Abstract
Background and Purpose─
Acute exposure to particulate matter with aerodynamic diameter <2.5 μm (PM2.5) is associated with acute cardiovascular and cerebrovascular mortality. The aim of this study was to evaluate these associations with specific causes of cardiovascular and cerebrovascular mortality in Mexico City.
Methods─
We obtained daily mortality records for Mexico City from 2004 to 2013 for cardiovascular and cerebrovascular causes in people ≥25 and ≥65 years old. Exposure to PM2.5 was assessed with daily estimates from a new hybrid spatiotemporal model using satellite measurements of Aerosol Optical Depth (AOD-PM2.5) and compared to ground level PM2.5 measurements with missing data estimated with Generalized Additive Models (GAM-PM2.5). We fitted Poisson regression models with distributed lags for all mortality outcomes.
Results─
An increase of 10 µg/m3 in AOD-PM2.5 was associated with increased cardiovascular, (1.22%; 95% CI:0.17─2.28) and cerebrovascular mortality (3.43%; 95% CI:0.10─6.28) for lag days 0 to 1 (lag0–1). Stronger effects were identified for hemorrhagic stroke and people ≥65 years. Associations were slightly smaller using GAM-PM2.5
Conclusions─
These results support the evidence that acute exposure to PM2.5 is associated with increased risk of specific cardiovascular and cerebrovascular mortality causes.
Keywords: particulate matter, ischemic stroke, hemorrhagic stroke, cardiovascular mortality
Introduction
Acute exposure to airborne particles with aerodynamic diameter ≤2.5 μm (PM2.5) can trigger cardiovascular and cerebrovascular mortality.1 In developing countries, the region with the highest burden of stroke,2 such evidence is limited3 and possibly related to lack of ground level monitoring of PM2.5.4
Mexico City, once considered the most polluted in the world, has improved its air quality because of various programs prioritizing public health.5 However, it continues to be among the most polluted cities in Latin America6 and over the past decades it has recorded an increasing rate of cardiovascular and cerebrovascular diseases, which are among the five leading causes of death.7
Despite the large body of scientific evidence about adverse health effects of particulate matter in Mexico City,8 epidemiologic research about cardiovascular and cerebrovascular mortality associated with PM2.5 exposure remains limited. We therefore evaluated acute PM2.5 exposure associated with specific causes of cardiovascular and cerebrovascular mortality.
Methods
The data that support the findings of this study are available from the corresponding author upon reasonable request. The present manuscript also adheres to the American Heart Association Journals’ implementation of the Transparency and Openness Promotion guidelines.
Mortality data and exposure assessment
We used an ecological design with public mortality records for Mexico City from 2004 to 2013. Our research was ruled exempt from human subjects review by the ethics board of the National Institute of Public Health of Mexico. Detailed methods are provided in the online-only Data Supplement. Deaths in people ≥25 and ≥65 years old, classified according to the tenth version of the International Classification of Diseases (ICD-10 codes), were aggregated to obtain daily counts for specific mortality causes. In order to improve the quality of ischemic heart disease mortality (IHM) data, we applied redistribution of misclassified cause of death by using the proportions estimated by Naghavi et al.9 (Table I in the online-only Data Supplement). Daily citywide exposure to PM2.5 was assessed with estimates from a new hybrid spatiotemporal model using satellite measurements of Aerosol Optical Depth (AOD-PM2.5) developed by Just et al.10 We also calculated daily PM2.5 averages from three monitoring stations of the Mexico City Atmospheric Monitoring System. On days with missing data, PM2.5 concentration was estimated with Generalized Additive Models (GAM-PM2.5), with methods described in the online-only Data Supplement.
Statistical analysis
Associations were estimated using Generalized Linear Models (GLM) with Poisson regressions and distributed lags. Our base models included dummy variables for season of year, day of the week, penalized splines to address long-term trends in mortality and a natural cubic spline of apparent temperature (AT). We alternatively included linear terms of NO2 and SO2 to investigate potential confounding effects of co-pollutants. Deviations from linearity in the concentration-response functions between PM2.5 and cardiovascular and cerebrovascular mortality featured comparison between models using linear PM2.5 and log-transformed PM2.5.
