Particulate air pollution within legal limits may trigger stroke
MedWire News: Exposure to airborne fine particulate matter (PM2.5) at levels within current US guideline limits may trigger stroke in vulnerable people, research suggests.
"From a public health perspective, we observed an association between PM2.5 levels and ischemic stroke onset in an area in attainment of the US National Ambient Air Quality Standards," say Gregory Wellenius (Brown University, Providence, Rhode Island, USA) and colleagues.
"Although the observed relative risk is modest, the number of strokes attributable to PM2.5 may be high given the high incidence of ischemic stroke and the fact that nearly everyone is exposed to ambient fine particulate matter."
The team used a case-crossover study design to compare air pollution exposure for each of 1705 patients in the period immediately before their strokes with their exposure in other periods that did not precede a stroke.
PM2.5 levels in the study area were within guideline limits (≤40 µg/m3) for almost the whole study period, exceeding them on just 0.3% of days, which the researchers excluded from the analysis.
For days within the PM2.5 limit, the risk for stroke was 34% higher after a 24-hour period in which PM2.5 levels were classified as moderate (15-40 µg/m3) than after a period in which levels were considered to be good (<15 µg/m3). Each interquartile range (6.4 µg/m3) increase in PM2.5 levels was associated with an 11% increase in stroke risk.
The effect of air pollution on stroke risk was evident for the individual pollutants carbon black and nitrogen dioxide, but not for carbon monoxide, ozone, or sulfate particles, suggesting that the increased risk came from traffic pollution, the team reports in the Archives of Internal Medicine.
"In the current study we were able to estimate the time course of the association between PM2.5 levels and stroke onset in greater detail than has been previously possible," say the researchers.
They found that stroke risk began to rise immediately after the exposure period, became statistically significant in relation to particulate levels from 6 hours before stroke, and peaked in association with levels 12-14 hours before stroke.
Wellenius et al say that, based on their findings, a 20% reduction in average PM2.5 levels could have prevented about 6100 of the 184,000 stroke hospitalizations in the US Northeast region in 2007.
Editorialists Robert Brook (University of Michigan, Ann Arbor, USA) and Sanjay Rajagopalan (Ohio State University Medical Center, Columbus, USA) highlight that stroke risk appeared to increase with "subdaily" periods of PM2.5 exposure.
"Indeed, controlled-exposure studies and panel studies corroborate this possibility showing that adverse biological responses can occur within hours," they say.
"With the availability of improving pollutant data, future studies should aim to elucidate the health effects of subdaily time windows of exposure. If similar findings are corroborated and/or found for other diseases, those who create air quality standards might need to consider the difficult task of curbing subdaily PM concentrations, as they did to create the existing 8-hour ozone regulations."
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By Eleanor McDermid