 1-unit increase in elemental carbon, organic compounds, and ammonium, nitrate, sulfate and dust particles (in µg/m3) and SO2 (in ppb); 0.1-unit increase in PM2.5 constituents (in µg/m3); 5-unit increase in PM10 (in µg/m3), NOx (in ppb) and ozone (in ppb); and 10-unit increase in CO (in ppb), adjusted for site, maternal age, race/ethnicity, insurance, smoking during pregnancy, and time of day.

bModel is further adjusted for total PM2.5 mass.

*Interaction term is significant (P < 0.05), suggesting that the response is different than in the normotensive women. Estimates that are statistically significant are given in bold.

Table 3.

Odds ratios (95% confidence intervals) of a higher blood pressure category on admission to labor/delivery after high exposure to polycyclic aromatic hydrocarbons, by hypertension statusa

Exposure by hypertensive status Time (hours)
0 1 2 3 4
Normotensive women
 Acenaphthene 1.067 (1.039–1.095) 1.065 (1.037–1.094) 1.072 (1.044–1.101) 1.070 (1.042–1.099) 1.073 (1.045–1.102)
 Acenaphthylene 1.077 (1.048–1.106) 1.072 (1.043–1.101) 1.073 (1.044–1.102) 1.068 (1.040–1.097) 1.069 (1.041–1.098)
 Anthracene 1.010 (0.983–1.037) 1.006 (0.979–1.033) 1.006 (0.979–1.033) 1.015 (0.988–1.042) 1.019 (0.992–1.046)
 Benz[a]anthracene 0.997 (0.971–1.024) 1.003 (0.977–1.031) 1.011 (0.985–1.039) 1.012 (0.985–1.040) 1.004 (0.978–1.032)
 Benz[a]pyrene 1.014 (0.986–1.043) 1.023 (0.995–1.052) 1.029 (1.001–1.058) 1.031 (1.003–1.06) 1.020 (0.992–1.049)
 Chrysene 1.006 (0.978–1.034) 1.009 (0.982–1.038) 1.016 (0.988–1.045) 1.022 (0.994–1.051) 1.020 (0.992–1.049)
 Fluoranthene 1.067 (1.039–1.096) 1.063 (1.035–1.092) 1.064 (1.035–1.093) 1.059 (1.031–1.088) 1.062 (1.034–1.091)
 Fluorene 1.053 (1.026–1.081) 1.049 (1.022–1.077) 1.047 (1.020–1.074) 1.053 (1.025–1.081) 1.060 (1.032–1.088)
 Indeno[1,2,3Cd]pyrene 0.994 (0.969–1.020) 0.998 (0.973–1.024) 1.001 (0.975–1.026) 0.995 (0.970–1.021) 0.999 (0.974–1.025)
 Naphthalene 1.012 (0.984–1.040) 1.026 (0.998–1.055) 1.039 (1.011–1.068) 1.051 (1.023–1.080) 1.038 (1.010–1.067)
 Phenanthrene 1.065 (1.037–1.094) 1.065 (1.037–1.094) 1.062 (1.033–1.090) 1.064 (1.036–1.093) 1.064 (1.036–1.093)
 Pyrene 1.053 (1.025–1.081) 1.058 (1.030–1.087) 1.063 (1.034–1.091) 1.056 (1.028–1.085) 1.060 (1.031–1.088)
Gestational hypertension
 Acenaphthene 1.134 (0.980–1.312) 1.115 (0.964–1.290) 1.106 (0.955–1.282) 1.174 (1.014–1.360) 1.135 (0.981–1.312)
 Acenaphthylene 1.259 (1.089–1.455)* 1.189 (1.029–1.374)* 1.154 (0.999–1.333) 1.151 (0.995–1.331) 1.109 (0.960–1.282)
 Anthracene 1.110 (0.973–1.267) 1.093 (0.958–1.247) 1.121 (0.983–1.279) 1.113 (0.975–1.270) 1.092 (0.957–1.247)
 Benz[a]anthracene 1.026 (0.889–1.183) 1.051 (0.911–1.214) 1.125 (0.976–1.297) 1.048 (0.909–1.207) 0.978 (0.850–1.125)
 Benz[a]pyrene 1.014 (0.874–1.175) 1.096 (0.945–1.271) 1.096 (0.946–1.270) 1.096 (0.945–1.271) 1.108 (0.955–1.285)
 Chrysene 1.050 (0.902–1.221) 1.108 (0.953–1.288) 1.201 (1.032–1.398)* 1.249 (1.073–1.455)* 1.259 (1.080–1.468)*
 Fluoranthene 1.148 (0.987–1.336) 1.127 (0.968–1.313) 1.138 (0.978–1.325) 1.188 (1.021–1.382) 1.172 (1.010–1.360)
 Fluorene 1.136 (0.984–1.311) 1.138 (0.985–1.313) 1.094 (0.947–1.262) 1.131 (0.979–1.307) 1.124 (0.975–1.295)
 Indeno[1,2,3Cd]pyrene 0.939 (0.809–1.089) 0.984 (0.847–1.142) 0.985 (0.849–1.144) 0.976 (0.840–1.134) 0.908 (0.782–1.054)
 Naphthalene 1.010 (0.861–1.185) 1.019 (0.867–1.198) 1.038 (0.884–1.218) 0.990 (0.844–1.161) 1.002 (0.857–1.171)
 Phenanthrene 1.162 (1.004–1.344) 1.193 (1.031–1.381) 1.188 (1.025–1.378) 1.166 (1.007–1.349) 1.119 (0.968–1.295)
 Pyrene 1.139 (0.978–1.327) 1.146 (0.983–1.336) 1.163 (0.996–1.357) 1.223 (1.051–1.424) 1.249 (1.076–1.451)*
Preeclampsia
 Acenaphthene 1.011 (0.906–1.129) 1.030 (0.923–1.149) 1.010 (0.905–1.129) 1.027 (0.919–1.147) 1.066 (0.955–1.190)
 Acenaphthylene 1.067 (0.955–1.192) 1.063 (0.952–1.188) 1.051 (0.940–1.174) 1.060 (0.948–1.185) 1.042 (0.933–1.163)
 Anthracene 1.052 (0.941–1.177) 1.086 (0.971–1.215) 1.067 (0.955–1.193) 1.071 (0.958–1.198) 1.045 (0.935–1.168)
 Benz[a]anthracene 1.158 (1.039–1.291)* 1.201 (1.077–1.339)* 1.164 (1.044–1.299)* 1.194 (1.071–1.331)* 1.081 (0.971–1.205)