Results
The mean±SD for all deaths due to cardiovascular and cerebrovascular causes per day was 39±8.2. IHM accounted for the highest proportion of all daily cardiovascular deaths (55%). Daily average counts for deaths due to ischemic and hemorrhagic stroke were 1.1±1.1 and 2.7±1.6, respectively. Daily averages for AOD-PM2.5 and GAM-PM2.5 were 24.4 µg/m3±8.2 and 25.9 µg/m3±10.3, respectively (Table II in the online-only Data Supplement describes mortality and environmental characteristics for the study period).
Table 1 shows results on same day exposure (lag0), cumulative effects over two (lag0–1) and six days (lag0–6) for AOD-PM2.5 and GAM-PM2.5. Significant mortality increments were observed for all cardiovascular and cerebrovascular mortality causes using AOD-PM2.5 (lag0–1). No deviations from linearity were observed in the concentration-response functions in the associations assessed (exposure range of 3─99.5 µg/m3). Slightly lower risks were observed using GAM-PM2.5 compared to AOD-PM2.5, but no significant differences in health effects parameters and standard errors were identified (Table IV in the online-only Data Supplement).
Table 1.
Percent increase and 95% confidence intervals (CIs) in mortality associated with 10-μg/m3 increase in AOD-PM2.5 and GAM- PM2.5
| AOD-PM2.5 | GAM- PM2.5 | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Overall | Base model | Fully adjusted | Base model | Fully adjusted | |||||
| % Change (95% CI) | % Change (95% CI) | % Change (95% CI) | % Change (95% CI) | ||||||
|
Cardiovascular ≥25
years old |
Lag0 | 1.32 | (0.50─2.15) | 1.02 | (0.04─2.02) | 1.04 | (0.38─1.70) | 0.76 | (−0.02─1.55) |
| Lag0–1 | 1.50 | (0.64─2.38) | 1.22 | (0.1─2.28) | 1.22 | (0.52─1.92) | 0.94 | (0.11─1.79) | |
| Lag0–6 | 1.24 | (0.25─2.24) | 0.99 | (−0.10─2.10) | 0.77 | (−0.09─1.64) | 0.59 | (−0.34─1.53) | |
|
Cerebrovascular ≥25
years old |
Lag0 | 2.92 | (0.36─5.55) | 3.16 | (0.13─6.27) | 2.05 | (0.04─4.11) | 2.21 | (−0.18─4.66) |
| Lag0–1 | 3.15 | (0.37─6.00) | 3.43 | (0.10─6.28) | 2.48 | (0.27─4.75) | 2.68 | (0.01─5.42) | |
| Lag0–6 | 2.54 | (−0.60─5.78) | 2.76 | (−0.68─6.33) | 2.07 | (−0.59─4.80) | 2.22 | (−0.69─5.22) | |
| Ischemic heart disease | Lag0 | 0.61 | (−0.47─1.70) | 0.51 | (−0.77─1.82) | 0.70 | (−0.16─1.57) | 0.66 | (−0.36─1.70) |
| Lag0–1 | 1.10 | (−0.07─2.29) | 1.00 | (−0.43─2.44) | 1.03 | (0.09─1.98) | 1.00 | (−0.14─2.14) | |
| Lag0–6 | 0.75 | (−0.55─2.07) | 0.67 | (−0.77─2.13) | 0.47 | (−0.65─1.61) | 0.44 | (−0.79─1.70) | |
|
Ischemic heart disease
(improved by potential misclassification in death cause) |
Lag0 | 0.89 | (−0.19─1.97) | 0.86 | (−0.41─2.14) | 0.77 | (−0.09─1.63) | 0.86 | (−0.16─1.89) |
| Lag0–1 | 1.37 | (0.21─2.54) | 1.32 | (−0.07─2.72) | 1.17 | (0.24─2.10) | 1.20 | (0.09─2.33) | |
| Lag0–6 | 0.78 | (−0.49─2.06) | 0.81 | (−0.60─2.23) | 0.41 | (−0.69─1.51) | 0.53 | (−0.69─1.75) | |
| Ischemic Stroke | Lag0 | -0.86 | (−5.64─4.15) | -0.60 | (−6.60─5.78) | -0.75 | (−4.45─3.09) | 1.01 | (−3.43─5.66) |
| Lag0–1 | 0.90 | (−4.19─6.25) | 1.12 | (−5.45─8.14) | 0.33 | (−3.70─4.53) | 2.28 | (−2.59─7.39) | |
| Lag0–6 | 5.68 | (−0.40─12.13) | 5.82 | (−1.27─13.41) | 4.87 | (−0.27─10.28) | 6.49 | (0.77─12.54) | |
| Hemorrhagic Stroke | Lag0 | 3.80 | (0.74─6.94) | 4.01 | (0.37─7.77) | 2.72 | (0.33─5.17) | 2.84 | (−0.01─5.77) |
| Lag0–1 | 3.16 | (−0.08─6.50) | 3.36 | (−0.58─7.46) | 2.72 | (0.16─5.35) | 2.86 | (−0.25─6.07) | |
