These data are consistent with the hypothesis that exposure to air pollution is adversely associated with measures of HDL.

We examined the cross-sectional relationship between air pollution and both HDL cholesterol and HDL particle number in the MESA Air study (Multi-Ethnic Study of Atherosclerosis Air Pollution). Study participants were 6654 white, black, Hispanic, and Chinese men and women aged 45 to 84 years. We estimated individual residential ambient fine particulate pollution exposure (PM 2.5 ) and black carbon concentrations using a fine-scale likelihood-based spatiotemporal model and cohort-specific monitoring. Exposure periods were averaged to 12 months, 3 months, and 2 weeks prior to examination. HDL cholesterol and HDL particle number were measured in the year 2000 using the cholesterol oxidase method and nuclear magnetic resonance spectroscopy, respectively. We used multivariable linear regression to examine the relationship between air pollution exposure and HDL measures. A 0.7×10 − 6 m − 1 higher exposure to black carbon (a marker of traffic-related pollution) averaged over a 1-year period was significantly associated with a lower HDL cholesterol (−1.68 mg/dL; 95% confidence interval, −2.86 to −0.50) and approached significance with HDL particle number (−0.55 mg/dL; 95% confidence interval, −1.13 to 0.03). In the 3-month averaging time period, a 5 μg/m 3 higher PM 2.5 was associated with lower HDL particle number (−0.64 μmol/L; 95% confidence interval, −1.01 to −0.26), but not HDL cholesterol (−0.05 mg/dL; 95% confidence interval, −0.82 to 0.71).

The relationship between air pollution and cardiovascular disease may be explained by changes in high-density lipoprotein (HDL).

Introduction

High-density lipoprotein (HDL) particles possess numerous potentially cardioprotective qualities.1 HDL particles transport cholesterol from lipid-carrying macrophages and are vital in the maintenance of net cholesterol balance in the arterial wall.1 Despite strong epidemiological evidence that HDL cholesterol (HDL-C) is inversely associated with cardiovascular events, recent clinical trials that raised HDL-C have failed to show benefit.2–5 Recent studies suggest that measurement of HDL particle number (HDL-P) may better reflect the cardioprotective qualities of HDL than HDL-C.6–8

Ambient air pollution is associated with atherosclerosis, heart failure, and cardiovascular death.9–15 Air pollution may affect HDL through inflammation and oxidative stress, promoting changes in HDL structure and function that results in proatherogenic or dysfunctional HDL.16,17 Exposures to both fine and ultrafine particulate matter (PM) have been associated with development of dysfunctional HDL and reduced HDL anti-inflammatory capacity in some (but not all) experimental studies.18–21 The association between ambient air pollution and HDL-C, HDL-P, and HDL particle size has not been well studied.

We examined the relation between long- and short-term concentration of air pollutants—PM 2.5 and black carbon (BC)—and measures of HDL structure in a multiethnic cohort of adults without clinical cardiovascular disease (CVD). We hypothesized that exposure to higher levels of air pollution would be associated not only with lower HDL-C, but also with lower HDL-P, which may better reflect HDL function. In a secondary analysis, we examined associations between short-term PM 2.5 concentrations and HDL. The MESA (Multi-Ethnic Study of Atherosclerosis)—an on-going study of risk factors that predict progression to clinically overt CVD or progression of the subclinical disease in a racially diverse population—provides a unique opportunity to combine highly refined measures of air pollution exposure with multiple measures of HDL, including particle number, size, and concentration, in a multiethnic population.

Materials and Methods

Materials and Methods are available in the online-only Data Supplement.

Results

Of the 6814 MESA participants, 160 participants had missing data on HDL, covariates, and exposure, leaving 6654 participants for analysis. Individually weighted exposure estimates were available for 5330 participants, and BC exposure estimates were available for 6557 participants. The current study population comprised 53% female and 18% with less than a high school education. Participants were 28% Black, 12% Chinese American, 22%, Hispanic and 39% White. Sixteen percent of the study population used lipid-lowering drugs, and 45% participants had hypertension. Predicted individually weighted PM 2.5 concentrations in the year 2000 ranged from 4.8 to 21.6 μg/m3, with an interquartile range of 4.1 μg/m3. BC concentrations ranged from 0.28 to 4.77 10−6 m−1, with an interquartile range of 0.7×10−6 m−1. Mean (SD) HDL-P in our study population was 34.0 (6.6) μmol/L. Mean (SD) HDL-C in our study population was 50.8 (15.5) mg/dL (Table 1).