 Benz[a]pyrene 0.938 (0.838–1.050) 0.952 (0.851–1.066) 0.936 (0.836–1.047) 0.942 (0.842–1.054) 0.957 (0.856–1.070)
 Chrysene 1.176 (1.051–1.317)* 1.154 (1.031–1.292)* 1.229 (1.098–1.375)* 1.174 (1.049–1.314)* 1.125 (1.005–1.258)*
 Fluoranthene 1.106 (0.991–1.234) 1.078 (0.966–1.203) 1.046 (0.937–1.168) 1.042 (0.933–1.164) 1.064 (0.954–1.187)
 Fluorene 1.135 (1.016–1.267) 1.124 (1.006–1.256) 1.096 (0.980–1.226) 1.077 (0.964–1.204) 1.081 (0.968–1.207)
 Indeno[1,2,3Cd]pyrene 0.861 (0.769–0.965)* 0.870 (0.775–0.975)* 0.891 (0.793–1.001) 0.878 (0.781–0.987)* 0.929 (0.827–1.045)
 Naphthalene 1.076 (0.965–1.199) 1.067 (0.957–1.190) 1.086 (0.974–1.212) 1.077 (0.966–1.200) 1.056 (0.948–1.176)
 Phenanthrene 1.115 (0.998–1.246) 1.124 (1.006–1.256) 1.117 (1.000–1.248) 1.075 (0.962–1.202) 1.083 (0.970–1.209)
 Pyrene 1.163 (1.041–1.299) 1.160 (1.037–1.297) 1.130 (1.011–1.262) 1.037 (0.928–1.158) 1.047 (0.938–1.168)
Chronic hypertension
 Acenaphthene 1.057 (0.883–1.267) 1.059 (0.884–1.269) 0.974 (0.812–1.169) 0.993 (0.829–1.189) 1.052 (0.877–1.261)
 Acenaphthylene 1.043 (0.871–1.250) 1.020 (0.851–1.221) 1.008 (0.842–1.208) 0.986 (0.822–1.182) 1.028 (0.856–1.234)
 Anthracene 1.011 (0.848–1.206) 0.977 (0.819–1.165) 0.990 (0.830–1.182) 0.958 (0.804–1.143) 1.005 (0.843–1.197)
 Benz[a]anthracene 0.925 (0.780–1.097) 0.918 (0.774–1.089) 0.868 (0.731–1.031) 0.949 (0.798–1.128) 1.042 (0.876–1.240)
 Benz[a]pyrene 0.938 (0.782–1.125) 0.933 (0.779–1.116) 0.960 (0.803–1.149) 0.992 (0.829–1.188) 1.027 (0.858–1.229)
 Chrysene 0.804 (0.680–0.951)* 0.789 (0.667–0.934)* 0.814 (0.687–0.964)* 0.893 (0.754–1.057) 0.917 (0.775–1.086)
 Fluoranthene 0.965 (0.807–1.154) 1.020 (0.851–1.222) 1.023 (0.853–1.227) 1.016 (0.848–1.218) 1.088 (0.910–1.300)
 Fluorene 1.010 (0.847–1.205) 0.929 (0.778–1.110) 0.940 (0.786–1.123) 0.965 (0.807–1.154) 0.996 (0.835–1.189)
 Indeno[1,2,3Cd]pyrene 0.967 (0.811–1.152) 1.001 (0.841–1.190) 1.029 (0.865–1.223) 1.075 (0.906–1.277) 1.013 (0.854–1.202)
 Naphthalene 0.957 (0.800–1.146) 0.935 (0.782–1.118) 1.025 (0.857–1.225) 1.116 (0.934–1.334) 1.114 (0.932–1.331)
 Phenanthrene 1.105 (0.924–1.321) 1.072 (0.896–1.283) 1.062 (0.888–1.270) 0.956 (0.801–1.142) 1.061 (0.891–1.262)
 Pyrene 0.915 (0.765–1.095) 0.933 (0.778–1.119) 1.014 (0.845–1.217) 1.046 (0.873–1.253) 1.068 (0.892–1.279)
Superimposed preeclampsia
 Acenaphthene 0.945 (0.648–1.378) 0.854 (0.575–1.270) 0.888 (0.601–1.310) 0.882 (0.596–1.306) 1.101 (0.745–1.627)
 Acenaphthylene 0.844 (0.569–1.251) 0.775 (0.521–1.152) 0.797 (0.536–1.187) 0.866 (0.588–1.275) 0.940 (0.636–1.388)
 Anthracene 1.259 (0.879–1.805) 1.494 (1.044–2.140)* 1.265 (0.879–1.821) 1.173 (0.819–1.680) 1.110 (0.776–1.586)
 Benz[a]anthracene 0.759 (0.542–1.064) 0.767 (0.549–1.072) 0.832 (0.594–1.166) 0.958 (0.688–1.335) 0.823 (0.593–1.142)
 Benz[a]pyrene 0.693 (0.451–1.063) 0.788 (0.509–1.219) 0.884 (0.571–1.368) 1.079 (0.701–1.662) 0.935 (0.593–1.474)
 Chrysene 0.700 (0.498–0.983)* 0.742 (0.524–1.050) 0.869 (0.612–1.233) 0.817 (0.581–1.150) 0.965 (0.690–1.348)
 Fluoranthene 0.728 (0.497–1.068) 0.685 (0.463–1.014)* 0.609 (0.409–0.908)* 0.703 (0.473–1.044)* 0.867 (0.584–1.288)
 Fluorene 0.881 (0.605–1.281) 0.819 (0.557–1.205) 0.870 (0.587–1.288) 0.847 (0.572–1.255) 0.991 (0.671–1.463)
 Indeno[1,2,3Cd]pyrene 0.765 (0.530–1.105) 0.951 (0.658–1.376) 0.938 (0.646–1.362) 0.982 (0.673–1.433) 1.141 (0.782–1.666)
 Naphthalene 0.688 (0.483–0.980)* 0.717 (0.502–1.023)* 0.827 (0.576–1.187) 0.988 (0.694–1.407) 0.903 (0.633–1.289)
 Phenanthrene 0.890 (0.597–1.325) 0.790 (0.531–1.174) 0.743 (0.498–1.107) 0.812 (0.545–1.212) 0.991 (0.668–1.470)
 Pyrene 0.702 (0.486–1.015)* 0.616 (0.426–0.891)* 0.635 (0.436–0.924)* 0.750 (0.515–1.092) 0.827 (0.572–1.198)
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aOrdinal logistic regression with linear function of air pollutant, hypertensive disorder, and their interaction and results clustered within woman estimated the effect of high air toxics exposure (≥75th percentile, in parts per billion), adjusted for site, maternal age, race/ethnicity, insurance, smoking during pregnancy, and time of day.