| Lag0–6 | 0.25 | (−3.38─4.02) | 0.33 | (−3.62─4.45) | -0.15 | (−3.19─2.98) | -0.08 | (−3.38─3.32) | |
|
Cardiovascular ≥65 years old |
Lag0 | 1.65 | (0.65─2.66) | 0.91 | (−0.27─2.10) | 1.20 | (0.41─1.99) | 0.53 | (−0.41─1.48) |
| Lag0–1 | 1.86 | (0.80─2.92) | 1.05 | (−0.20─2.32) | 1.41 | (0.57─2.25) | 0.67 | (−0.33─1.67) | |
| Lag0–6 | 2.92 | (1.72─4.13) | 2.29 | (0.98─3.63) | 1.84 | (0.83─2.86) | 1.22 | (0.11─2.34) | |
|
Cerebrovascular ≥65
years old |
Lag0 | 2.59 | (−0.52─5.81) | 2.66 | (−1.02─6.47) | 1.82 | (−0.60─4.31) | 1.91 | (−0.98─4.89) |
| Lag0–1 | 4.01 | (0.54─7.60) | 4.24 | (0.12─8.52) | 3.19 | (0.44─6.01) | 3.37 | (0.09─6.76) | |
| Lag0–6 | 4.54 | (0.56─8.68) | 4.70 | (0.39─9.19) | 3.87 | (0.49─7.37) | 3.97 | (0.32─7.75) | |
Base model: adjusted by Apparent Temperature, day of the week, season of year, holidays, long-term trends
Fully adjusted: includes linear terms of SO2 and NO2 stratified at the mean
Discussion
This is the first study showing that acute exposure to PM2.5 is associated with specific cardiovascular and cerebrovascular mortality causes in Mexico City, the most populated city in North America. We found results consistent with previous studies with daily increments in cardiovascular mortality of 1%─2% for every 10-μg/m3 increase in PM2.5 (lag0–1).11 For cerebrovascular mortality, our findings seemed larger than the 1.4% summary (95% CI:0.9─1.9%) reported for cities in Europe, Asia and North America (lag0–1).12 Also, our results for people ≥65 years old are consistent with the conclusions from a meta-analysis, pointing out strong evidence of higher mortality risks in older populations associated with acute exposure to particulate matter than in younger populations.13
Most studies have presented combined results for stroke types associated with acute exposure to PM2.5; with weaker evidence for associations with different types of stroke. We observed greater effects for hemorrhagic stroke than for ischemic stroke. Possible explanations are: higher frequency of hemorrhagic stroke observed in Mexico City and distribution of cofactors making its inhabitants more liable to suffer hemorrhagic stroke.
Limitations in our investigation include citywide exposure assessment to PM2.5. We may have failed to capture the spatio-temporal PM2.5 variability within Mexico City possibly biasing point estimates toward the null (Berkson type error).14 Also, the association between exposure to PM2.5 and onset of acute cardiovascular events might be subject to substantial underestimation related to exposure misclassification. We used date of death instead of time of symptom onset to assign exposure to PM2.5
Time-series studies in air pollution epidemiology generally rely on correct classification of death causes from government records. Even though we carried out proportional redistribution of potentially misclassified death causes to IHM in order to reduce measurement error, there are other mortality outcomes subject to correction that were not addressed in our research.
PM2.5 toxicity depends on different factors besides concentration levels. Further research assessing the spatial distribution and composition of PM2.5 within Mexico City is needed to further refine our findings.
Supplementary Material
Acknowledgments
Sources of Funding
I.G.A. was supported by the Fulbright and CONACyT student-grants. A.C.J. was supported by NIH grants R00ES023450 and P30ES023515.
Footnotes
Disclosures
None
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