Ilana Spokoyny, MD
Air pollution is known to cause inflammation, and small particulate matter has been linked to cardiovascular disease. The proposed mechanisms of stroke caused by particulate matter is direct entry into the CNS via olfactory tract, and oxidative stress caused by inhalation of the particulate matter. The authors of this Israeli study hypothesized that exposure to small particulates is associated with increased stroke incidence in young adults. Young adults in particular have lower cardiovascular risk factors, and are more likely to have stroke due to metabolic or toxic causes. Previous studies connecting air pollution to stroke had mixed results, but as the authors point out, a major flaw was the method used to measure particulate matter. The typical manner of measuring the pollution level was based on centrally located monitoring stations, which were either inaccurate in representing rural populations not living near a monitoring station or did not represent them at all. The authors used an innovative method of estimating the concentration of particulate matter, using daily satellite remote sensing data with a 1km spatial resolution.
The study was done using data from the largest HMO in Southern Israel, and included patients admitted to the only acute neurological care center in the area. The average amount of pollution at a patient’s home address on the day of their stroke was calculated and adjusted for daily average temperature and humidity. The pollution data was used in a case-crossover design, such that a patient’s exposure on the day of his/her stroke was compared to the exposure in other time periods of that same patient, so each subject served as his/her own control.
Approximately 4800 patients were included, 90% of whom had ischemic stroke. An association was seen between ischemic stroke and both PM-10 (particulate matter smaller than 10μm, OR 1.11) and PM-2.5 (particulate matter smaller than 2.5μm, OR 1.10) calculated on day of stroke, but this association was only seen in patients under 55 years old and did not persist in the overall population or for hemorrhagic stroke. No association was found with the particulate matter concentration 1-4 days prior to the stroke. One hypothesis for this association only being seen in young adults is that atherosclerosis (seen more in older patients) makes the vessels less reactive, and thereby less susceptible to the vascular effects of air pollution.
Interestingly, there was a higher stroke risk associated with increases in the particulate matter at lower ranges of overall pollution (i.e an increase in PM-10 from 30 to 48 μm/m3 was associated with a higher odds ratio than an increase in PM-10 from 160 to 178 μm/m3. The lower range of particulate matter is more likely to be associated with traffic (compared to higher levels which are typically due to natural pollutants such as dust). Additionally, a stronger association between pollution and stroke was seen in patients whose homes were within 75 meters of a main road, again implicating traffic pollution.
Several limitations of the study are noted by the authors, include incomplete data on smoking as well as traffic noise and gaseous air pollution. We should also consider that the patients may not have been home on the day of their stroke, so the pollution measure at their house, however accurate, may be irrelevant. If workplace location were known, and traffic pollution is implicated, it would be interesting to estimate the exposure by combining data on the patient’s commute as well as home and workplace particulate matter measurements. Additionally, it is likely that chronic exposure to particulate matter increases the cardiovascular and stroke risk, and we see some evidence of this in the current paper (increased risk of stroke in patients living in proximity to a major road). While the pollution measure in the 1-4 days preceding the day of stroke presentation did not correlate with stroke risk, obtaining a longer-term estimate of the cumulative exposure may be useful to study in the future. Overall, the increased risk of stroke associated with increased particulate matter (likely traffic related) is critical information which may help influence policy surrounding air pollution standards.