*Interaction term is significant (P < 0.05), suggesting that the response is different than in the normotensive women. Estimates that are statistically significant are given in bold.

Table 4.

Odds ratios (95% confidence intervals) of higher blood pressure category on admission to labor/delivery by volatile organic compoundsa

Exposure by hypertensive status Time (hours)
0 1 2 3 4
Normotensive
 Benzene 1.001 (0.973–1.030) 1.009 (0.981–1.038) 1.006 (0.978–1.035) 0.997 (0.970–1.026) 0.995 (0.967–1.023)
 1,3-butadiene 1.013 (0.985–1.043) 1.018 (0.990–1.047) 1.038 (1.010–1.067) 1.051 (1.022–1.081) 1.058 (1.028–1.089)
 Ethylbenzene 0.997 (0.968–1.028) 1.001 (0.971–1.032) 0.999 (0.969–1.030) 0.999 (0.969–1.030) 0.986 (0.956–1.016)
 Cyclohexane 1.008 (0.977–1.040) 1.016 (0.985–1.048) 1.013 (0.982–1.045) 1.016 (0.985–1.048) 1.015 (0.984–1.047)
 Methyl-tertiary-butyl ether 0.983 (0.956–1.010) 0.992 (0.965–1.019) 0.997 (0.971–1.025) 0.994 (0.968–1.022) 0.980 (0.953–1.007)
N-hexane 1.000 (0.972–1.029) 1.010 (0.982–1.039) 1.007 (0.979–1.036) 1.011 (0.983–1.040) 0.994 (0.966–1.023)
 Ethyl-methyl ketone 0.979 (0.951–1.008) 1.001 (0.972–1.030) 0.999 (0.970–1.028) 0.995 (0.967–1.025) 0.988 (0.959–1.017)
 M-xylene 0.996 (0.966–1.027) 1.009 (0.979–1.040) 1.005 (0.975–1.036) 1.002 (0.972–1.033) 1.001 (0.971–1.032)
 O-xylene 0.993 (0.962–1.024) 0.985 (0.955–1.017) 1.000 (0.969–1.032) 1.007 (0.976–1.039) 0.998 (0.968–1.030)
 P-xylene 0.996 (0.966–1.026) 1.008 (0.978–1.039) 1.003 (0.973–1.033) 1.000 (0.970–1.030) 0.983 (0.954–1.014)
 Propene 1.000 (0.974–1.027) 1.014 (0.988–1.041) 1.036 (1.009–1.064) 1.047 (1.020–1.075) 1.039 (1.012–1.067)
 Sesquiterpene 0.971 (0.948–0.995) 0.979 (0.955–1.003) 0.992 (0.968–1.017) 0.992 (0.969–1.016) 0.992 (0.968–1.016)
 Styrene 0.986 (0.958–1.016) 1.000 (0.972–1.029) 1.017 (0.989–1.046) 1.031 (1.002–1.061) 1.035 (1.005–1.066)
 Toluene 0.988 (0.958–1.019) 0.982 (0.952–1.013) 0.995 (0.964–1.026) 1.004 (0.973–1.035) 1.004 (0.973–1.036)
Gestational hypertension
 Benzene 1.153 (0.992–1.340) 1.183 (1.014–1.379)* 1.216 (1.045–1.415)* 1.190 (1.025–1.383)* 1.203 (1.037–1.394)*
 1,3-butadiene 0.960 (0.837–1.101) 0.991 (0.861–1.140) 1.008 (0.874–1.162) 1.010 (0.875–1.165) 1.077 (0.932–1.244)
 Ethylbenzene 1.163 (0.989–1.369) 1.119 (0.951–1.318) 1.103 (0.935–1.300) 1.141 (0.972–1.340) 1.143 (0.974–1.342)
 Cyclohexane 1.164 (0.986–1.375) 1.140 (0.966–1.345) 1.176 (0.996–1.387) 1.198 (1.015–1.415) 1.132 (0.961–1.332)
 Methyl-tertiary-butyl ether 1.144 (0.992–1.320)* 1.221 (1.059–1.409)* 1.259 (1.092–1.451)* 1.186 (1.031–1.366)* 1.154 (1.003–1.328)*
 N-hexane 1.266 (1.088–1.472)* 1.252 (1.076–1.456)* 1.270 (1.092–1.476)* 1.275 (1.097–1.481)* 1.218 (1.048–1.416)*
 Ethyl-methyl ketone 1.055 (0.901–1.236) 1.080 (0.921–1.266) 1.096 (0.936–1.284) 1.093 (0.933–1.280) 1.080 (0.924–1.262)
 M-xylene 1.132 (0.961–1.332) 1.203 (1.019–1.422)* 1.190 (1.010–1.402)* 1.235 (1.048–1.456)* 1.229 (1.046–1.443)*