Table 1. Characteristics of MESA Participants in Study All Participants (N=6814) Participants With Complete Data (N=6654) Participants With Complete Individually Weighted Estimates (N=5330) Age, mean±SD, y 62.2±10.2 62.2±10.2 61.4±10.0 Male, N (%) 3213 (47.2) 3134 (47.1) 2527 (47.4) Race/ethnicity, N (%) Non-Hispanic white 2622 (38.5) 2567 (38.6) 2118 (39.7) Black 1893 (27.8) 1831 (27.5) 1427 (26.8) Hispanic 1496 (21.9) 1466 (22.0) 1132 (21.2) Chinese 803 (11.8) 790 (11.9) 653 (12.2) Smoker, N (%) Never 3417 (50.3) 3344 (50.3) 2716 (51.1) Former 2487 (36.6) 2448 (36.8) 1957 (36.8) Current 887 (13.1) 862 (13.0) 644 (12.1) Current alcohol users, N (%) 3749 (68.5) 3677 (68.6) 3044 (70.6) Physical activity, mean±SD, h/d 12.6±5.9 12.6±5.9 12.8±5.8 Body mass index, mean±SD, kg/m2 28.3±5.5 28.3±5.5 28.3±5.4 Diabetes mellitus, N (%) Normal 4992 (73.5) 4895 (73.6) 3993 (75.1) Impaired fasting glucose 939 (13.8) 922 (13.9) 725 (13.6) Nontreated DM 179 (2.6) 177 (2.7) 120 (2.3) Treated DM 680 (10.0) 660 (9.9) 478 (9.0) Hypertension, N (%) 3058 (44.9) 2984 (44.9) 2289 (43.0) Any lipid-lowering medication, N (%) 1100 (16.1) 1080 (16.2) 873 (16.4) Postmenopausal, N (%) 2949 (82.0) 2884 (82.0) 2259 (80.7) Systolic blood pressure, mean±SD, mm Hg 126.6±21.5 126.6±21.5 125.4±20.7 Diastolic blood pressure, mean±SD, mm Hg 72.0±10.3 72.0±10.3 71.8±10.2 Homeostatic model assessment of insulin resistance (HOMA-IR), mean (SD), mg/dL 2.7±6.4 2.7±6.5 2.6±6.5 C-reactive protein, mean±SD, mg/L 3.8±5.9 3.8±5.8 3.6±5.4 Triglycerides, mean±SD, mg/dL 131.6±88.8 132.0±89.4 131.0±87.4 Low-density lipoprotein, mean±SD, mg/dL 117.2±31.5 117.2±31.4 117.1±31.1 Total cholesterol, mean±SD, mg/dL 194.2±35.7 194.3±35.7 194.1±35.4 HDL cholesterol, mean±SD, mg/dL 51.0±14.8 51.0±14.8 51.1±14.8 HDL particle number, mean±SD, μmol/L 34.0±6.6 34.1±6.6 34.2±6.7 PM 2.5 individual year 2000, mean±SD, μg/m3 10.9±3.3 10.9±3.3 10.9±3.3 PM 2.5 outdoor year 2000, mean±SD, μg/m3 16.7±2.9 16.7±2.9 16.6±2.8 Black carbon year 2000, mean±SD, 10–6/m-1 0.9±0.5 0.9±0.5 0.9±0.5

HDL Cholesterol

We found a nonsignificant association between higher PM 2.5 concentrations and lower HDL-C concentrations (Table 2). In the 2-week averaging period, adjusting for age, sex, race/ethnicity, and site only, we observed a significant −0.86 mg/dL (95% confidence interval [CI], −1.38 to −0.34) difference in HDL-C for a 5 μg/m3 higher PM 2.5 ; however, this association attenuated (and became nonsignificant) after adjustment for other covariates. We found no significant association between short-term (0–5 days prior) PM 2.5 exposure or outdoor PM 2.5 exposure and HDL-C (Table 3).

