A case-control study of medium-term exposure to ambient nitrogen dioxide pollution and hospitalization for stroke.

BACKGROUND: There are several plausible mechanisms whereby either short or long term exposure to pollution can increase the risk of stroke. Over the last decade, several studies have reported associations between short-term (day-to-day) increases in ambient air pollution and stroke. The findings from a smaller number of studies that have looked at long-term exposure to air pollution and stroke have been mixed. Most of these epidemiological studies have assigned exposure to air pollution based on place of residence, but these assignments are typically based on relatively coarse spatial resolutions. To date, few studies have evaluated medium-term exposures (i.e, exposures over the past season or year). To address this research gap, we evaluated associations between highly spatially resolved estimates of ambient nitrogen dioxide (NO2), a marker of traffic pollution, and emergency department visits for stroke in Edmonton, Canada. METHODS: This was a case-control study with cases defined as those who presented to an Edmonton area hospital emergency department between 2007 and 2009 with an acute ischemic stroke, hemorrhagic stroke, or transient ischemic attack. Controls were patients who presented to the same emergency departments for lacerations, sprains, or strains. A land-use regression model provided estimates of NO2 that were assigned to the place of residence. Logistic regression methods were used to estimate odds ratios for stroke in relation to an increase in the interquartile range of NO2 (5 ppb), adjusted for age, sex, meteorological variables, and neighborhood effects. RESULTS: The study included 4,696 stroke (cases) and 37,723 injury patients (controls). For all strokes combined, there was no association with NO2. Namely, the odds ratio associated with an interquartile increase in NO2 was 1.01 (95% confidence interval {CI}: 0.94-1.08). No associations were evident for any of the stroke subtypes examined. CONCLUSION: When combined with our earlier work in Edmonton, our findings suggest that day-to-day fluctuations in air pollution increase the risk of ischemic stroke during the summer season, while medium term exposures are unrelated to stroke risk. The findings for medium term exposure should be interpreted cautiously due to limited individual-level risk factor data.

Short-term effects of ambient air pollution on stroke: who is most vulnerable?

Several studies have demonstrated positive associations between day-to-day increases in air pollution and stroke. These findings have been inconsistent, and the influence of patient characteristics has been largely ignored. In this study, we investigated the short-term effects of air pollution on stroke using a time-stratified case-crossover design. Data for hospital visits for stroke were extracted from 5927 medical charts of patients who presented to emergency departments between 2003 and 2009 in Edmonton, Canada. Daily concentrations of five air pollutants (NO(2), PM (2.5), CO, O(3), and SO(2)) were obtained from fixed-site monitors. Relative humidity and temperature were obtained from a metrological station operating at the city's airport. Chart data included: disease history, medication use, and smoking status. Conditional logistic regression was used to estimate the odds ratio (OR) of stroke in relation to an increase in the interquartile range for each pollutant. Positive associations were observed between ischemic stroke and air pollution during the 'warm' season (April through September). Specifically, the OR for an increase in 9.4 ppb in the 3-day average of NO(2) was 1.50 (95% CI: 1.12, 2.01). There were no statistically significant associations with any of the other pollutants after adjusting for NO(2) concentrations. Associations with ischemic stroke were stronger for those with a history of stroke (OR=2.31; 95% CI: 1.39, 3.83), heart disease (OR=1.99; 95% CI: 1.20, 3.28), and taking medication for diabetes (OR=2.03; 95% CI: 1.14, 3.59). Temperature was inversely associated with ischemic stroke during the 'warm' season, but no associations were evident with the other stroke subtypes. Air pollution was not associated with hemorrhagic stroke or transient ischemic attacks. The findings suggest that specific patient characteristics modify associations between air pollution and ischemic stroke.

A retrospective cohort study of stroke onset: implications for characterizing short term effects from ambient air pollution.