 O-xylene 1.184 (1.001–1.402)* 1.183 (0.997–1.404)* 1.179 (0.995–1.398) 1.178 (0.995–1.395) 1.198 (1.015–1.413)*
 P-xylene 1.085 (0.920–1.279) 1.110 (0.942–1.307) 1.144 (0.971–1.347) 1.153 (0.981–1.355) 1.098 (0.935–1.288)
 Propene 1.203 (1.048–1.382)* 1.222 (1.06–1.408)* 1.211 (1.049–1.397)* 1.255 (1.085–1.452)* 1.242 (1.073–1.437)*
 Sesquiterpene 1.017 (0.888–1.165) 0.992 (0.866–1.136) 1.044 (0.913–1.195) 1.092 (0.954–1.249) 1.010 (0.881–1.156)
 Styrene 1.008 (0.878–1.158) 1.044 (0.907–1.203) 1.062 (0.922–1.224) 1.026 (0.890–1.182) 1.054 (0.914–1.214)
 Toluene 1.174 (0.992–1.390) 1.171 (0.988–1.387)* 1.219 (1.028–1.444)* 1.213 (1.027–1.431)* 1.209 (1.025–1.426)*
Preeclampsia
 Benzene 1.162 (1.046–1.291)* 1.179 (1.061–1.309)* 1.194 (1.074–1.327)* 1.160 (1.043–1.289)* 1.115 (1.003–1.238)*
 1,3-butadiene 0.946 (0.842–1.063) 0.936 (0.831–1.056) 0.947 (0.836–1.073) 0.992 (0.874–1.127) 0.966 (0.849–1.100)
 Ethylbenzene 1.110 (0.998–1.234) 1.101 (0.990–1.224) 1.078 (0.970–1.199) 1.053 (0.947–1.171) 1.029 (0.925–1.144)
 Cyclohexane 1.104 (0.992–1.228) 1.129 (1.014–1.256) 1.145 (1.029–1.274)* 1.129 (1.015–1.256) 1.096 (0.985–1.219)
 Methyl-tertiary-butyl ether 1.084 (0.976–1.205) 1.107 (0.997–1.230)* 1.110 (0.999–1.234) 1.074 (0.966–1.193) 1.028 (0.925–1.142)
 N-hexane 1.265 (1.138–1.406)* 1.236 (1.112–1.375)* 1.180 (1.060–1.312)* 1.193 (1.073–1.326)* 1.142 (1.027–1.270)*
 Ethyl-methyl ketone 1.070 (0.962–1.191) 1.104 (0.993–1.229) 1.067 (0.959–1.187) 1.039 (0.934–1.156) 1.030 (0.926–1.146)
 M-xylene 1.117 (1.004–1.242)* 1.128 (1.014–1.255)* 1.106 (0.995–1.231) 1.042 (0.937–1.159) 1.006 (0.905–1.119)
 O-xylene 1.058 (0.951–1.176) 1.042 (0.936–1.159) 1.026 (0.922–1.141) 1.013 (0.911–1.127) 0.980 (0.881–1.089)
 P-xylene 1.101 (0.990–1.224) 1.125 (1.012–1.251)* 1.088 (0.979–1.211) 1.043 (0.938–1.159) 0.992 (0.892–1.103)
 Propene 1.019 (0.914–1.136) 1.017 (0.911–1.135) 1.079 (0.965–1.206) 1.096 (0.979–1.227) 1.027 (0.916–1.15)
 Sesquiterpene 1.130 (1.010–1.264)* 1.125 (1.005–1.258)* 1.122 (1.002–1.256)* 1.066 (0.953–1.192) 1.112 (0.996–1.242)
 Styrene 0.980 (0.871–1.103) 0.939 (0.832–1.059) 0.996 (0.879–1.128) 1.022 (0.899–1.161) 1.033 (0.908–1.176)
 Toluene 1.057 (0.950–1.176) 1.047 (0.941–1.165) 1.050 (0.944–1.168) 1.021 (0.918–1.135) 0.998 (0.898–1.109)
Chronic hypertension
 Benzene 0.890 (0.743–1.065) 0.957 (0.799–1.147) 0.975 (0.811–1.171) 0.996 (0.829–1.197) 1.041 (0.868–1.249)
 1,3-butadiene 1.022 (0.862–1.213) 0.946 (0.800–1.119) 0.972 (0.823–1.149) 0.954 (0.806–1.129) 0.903 (0.763–1.068)
 Ethylbenzene 0.911 (0.760–1.093) 0.917 (0.765–1.099) 0.922 (0.767–1.109) 1.060 (0.883–1.273) 1.070 (0.893–1.282)
 Cyclohexane 0.885 (0.741–1.057) 0.931 (0.777–1.115) 0.919 (0.767–1.102) 0.947 (0.791–1.133) 1.011 (0.847–1.207)
 Methyl-tertiary-butyl ether 0.961 (0.810–1.140) 0.972 (0.820–1.153) 1.024 (0.864–1.213) 0.976 (0.824–1.155) 1.101 (0.929–1.304)
 N-hexane 0.852 (0.714–1.017) 0.908 (0.760–1.085) 0.889 (0.743–1.063) 1.007 (0.844–1.202) 1.036 (0.868–1.237)