Table 2. Associations Between Long and Medium-Term Air Pollutants and HDL–MESA Air Individually Weighted PM 2.5 (5 μg/m3) Outdoor PM 2.5 (5 μg/m3) Black Carbon (0.7×10−6/m−1) Year 2000 average HDL-C, mg/dL Minimally adjusted model −0.13 (−1.24 to 0.98) 0.85 (−0.69 to 2.40) −1.40 (−2.58 to −0.22) Final model −0.50 (−1.61 to 0.61) 0.86 (−0.67 to 2.38) −1.68 (−2.86 to −0.50) HDL-P, μmol/L Minimally adjusted model −0.15 (−0.65 to 0.34) 0.33 (−0.35 to 1.02) −0.47 (−1.00 to 0.05) Final model −0.21 (−0.75 to 0.33) 0.42 (−0.32 to 1.17) −0.55 (−1.13 to 0.03) Three-month average* HDL-C, mg/dL Minimally adjusted model −0.002 (−0.75 to 0.74) 0.88 (0.27 to 1.48) Final model −0.05 (−0.82 to 0.71) 0.47 (−0.17 to 1.10) HDL-P, μmol/L Minimally adjusted model −0.64 (−0.97 to −0.31) −0.28 (−0.55 to −0.01) Final model −0.64 (−1.02 to −0.26) −0.27 (−0.58 to 0.04) Two-week average* HDL-C, mg/dL Minimally adjusted model −0.86 (−1.38 to −0.34) −0.01 (−0.38 to 0.37) Final model −0.39 (−0.97 to 0.18) 0.14 (−0.26 to 0.55) HDL-P, μmol/L Minimally adjusted model −0.35 (−0.58 to −0.12) −0.13 (−0.29 to 0.04) Final model −0.29 (−0.57 to −0.01) −0.07 (−0.27 to 0.12)

Table 3. Associations Between Measures of HDL and Short-Term Exposure to Fine Particulate Air Pollution on the Day of Blood Collection and the Days Prior Model Change in HDL per 5 μg/m3 Higher Average PM 2.5 Prior to Blood Draw Day of Blood Draw 1 Day Prior 3 Days Prior 5 Days Prior HDL-C, mg/dL Minimally adjusted model −0.14 (−0.37 to 0.10) −0.06 (−0.28 to 0.16) −0.06 (−0.34 to 0.23) −0.20 (−0.54 to 0.14) Final model 0.02 (−0.21 to 0.26) 0.05 (−0.17 to 0.28) 0.01 (−0.28 to 0.30) −0.06 (−0.42 to 0.29) HDL-P (μmol/L) Minimally adjusted Model −0.04 (−0.15 to 0.06) −0.06 (−0.15 to 0.04) −0.11 (−0.23 to 0.02) −0.17 (−0.32 to −0.02) Final model −0.06 (−0.17 to 0.06) −0.06 (−0.17 to 0.04) −0.13 (−0.27 to 0.01) −0.21 (−0.38 to −0.04)

We found a significant association between higher concentrations of BC and lower HDL-C levels. A 0.7×10−6 m−1 higher BC at the 1-year averaged time period was associated with a −1.68 mg/dL (95% CI, −2.86 to −0.50; P=0.001) lower HDL-C when adjusted for covariates in our final model (Table 2). Sensitivity analyses additionally adjusting for HDL-P, pack-years smoked, income, niacin use, short-term pollutant concentrations, both annual averaged PM 2.5 and BC simultaneously, and models with random slopes and intercepts for site did not meaningfully change the results.

HDL Particle Number

We found a significant inverse association between medium-term (3-month and 2-week) PM 2.5 concentrations and HDL-P, but not in the 1-year period, although the association in that period was of similar magnitude and direction (Table 2). A 5-μg/m3 higher 3-month average PM 2.5 concentration was associated with a −0.64 μmol/L (95% CI, −1.02 to −0.26) lower HDL-P, and a 5-μg/m3 higher 2-week average PM 2.5 was associated with a −0.29 μmol/L (95% CI, −0.57 to −0.01) lower HDL-P in multivariate-adjusted models (Table 2). In the short-term PM 2.5 analysis, we found a significant inverse association between higher PM 2.5 in the 5 days before blood draw and HDL-P (−0.21 μmol/L per 5 μg/m3; 95% CI, −0.38 to −0.04; Table 3). Averaging periods that included fewer days before the blood draw had no association with HDL-P.

