Short-Term Exposure to Air Pollutants Ups MI Risk: Meta-Analysis

February 15, 2012

February 15, 2012 (Paris, France) — The risk of MI went up significantly with rising concentrations of most of major air pollutants included in a new meta-analysis of studies in which pollution-exposure times were measured for no more than a week [1]. The individual culprits included carbon monoxide and sulfur dioxide but not ozone, which appeared unrelated to MI risk.

"Compared with traditional cardiovascular risk factors such as tobacco use, hypertension, or diabetes, the relative risks of myocardial infarction [observed in the meta-analysis] are rather low," observed lead author Dr Hazrije Mustafic (University Paris Descartes, France) for heartwire .

"But the difference is in the level of exposure. For example, in France, 20% of the adult population smokes, but the entire population is exposed to air pollution." So with the pollutants, she said, "the risk is low at the individual level, but if you consider the entire population, the impact is not negligible."

Moreover, observed Mustafic, "the majority of studies included in our meta-analysis were conducted in countries where concentrations of air pollutants are less than [World Health Organization] WHO recommendations. Yet even under those conditions, we found significant associations."

The meta-analysis, which is published in the February 15, 2012 Journal of the American Medical Association, examined studies of MI risk with exposure to at least one air pollutant for up to seven days. The number of studies per pollutant ranged from 13 for particulate matter (PM) of <2.5 µm in diameter (PM2.5) to 21 for nitrogen dioxide.

All of the individual air pollutants in the analysis except ozone showed a significant bump in MI relative risk for every increment in concentration. The increments were 10 µg/m3 for all pollutants except carbon monoxide, for which the increment was 1 mg/m3.

Relative Risk (95% CI) for Acute MI Following Up to One Week of Exposure to Pollutants, Per Increment in Concentration*

Pollutant (increment) RR (95% CI) p
Carbon monoxide 1.048 (1.026–1.070) <0.001
Nitrogen dioxide 1.011 (1.006–1.016) <0.001
Sulfur dioxide 1.010 (1.003–1.017) 0.007
PM10 1.006 (1.002–1.009) 0.002
PM2.5 1.025 (1.015–1.036) 0.004
Ozone 1.003 (0.997–1.010) 0.36

*10 µg/m3 for all pollutants except carbon monoxide (1 mg/m3)

PM10=particulate matter <10 µm in diameter

PM2.5=particulate matter <2.5 µm in diameter

Mustafic and her coauthors point to several potential mechanisms that might trigger an MI with short-term exposure to air pollution. They include induction of systemic inflammation, dysregulation of the cardiac autonomic system, increased blood viscosity, vasoconstriction or vasospasm, and promotion of arrhythmias. Experimental evidence suggests a number of mechanisms are probably involved, they write.

Ozone didn't emerge from the meta-analysis as a significant influence on MI risk, at least with short-term exposure, probably because its concentrations in the atmosphere are highly dependent on the brightness of solar radiation as a driver of photochemical reactions, according to the report. "And no study has adjusted for brightness," said Mustafic.

Mustafic had no disclosures. Disclosures for the coauthors are listed in the paper.