 Ethyl-methyl ketone 0.967 (0.808–1.158) 0.853 (0.712–1.023) 0.879 (0.735–1.052) 1.004 (0.842–1.198) 0.973 (0.816–1.160)
 M-xylene 0.905 (0.755–1.084) 0.904 (0.755–1.082) 0.956 (0.797–1.149) 0.986 (0.821–1.183) 1.054 (0.880–1.261)
 O-xylene 0.946 (0.788–1.137) 0.983 (0.819–1.181) 1.032 (0.858–1.241) 1.094 (0.911–1.313) 1.057 (0.878–1.272)
 P-xylene 0.915 (0.765–1.095) 0.942 (0.787–1.127) 0.934 (0.777–1.123) 1.052 (0.876–1.264) 1.092 (0.911–1.308)
 Propene 0.953 (0.800–1.135) 0.919 (0.771–1.096) 0.913 (0.766–1.088) 0.941 (0.789–1.123) 1.038 (0.871–1.237)
 Sesquiterpene 0.891 (0.747–1.062) 0.957 (0.804–1.141) 1.050 (0.881–1.253) 0.968 (0.814–1.152) 0.978 (0.821–1.166)
 Styrene 1.000 (0.845–1.184) 0.989 (0.836–1.169) 0.921 (0.779–1.088) 0.853 (0.722–1.008) 0.892 (0.755–1.053)
 Toluene 1.006 (0.837–1.208) 1.012 (0.843–1.216) 1.037 (0.862–1.248) 1.084 (0.904–1.301) 1.062 (0.884–1.275)
Superimposed preeclampsia
 Benzene 0.778 (0.547–1.105) 0.793 (0.554–1.135) 0.870 (0.606–1.249) 0.928 (0.652–1.321) 0.910 (0.640–1.292)
 1,3-butadiene 0.765 (0.553–1.057) 0.767 (0.551–1.067) 0.809 (0.580–1.129) 0.797 (0.569–1.115) 0.702 (0.504–0.979)*
 Ethylbenzene 0.748 (0.527–1.061) 0.774 (0.544–1.100) 0.793 (0.558–1.128) 0.887 (0.629–1.253) 0.909 (0.646–1.280)
 Cyclohexane 0.774 (0.545–1.099) 0.867 (0.612–1.227) 0.814 (0.571–1.160) 0.886 (0.630–1.248) 0.891 (0.634–1.253)
 Methyl-tertiary-butyl ether 0.960 (0.692–1.333) 1.077 (0.775–1.497) 1.126 (0.809–1.567) 1.285 (0.928–1.779) 1.250 (0.901–1.734)
 N-hexane 0.794 (0.561–1.123) 0.759 (0.535–1.076) 0.731 (0.513–1.043) 0.895 (0.632–1.266) 0.849 (0.603–1.195)
 Ethyl-methyl ketone 0.812 (0.573–1.150) 0.858 (0.608–1.212) 0.847 (0.598–1.200) 0.868 (0.618–1.218) 0.916 (0.657–1.276)
 M-xylene 0.719 (0.507–1.017) 0.775 (0.544–1.105) 0.889 (0.618–1.280) 0.898 (0.634–1.272) 0.854 (0.603–1.208)
 O-xylene 0.716 (0.501–1.024) 0.770 (0.539–1.098) 0.850 (0.590–1.224) 0.992 (0.694–1.417) 0.986 (0.693–1.403)
 P-xylene 0.762 (0.537–1.081) 0.820 (0.578–1.163) 0.871 (0.613–1.238) 0.892 (0.634–1.255) 0.870 (0.618–1.224)
 Propene 0.914 (0.655–1.275) 1.026 (0.732–1.438) 0.903 (0.634–1.287) 0.797 (0.562–1.130) 0.916 (0.652–1.286)
 Sesquiterpene 0.655 (0.467–0.918) 0.727 (0.514–1.027) 0.734 (0.515–1.046) 0.795 (0.563–1.123) 0.926 (0.658–1.304)
 Styrene 0.872 (0.629–1.209) 0.830 (0.596–1.155) 0.764 (0.549–1.062) 0.763 (0.547–1.065) 0.694 (0.498–0.967)*
 Toluene 0.745 (0.520–1.066) 0.738 (0.516–1.055) 0.860 (0.599–1.236) 0.944 (0.661–1.350) 1.013 (0.713–1.440)
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aOrdinal logistic regression with linear function of air pollutant, hypertensive disorder, and their interaction and results clustered within woman estimated the effect of high air toxics exposure (≥75th percentile in parts per billion), adjusted for site, maternal age, race/ethnicity, insurance, smoking during pregnancy, and time of day.