Adjustment for HDL-C did not significantly change the association between PM 2.5 and HDL-P in the 3-month averaging time period, although the association in the 2-week time was attenuated and not significant (Table I in the online-only Data Supplement). We found no significant associations between outdoor PM 2.5 and HDL-P in fully adjusted models. Sensitivity analyses additionally adjusting for HDL-C, pack-years smoked, income, niacin use, short-term pollutant concentrations, models adjusting for both pollutants simultaneously, and models with random slopes and intercepts for site did not meaningfully change the results. The findings related to the 3-month exposure period were not affected, in direction or significance, by adjusting for the 3-day average PM 2.5 concentration. Adjusted for 3-day average PM 2.5 , a 5 μg/m3 higher PM 2.5 over the 3-month time period was associated with a −0.50 (95% CI, −0.92 to −0.09) change in HDL-P.

We did not find a significant association between BC and HDL-P in the 1-year averaging period, although it borderline and in the same direction as previous associations. A 0.7×10−6 m−1 higher BC exposure was associated with −0.55 μmol/L (95% CI, −1.13 to 0.03) lower HDL-P. This association did not change with adjustment for mean HDL particle size, HDL-C, pack-years smoked, income, and niacin use. Models using random intercepts and slopes to control for site also showed no change.

We found a significant interaction between sex and the association between BC and HDL-C (P for interaction <0.001), with the association stronger in women for BC and HDL-C (−2.63 mg/dL; 95% CI, −4.46 to −0.81) than in men (−0.65 mg/dL; 95% CI, −2.14 to 0.84). We observed a stronger relationship in women for 3-month exposure to PM 2.5 and HDL-P (−0.71 μmol/L; 95% CI, −1.33 to −0.09 for women compared with −0.48 μmol/L; 95% CI, −0.93 to −0.02 for men [P for interaction =0.02]). The same phenomenon was observed for 2-week exposure to PM 2.5 and HDL-P (−0.42 μmol/L; 95% CI, −0.89 to 0.05 for women compared with −0.15 μmol/L; 95% CI, −0.49 to 0.18 for men [P for interaction =0.03]). While the interaction was not significant, we observed that among those taking lipid lowering medications, 1-year averaged BC was associated with a −2.60 mg/dL (95% CI, −5.18 to −0.02) change in HDL-C compared with a −1.38 mg/dL (95% CI, −2.70 to −0.05) change in HDL-C among those not taking lipid-lowering medication. Among those with hypertension 1-one year averaged BC was associated with a −1.94 mg/dL (95% CI, −3.76 to −0.13) change in HDL-C compared with a −1.48 mg/dL (95% CI, −3.04 to 0.09) change in HDL-C among those without hypertension. Among those taking lipid-lowering medications, 3-month averaged PM2.5 was associated with a −1.36 mg/dL (95% CI, −2.26 to −0.46) change in HDL-P, compared with a −0.47 mg/dL (95% CI, −0.89 to −0.06) change in HDL-P among those not taking lipid-lowering medication. Among those with hypertension, 3-month averaged PM2.5 was associated with a −0.75 mg/dL (95% CI, −1.35 to −0.15) change in HDL-P compared with a −0.56 mg/dL (95% CI, −1.04 to −0.07) change in HDL-P among those without hypertension. We found no other significant interactions.

Discussion

In a large, multiethnic cohort study of men and women free of prevalent clinical CVD, we found that higher concentrations of PM 2.5 over a 3-month time period was associated with lower HDL-P, and higher annual concentrations of BC were associated with lower HDL-C. Lower HDL particle numbers have been associated with increasing carotid intima-medial thickness and cardiovascular events in previous studies, and lower HDL-C is a traditional risk factor for CVD.1,6,8,22

In MESA participants, a 5 μg/m3 higher concentration of PM 2.5 over a 3-month time period was associated with a −0.64 μmol/L lower HDL-P. This observed lower HDL-P is comparable to other traditional risk factors such as those observed in smoking cessation studies (1.0 μmol/L change).23 Lower HDL-P levels in MESA participants have been independently associated with carotid atherosclerosis and coronary heart disease. A 0.7×10−6 m−1 higher exposure to BC over a 1-year period was also associated with a −1.68 mg/dL lower concentration of HDL-C. This lower HDL-C can be compared with the effect of quitting smoking (2.4 mg/dL change) on HDL-C in smoking cessation programs.23 These associations persisted after control for smoking and other risk factors for HDL. Effect sizes for PM 2.5 and HDL-P increased from day of blood draw to 5-day, 2-week, and finally to 3-month averaged time periods, which may indicate a potential cumulative exposure effect. However, we did not see an association between 1-year averaged PM 2.5 and HDL-P, suggesting that potential effects of air pollution on HDL may be more short term or medium term.