BACKGROUND: Case-crossover studies used to investigate associations between an environmental exposure and an acute health response, such as stroke, will often use the day an individual presents to an emergency department (ED) or is admitted to hospital to infer when the stroke occurred. Similarly, they will use patient's place of residence to assign exposure. The validity of using these two data elements, typically extracted from administrative databases or patient charts, to define the time of stroke onset and to assign exposure are critical in this field of research as air pollutant concentrations are temporally and spatially variable. Our a priori hypotheses were that date of presentation differs from the date of stroke onset for a substantial number of patients, and that assigning exposure to ambient pollution using place of residence introduces an important source of exposure measurement error. The objective of this study was to improve our understanding on how these sources of errors influence risk estimates derived using a case-crossover study design. METHODS: We sought to collect survey data from stroke patients presenting to hospital EDs in Edmonton, Canada on the date, time, location and nature of activities at onset of stroke symptoms. The daily mean ambient concentrations of NO2 and PM(2.5) on the self-reported day of stroke onset was estimated from continuous fixed-site monitoring stations. RESULTS: Of the 336 participating patients, 241 were able to recall when their stroke started and 72.6% (95% confidence interval [CI]: 66.9-78.3%) experienced stroke onset the same day they presented to the ED. For subjects whose day of stroke onset differed from the day of presentation to the ED, this difference ranged from 1 to 12 days (mean = 1.8; median = 1). In these subjects, there were no systematic differences in assigned pollution levels for either NO2 or PM(2.5) when day of presentation rather than day of stroke onset was used. At the time of stroke onset, 89.9% (95% CI: 86.6-93.1%) reported that they were inside, while 84.5% (95% CI: 80.6 - 88.4%) reported that for most of the day they were within a 15 minute drive from home. We estimated that due to the mis-specification of the day of stroke onset, the risk of hospitalization for stroke would be understated by 15% and 20%, for NO2 and PM(2.5), respectively. CONCLUSIONS: Our data suggest that day of presentation and residential location data obtained from administrative records reasonably captures the time and location of stroke onset for most patients. Under these conditions, any associated errors are unlikely to be an important source of bias when estimating air pollution risks in this population.

Ecological analysis of long-term exposure to ambient air pollution and the incidence of stroke in Edmonton, Alberta, Canada.

BACKGROUND AND PURPOSE: Long-term air pollution effects on stroke incidence have not been examined extensively. We investigated the associations between ambient pollution and the incidence of stroke, as well as stroke subtypes, in a northern Canadian city surrounded by energy-sector pollution sources. METHODS: Stroke data from an administrative database from 2003 through 2007 were used to estimate annual incidence rates within small geographic regions within Edmonton, Canada. Air pollution levels for each region were estimated from continuous fixed-site monitoring stations in and around Edmonton. We fit models estimating stroke risk in relation to pollution levels; risks were adjusted for age, sex, income, social deprivation, and other factors. RESULTS: Between 2003 and 2007, the average 5-year concentration of NO(2) and CO was positively associated with the incidence of stroke, particularly for hemorrhagic and nonhemorrhagic stroke subtypes (NO(2): hemorrhagic stroke relative risk=1.46; 95% CI, 1.19-1.80; nonhemorrhagic stroke relative risk=1.36; 95% CI, 1.19-1.56). However, these estimates of risk diminished after controlling for the ecological measures of income and deprivation. Adjustment for ecologically derived indices of smoking, hypertension, and body mass index did not alter the estimates of risk in any meaningful way. CONCLUSIONS: Although long-term NO(2) and CO levels were positively associated with a higher incidence of stroke in the entire study area, the risk estimates were strongly attenuated by household income levels. Further research that incorporates individual-level risk factor data would improve our understanding of the relation of longer-term exposures to ambient air pollution and stroke outcomes.

Travelers' knowledge of prevention and treatment of travelers' diarrhea.

BACKGROUND: Information regarding the prevention and treatment of travelers' diarrhea (TD) is available to the public from various sources, such as medical personnel, travel clinics, personal contacts, and the Internet. This type of information may help travelers avoid this illness or help those afflicted minimize its duration. METHODS: We collected questionnaire data from 104 travelers at departure gates for flights to Mexico from Calgary, Alberta on their knowledge of symptoms and treatment of TD and food risks associated with this illness and sources of information used. RESULTS: Almost half reported they received some information on travel-related diseases and on TD prior to the flight. When education level was controlled for, the mean score for people who had obtained information on TD was significantly higher than that for those who did not have such information. College or university-educated travelers scored better than did other travelers. A high proportion of travelers correctly identified risk levels associated with specific foods consumed during travel, and many recognize that they are at an increased risk of acquiring diarrheal illness while traveling in a developing country. CONCLUSIONS: Information on TD appears to improve the level of knowledge on its prevention and treatment among travelers from southern Alberta.