*Interaction term is significant (P < 0.05), suggesting that the response is different than in the normotensive women. Estimates that are statistically significant are given in bold.

Women with gestational hypertension had increased odds of higher blood pressure category with exposure to dust particles (by 0.3%/0.1 units), acenaphthene, acenaphthylene, chrysene, fluoranthene, phenanthrene, pyrene, benzene, methyl-tertiary-butyl ether, n-hexane, xylenes, propene, and toluene (by 18%–27%/high exposure) (Tables 2–4). Additionally, in the linear models, higher systolic or diastolic blood pressure was associated with exposures to elemental carbon, cyclohexane, NOx, anthracene, ethylbenzene, and toluene, and lower diastolic blood pressure was associated with exposure to CO (Supplementary Tables S2–S7).

Preeclamptic women had increased odds of higher blood pressure with exposure to PM10 (by 3%/5 units), elemental carbon (by 1%/0.1 units), nitrate and dust particles (by 0.4%–0.6%/0.1 units), benz[a]anthracene, chrysene, fluorene, phenanthrene, pyrene, benzene, cyclohexane, n-hexane, xylenes, and sesquiterpene (by 12%–23%/high exposure) but lower odds with exposure to sulfate particles, ozone, CO, and indeno[1,2,3-Cd]pyrene (Tables 2–4). In the linear models, NOx, SO2, and benz[a]pyrene were also associated with lower systolic or diastolic blood pressure, and methyl-tertiary-butyl ether was associated with higher diastolic blood pressure (Supplementary Tables S2–S7).