This is the first large observational study to suggest an association between air pollution exposure and HDL particle number. Our results build on previous work suggesting an association between air pollution and HDL through the use of additional properties of HDL, which may be potentially more clinically relevant, and examining them in a multiethnic cohort with excellent measurement of covariates and cutting edge assessment of air pollution exposure. This study contributes to the hypothesis that air pollution may act through HDL to contribute to CVD at comparably low levels found in developed countries. In examining different pollutant averaging times, our study also adds information suggesting that air pollution may be associated with changes in HDL in both the short term and the medium term and that both time periods may be relevant for examining the effect of air pollution on CVD risk factors.

Two cohort studies, conducted by Chuang et al,24 in Taiwan have previously investigated the relationship between air pollution and HDL in humans. In a population-based survey, they reported a decrease in HDL-C per interquartile range increase in PM 10 over a 1-day averaging period before blood draw.24 In a separate cross-sectional analysis, they found no association between a 1 interquartile range increase in 1-year averaged air pollution exposure before blood draw and HDL-C in a cohort of 1023 subjects aged 54 to 90 years living in Taiwan.25 These studies relied on central-site monitoring data and had limited data on likely confounding variables, such as socioeconomic status, smoking, physical activity, and use of lipid-lowering medications, which are associated with both air pollution and HDL, and may explain why our study found differing results. Our results are consistent with a prior occupational study of PM 2.5 and HDL-C in a repeated measures panel study of welders.26 Those exposed to high PM 2.5 during welding experienced an acute decrease of −2.6 mg/dL (95% CI, −5.3 to −0.0) in circulating HDL-C levels 18 hours after exposure compared with their baseline levels.26 These magnitude of effect sizes were higher than those observed in our study; however, welders were exposed to much higher concentrations of pollutants.

Characteristics of HDL beyond total concentration are likely important in the protective effects of the lipoprotein. In our study, we focused on particle number as an alternate characteristic, though there is reason to think that functional characteristics—not assessed here—are also important and may be affected by air pollutant or other exposures, such as tobacco smoke.18–20,27–31 For example, in Yin et al,19 samples of HDL from mice exposed to fine and ultrafine PM were found to have significantly reduced HDL anti-inflammatory properties compared with those of unexposed mice, suggesting that PM exposure may reduce the ability of HDL to protect against atherosclerosis.19 Experimental study of HDL function and structure is needed to confirm and further characterize the effect of air pollution on HDL.

As a whole, these studies are consistent with the hypothesis that exposure to air pollution increases risk of CVD. Our findings support hypotheses that HDL may play a role in the biological pathway explaining the association between air pollution and CVD.17 PM has been shown to induce creation of dysfunction HDL, which is associated with reduced protection against atherosclerosis through inability to participate in reverse cholesterol transport and antioxidation.17,19,32

Both the relationship between PM 2.5 and HDL-P and the relationship between BC and HDL-C were modified by sex. In both cases, the association between air pollution and HDL was stronger in women, although the association in men was still negative. Women typically have higher levels of both HDL-P and HDL-C than men, which has been attributed to higher estrogen production in women.33 Some research has suggested that air pollutants may induce estrogen-disrupting effects, acting as a potential xenoestrogen involved in the generation of reactive oxygen species and induction of oxidative stress.34,35 Although many of the women in our study were postmenopausal, air pollution–related disruption of the HDL-raising effects of estrogen may explain the stronger results observed in women. Our study found no significant evidence of effect modification by age, race/ethnicity, diabetes mellitus, smoking, obesity, or site. Our sensitivity analyses examining adjustment for other aspects of HDL did not strongly change the conclusions of our study. The positive association between 3-month average of PM 2.5 and HDL-C when controlling for HDL-P is difficult to interpret; however, it may be explained by a reduction in the number of small, cholesterol-depleted HDL particles, leaving the average amount of cholesterol in HDL particles higher on a per-particle basis. Smaller HDL particles may play a more important role in cholesterol efflux than larger particles, so a reduction in the number of smaller HDL particles supports the hypothesis that the relationship between air pollution and CVD could be mediated through change in cholesterol efflux.36 We also observed associations between HDL and individually weighted PM 2.5 —which takes into account participant’s time spent indoors—but not outdoor PM 2.5 , which simply estimates PM 2.5 outside a participant’s home. Because many study subjects spend the majority of their time indoors, this estimate will contain some error because only a fraction of outdoor air pollution penetrates into homes. Subjects also spend time away from their homes and may also spend time in traffic on roadways, which can be significant source of air pollution exposure itself, further exposing them to air pollution that is generally not taken into account by outdoor air pollution exposure estimates.

