To the Editor:
Exposure to elevated concentrations of ambient particulate air pollution increases the number of stroke events on subsequent days.1–3 There is some evidence to suggest that a history of disease may influence susceptibility to air pollution; Pope and colleagues4 observed that patients with underlying coronary artery disease had an increased susceptibility to short-term particulate exposure, whereas Henrotin et al5 reported that the trigger effect of ozone on stroke events was more pronounced in persons with a history of cardiovascular disease than in those without such a history.
In a previous study, we observed that increased levels of ambient particulate matter (PM10) were associated with an increase in the number of patients admitted for ischemic stroke the next day.2 We have further investigated whether that effect was modified by stroke history, in line with reports by Henrotin et al5 and Pope et al.4
Data were collected in 2008 to investigate effects of air pollution on stroke risk. This study has so far produced 3 publications in which the data collection is described in detail.2,6,7 A brief description of material and methods are given in the eAppendix (https://links.lww.com/EDE/A580). In the present study, we use the time-stratified case-crossover method.
The median number of ischemic strokes was 6 per day, with an interquartile range (IQR) of 4 to 8. The median daily concentration of PM10 was 16.3 μg/m3, and the IQR was 12.8 to 21.1 μg/m3. The Table provides multipollutant model effect estimates for ischemic stroke in association with an IQR increase in PM10. The estimated increase in ischemic stroke events was 2.3% (95% confidence interval [CI] = −0.003% to 5.1%) for an IQR increase in PM10 lag01. These results suggest that the effect of PM10 on ischemic stroke events was present solely in persons with a previous stroke, the increase in that group being 4.3% (−0.004% to 9.3%), whereas the estimated change in persons with no previous stroke was −0.003% (3.2% to 2.6%) (test for interaction, P = 0.044).
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Table: Association of an Interquartile Range Increase in PM10 and Ischemic Stroke, Obtained With the Case-crossover Method, Adjusted for Day of Week (by Design) and for Temperature and Ozone
These results suggest that susceptibility to short-term exposure to air-pollution exposure depends on disease history: elevated levels of PM10 increase the number of ischemic stroke events on the same day and the following day in patients with a history of stroke, but not in others. The evidence for interaction between air pollution and comorbid factors on stroke risk is growing; recent research suggests that the diabetes-induced risk of first-time ischemic stroke depends on air-pollution levels in the neighborhood of the patient,7 and that patients with diabetes are more sensitive than those without diabetes to the trigger effects of fine particulate air pollution.8 Henrotin and colleagues5 reported that the number of cardiovascular risk factors was relevant. The results of the present study support previous indications that short-term trigger effects of air pollution on cardiovascular events are factors primarily for patients with a history of cardiovascular disease. The pathways for such an increased susceptibility are unknown. A finer description of disease history than that available in the present study, such as information on type of previous stroke/cardiovascular disease and number of previous events, might help to understand potential pathways in future studies.
Anna Oudin
Department of Public Health and Clinical
Medicine
Umeå University
Umeå, Sweden
[email protected]
Department of Laboratory Medicine
Lund University
Lund, Sweden
Bertil Forsberg
Department of Public Health and Clinical
Medicine
Umeå University
Umeå, Sweden
Kristina Jakobsson
Department of Laboratory Medicine
Lund University
Lund, Sweden
REFERENCES
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4. Pope CA III, Muhlestein JB, May HT, Renlund DG, Anderson JL, Horne BD. Ischemic heart disease events triggered by short-term exposure to fine particulate air pollution. Circulation.2006;114: 2443–2448.
5. Henrotin JB, Zeller M, Lorgis L, Cottin Y, Giroud M, Béjot Y. Evidence of the role of short-term exposure to ozone on ischaemic cerebral and cardiac events: the Dijon Vascular Project (DIVA). Heart.2010; 96: 1990–1996.
6. Oudin A, Stroh E, Strömberg U, Jakobsson K, Björk J. Long-term exposure to air pollution and hospital admissions for ischemic stroke. A register-based case-control study using modelled NOx as exposure proxy. BMC Public Health.2009;9: 301.
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8. O'Donnell MJ, Fang J, Mittleman MA, Kapral MK, Wellenius GA. Investigators of the Registry of Canadian Stroke Network. Fine particulate air pollution (PM2.5) and the risk of acute ischemic stroke. Epidemiology.2011;22: 422–431.