Women with chronic hypertension had increased odds of higher blood pressure category with exposure to CO (by 0.3%–0.4%/10 units), NOx (by 1%–2%/5 units), and SO2 (by 2%–3%/1 unit), whereas opposing effects were observed with exposure to sulfate and dust particles and chrysene (Tables 2–4). Acenaphthene, o-xylene, and toluene exposures were also associated with higher systolic or diastolic blood pressure in the linear models whereas ammonium particles associated with lower diastolic blood pressure (Supplementary Tables S2–S7).

Women with superimposed preeclampsia had increased odds of higher blood pressure with exposure to CO (by 0.6%–0.7%/10 units), ozone (by 8%–11%/5 units), and anthracene (by 49%/high exposure) but lower odds with exposure to SO2, chrysene, fluoranthene, naphthalene, pyrene, sesquiterpene, and styrene (Tables 2–4). In the linear models, benz[a]anthracene, indeno[1,2,3-Cd]pyrene, naphthalene, 1,3-butadiene, xylenes, and toluene were also associated with lower systolic blood pressure, whereas naphthalene, methyl-tertiary-butyl ether, and xylenes increased diastolic blood pressure (Supplementary Tables S2–S7).

Sensitivity analyses

All results were similar after excluding women who took antihypertensive medications (data not shown). Results among normotensive women persisted after restricting data to spontaneous labors, with some loss of precision due to smaller sample sizes. In women with hypertensive disorders, the results were more attenuated, but the association between PM10 and SO2 exposure and higher blood pressure category persisted in women with preeclampsia and chronic hypertension, respectively, and novel association between PM2.5 and higher blood pressure category was seen in women with preeclampsia and superimposed preeclampsia (data not shown).

DISCUSSION

In this large study of women with term deliveries, acute exposure to several air pollutants increased the odds of having higher blood pressure on admission to labor/delivery. Although most associations with blood pressure elevations were seen among normotensive women, we also observed air pollutant effects among women with preeclampsia and gestational hypertension. To our knowledge, this is the first large study evaluating the association between acute air pollution exposure and blood pressure during pregnancy. Pregnancy is associated with changes in the cardiovascular system and blood pressure, but blood pressure measured at term typically is comparable with or exceeds prepregnancy values.3

We found higher blood pressure on admission to labor/delivery among normotensive women after exposure to CO, NOx, SO2, some PM2.5 constituents, and several air toxics. In the general population, acute and recent exposures to PM10, PM2.5, NOx, SO2, and ozone increased blood pressure in most studies,20–24 although in nonsmoking adults, PM10 and ozone exposure decreased systolic BP.23 Previous studies have also consistently shown that long-term air pollution exposure increases blood pressure and/or risk of hypertensive disorders in pregnancy.6–11 Acute or recent PM10 or NOx exposures increased blood pressure during pregnancy in some, but not all, studies.7,9,14 These studies in pregnant women have generally estimated the effect of short-term air pollution (e.g., daily pollution averages), excluded women with chronic hypertension, and lacked power to evaluate specific hypertensive disorders, which may explain why their results are somewhat different from ours.

We found that normotensive women and those with hypertensive disorders of pregnancy often had blood pressure effects in similar direction with air pollution exposure, but the magnitude of effect was often stronger in women with hypertensive disorders. Preeclamptic women seemed susceptible to the effects of particulates because total PM10 and several PM2.5 constituents increased blood pressure, whereas opposing effects were sometimes seen among normotensive women. In women with chronic hypertension and superimposed preeclampsia, air pollution exposure generally was either associated with no effect on blood pressure or lower odds of high blood pressure. Most of our observed negative effects remained after excluding women using antihypertensive medication. Results of previous studies have also been inconsistent regarding air pollution effects on blood pressure among people with chronic hypertension, as in 1 study, air pollution increased blood pressure only in elderly persons taking antihypertensive medication,25 whereas opposing effects were seen in another study.26 The underlying chronic hypertension and associated vascular changes may mask any small effects air pollutants have on blood pressure among women with chronic hypertension and superimposed preeclampsia, leading to seemingly negative or null effects.

We observed several novel associations between high blood pressure on admission to labor/delivery and air toxics, mostly among normotensive women and/or among women with gestational hypertension or preeclampsia. Previously, increased blood pressure was observed after acute exposure to oxygenated polycyclic aromatic hydrocarbons in particulates among elderly people using antihypertensive medication,25 and long-term exposure to some air toxics has been associated with ischemic heart disease and total mortality.27,28 Our results support increases in blood pressure associated with acute air toxics exposure in pregnancy.

Inhaled air pollutants have been found to have systemic effects in the general and pregnant populations, including elevations in inflammatory markers, oxidative stress, plasma viscosity, fibrinogen, and atherosclerosis, and endothelial dysfunction.1,21,29–31 The very acute effects of air pollution are likely mediated by autonomic nervous system imbalance, leading to vasoconstriction.21 Women with preeclampsia have increases in peripheral vascular resistance and may be hyperresponsive to vasoactive factors and even have reversal of the circadian rhythm of blood pressure.5 Additionally, women with hypertensive disorders of pregnancy have higher rates of cardiovascular risk factors, and especially women with preeclampsia have higher inflammation and more endothelial dysfunction,5 which may precipitate and enhance any effects of air pollution on blood pressure. Some of these underlying differences may explain why women with preeclampsia and gestational hypertension at term seemed to experience stronger effects of air pollution than normotensive women.