This study has several strengths as a large, population-based, multiethnic cohort able to examine the relationship between air pollution and advanced measures of HDL. The MESA study features measurement of numerous covariates, with multiple levels of quality control.37 Great care was taken in producing high-quality air pollution estimates, with cohort-specific monitoring and modeling and with special attention paid to minimizing measurement error in the estimates.38,39 This is the first study to examine air pollution and measures of HDL particle number and size in a large cohort setting. However, while our exposure models are more accurate and less susceptible to measurement error than distance to monitor or nearest roadway analyses, we cannot rule out the limitation of measurement error in our models. The measurement error in air pollution estimates is likely to be independent of HDL measurements, so we would generally expect this error to be nondifferential and bias estimates toward the null, representing an underestimation of the true measure of effect. Air pollution is a complex mixture of particles and gases, and it is possible that our estimates may be driven by a different, highly correlated pollutant that is unmeasured, or an interaction between the pollutants, rather than PM 2.5 or BC per se. Further study of multipollutant models and mixtures will be needed to confirm these results. Another limitation of our study is its cross-sectional design. HDL-P was only been measured at one point in time, and a snapshot analysis of air pollution and HDL cannot provide valid inference on the effect of air pollution on HDL over time. Although estimates of air pollution represent participants’ exposure in the time period before HDL was measured, associations from this study should be interpreted with caution. Finally, while covariates were measured carefully during exams, we cannot rule out the possibility that residual confounding exists because of potentially important covariates being unmeasured or measured with error.

In summary, we found evidence that exposure to PM 2.5 and BC were associated with changes in several measures of HDL, and short-term exposure was associated with lower HDL-P in our study of a multiethnic population free of CVD.

Nonstandard Abbreviations and Acronyms BC black carbon CVD cardiovascular disease HDL high-density lipoprotein HDL-C high-density lipoprotein cholesterol HDL-P high-density lipoprotein cholesterol particle matter MESA Multi-Ethnic Study of Atherosclerosis PM particulate matter

Acknowledgments

The views expressed in this document are solely those of the authors and the US Environmental Protection Agency (EPA) does not endorse any products or commercial services mentioned in this publication. The authors thank other investigators, staff, and participants of the MESA study (Multi-Ethnic Study of Atherosclerosis) for their valuable contributions. A full list of participating MESA investigators and institutions can be found at http://www.mesa-nhlbi.org.

Sources of Funding This research was supported by National Heart, Lung, and Blood Institute contracts N01-HC-95159 through N01-HC-95169, R01-HL-077612, and HL-075476, National Center for Research Resources grants UL1-TR-000040 and UL1-TR-00107, National Institute for Environmental Health Sciences grants F31ES025096, K24ES013195, P50ES015915, and P30ES07033, and an unrestricted grant from LipoScience, Inc. This publication was developed under STAR research assistance agreements RD831697 (MESA Air [Multi-Ethnic Study of Atherosclerosis Air Pollution]) and R834796 by the US Environmental Protection Agency (EPA). It has not been formally reviewed by the EPA.

Disclosures S. Mora reports nonfinancial support from LipoScience (now LabCorp) and Quest Diagnostics during the conduct of the study; grants from Atherotech Diagnostics; and modest consulting fees from Lilly, Pfizer, and Cerenis Therapeutics; all are outside the submitted work. The other authors report no conflicts.

Footnotes