Our study had several strengths, including large sample size, which allowed us to study women with specific hypertensive disorders of pregnancy. We used established clinical cut points in pregnancy for our blood pressure categories to ensure our findings would be clinically meaningful.5,17,18 The modeled air pollution data allowed us to include women even from areas without air monitors. In addition, the models accounted for atmospheric reactions between pollutants, the effects of weather and season on the pollutant levels, and source pollution, rendering a sophisticated model estimate of pollutant mixtures at a given time. The biggest limitation in our study was the use of clinical blood pressure, measured only once during pregnancy. However, we expect that blood pressure was measured with good practice by experienced staff using validated instruments because these measures are routinely taken and can be used in clinical decision-making concerning labor and delivery. Also, because we used data on blood pressure at labor/delivery admission, these measures may be influenced by the pain and stress associated with labor and uterine contractions. Indeed, only 40% of all normotensive women had normal blood pressure on admission to labor/delivery. We believe that these 1-time measurements at labor/delivery are most comparable with blood pressure measured at emergency department visits, which also are prone to be in the high-normal or hypertensive range.32 One-time hypertensive blood pressure during pregnancy has been shown to increase risk of subsequent hypertension and cardiac events,15 similar to the effects of high blood pressure measured at emergency department visits,32 and even high-normal blood pressures are known to increase risk of chronic hypertension.18 In our study, 0.3% of all hypertensive admission blood pressures among normotensive women could be attributed to small increases in SO2, whereas 6.5% of all hypertensive admissions were attributable to air toxics. Blood pressures this high could trigger a preeclampsia work-up in normotensive women, which has implications on a population level. Among the 4 million US births per year, at least 28,000 hypertensive admissions and/or preeclampsia work-ups can be attributed to air toxics.

In our large study of women with singleton term deliveries, exposure to several air pollutants in the hours before admission was associated with increased odds of normotensive women having high normal or hypertensive blood pressure. We also found that women with gestational hypertension and preeclampsia had higher blood pressure after air pollution exposure, whereas small or no effects were seen among women with chronic hypertension or superimposed preeclampsia. Acute air pollution exposure may trigger a hypertensive response especially in relation to the stresses of labor and delivery, which may trigger a preeclampsia workup in previously normotensive women, lead to more intense follow-up during labor, and even have long-term implications by increasing risk of subsequent hypertension and cardiac events.

SUPPLEMENTARY MATERIAL

Supplementary materials are available at American Journal of Hypertension (http://ajh.oxfordjournals.org).

DISCLOSURE

The authors declared no conflict of interest.

Supplementary Material

Supplementary Data
supp_28_1_58__index.html (1.7KB, html)

ACKNOWLEDGMENTS

This work was supported by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health; including Contract No. HHSN267200603425C (Consortium on Safe Labor) and Contract No. HHSN275200800002I; Task Order No. HHSN27500008 (Air Quality and Reproductive Health). Institutions involved in the consortium include: Baystate Medical Center, Springfield, MA; Cedars-Sinai Medical Center Burnes Allen Research Center, Los Angeles, CA; Christiana Care Health System, Newark, DE; Georgetown University Hospital, MedStar Health, Washington, DC; Indiana University Clarian Health, Indianapolis, IN; Intermountain Healthcare and the University of Utah, Salt Lake City, Utah; Maimonides Medical Center, Brooklyn, NY; MetroHealth Medical Center, Cleveland, OH; Summa Health System, Akron City Hospital, Akron, OH; The EMMES Corporation, Rockville, MD (Data Coordinating Center); University of Illinois at Chicago, Chicago, IL; University of Miami, Miami, FL; and University of Texas Health Science Center at Houston, Houston, Texas. The Texas A&M Supercomputing Facility (http://sc.tamu.edu) and the Texas Advanced Computing Center (http://www.tacc.utexas.edu/) provided computing resources essential for completing the air quality simulations for exposure estimations in this study.

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Associated Data

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Supplementary Materials

Supplementary Data
supp_28_1_58__index.html (1.7KB, html)
supp_hpu077_Am_J_Hypertens_supplemental_table_1.doc (107KB, doc)
supp_hpu077_Am_J_Hypertens_supplemental_table_5.doc (111.5KB, doc)
supp_hpu077_Am_J_Hypertens_supplemental_table_7.doc (123KB, doc)
supp_hpu077_Am_J_Hypertens_supplemental_table_3.doc (126.5KB, doc)
supp_hpu077_Am_J_Hypertens_supplemental_table_4.doc (112.5KB, doc)
supp_hpu077_Am_J_Hypertens_supplemental_table_2.doc (117.5KB, doc)
supp_hpu077_Am_J_Hypertens_supplemental_table_6.doc (124KB, doc)

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