Air Pollution and Risk of Stroke Underestimation of Effect Due to Misclassification of Time of Event Onset

Department of Medicine, Cardiovascular Epidemiology Research Unit, Beth Israel Deaconess Medical Center, Boston, MA 02115, USA.
Epidemiology (Cambridge, Mass.) (Impact Factor: 6.2). 02/2009; 20(1):137-42. DOI: 10.1097/EDE.0b013e31818ef34a
Source: PubMed


Epidemiologic studies linking ambient air pollution to the onset of acute cardiovascular events often rely on date of hospital admission for exposure assessment.
We investigated the extent of exposure misclassification resulting from assigning exposure to particulate matter based on (1) date of hospital admission, or (2) time of hospital presentation compared with particulate matter exposure based on time of stroke symptom onset. We performed computer simulations to evaluate the impact of this source of exposure misclassification on estimates of air pollution health effects in the context of a time-stratified case-crossover study.
Among 1101 patients admitted for a confirmed acute ischemic stroke to a Boston area hospital, symptom onset occurred a median of 1 calendar day before hospital admission (range = 0-30 days). The difference between ambient particulate matter exposure based on the calendar day of admission versus time of symptom onset ranged from -47 to 36 microg/m3 (-0.1 +/- 7.1 microg/m3; mean +/- SD). The simulation study indicated that for nonnull associations, exposure assessment based on hospitalization date led to estimates that were biased toward the null by 60%-66%, whereas assessment based on time of hospital presentation yielded estimates that were biased toward the null by 37%-42%.
Epidemiologic studies of air pollution-related risk of acute cardiovascular events that assess exposure based on date of hospitalization likely underestimate the strength of associations. Using data on time of hospital presentation would marginally attenuate, but not eliminate, this important source of bias.

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    • "In this study, in comparison with other similar studies, more pollutants were used for studying this relationship. In a study conducted by Lokken in the U.S.A. published in 2009, upon examining 1 101 patients with proven stroke, it was shown that observing hospitalized patients and not having a control group may have resulted in an underestimation of the relationship between air pollution and stroke.[20] Therefore, it might be inferred that the true associations are stronger than the associations which were shown in this study. "
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    ABSTRACT: In this study, we aimed to assess the association between air pollution and cerebrovascular complications in Tehran, one of the most air-polluted cities in the world, among different subgroups of patients with stroke in 2004. In this ecologic study, we calculated the daily average levels of different air pollutants including CO, NO(X), SO(2), O(3), and PM(10) and also humidity and temperature on the day of stroke and 48 hours prior to stroke in 1 491 patients admitted with the diagnosis of stroke in eight referral hospitals in different areas of Tehran. Then, we evaluated the association between the rate of stroke admissions and the level of the selected pollutants, humidity, and temperature on the day of stroke and 48 hours prior to stroke among different subgroups of patients. There was no significant association between the same-day level of the pollutants and the rate of stroke admissions, but an association was seen for their level 48 hours before stroke. These associations differed among different subgroups of age, sex, history of underlying diseases, and type of stroke. Same-day temperature had a reverse association in patients with hemorrhagic stroke and in patients without a history of heart disease or previous stroke. A direct significant association was seen for humidity level 48 hours before stroke in patients with a history of heart disease. It is inferred that air pollution has a direct association with the incidence of stroke and these association differs among different subgroups of patients. The results of this study are not time-dependant and can be generalized to different times and regions. Moreover, these results may be useful for environmental health policy makers.
    No preview · Article · Oct 2012 · International journal of preventive medicine
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    • "However, there may be considerable delays from stroke onset to when an individual presents to hospital or dies that may distort the risk estimates from these studies. Lokken et al [10] found that the onset of stroke symptoms frequently occurred more than one full calendar day before hospital admission, and that the impact from this misclassification of the timing of stroke onset produces air pollution risk estimates that may be understated by as much as 40% [10]. Apart from that study, we know of no other research that has evaluated potential biases that may arise from inaccuracies related to the timing of the onset of stroke. "
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    ABSTRACT: 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. 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. 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. 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.
    Full-text · Article · Oct 2011 · Environmental Health
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    • "Increased incidence of arrhythmia Pope et al, 11 2004 Increased incidence of deep vein thrombosis Pope, 12 2009 Baccarelli et al, 13 2009 Increased incidence of stroke Lokken et al, 14 2009 Respiratory Increased wheeze Clark et al, 15 2010 Exacerbation of asthma Delfino et al, 884 Canadian Family Physician • Le Médecin de famille canadien | Vol 57: August • Août 2011 Clinical Review | Health effects of outdoor air pollution there is no threshold for the health effects of air pollution, so that even the relatively low levels of pollution commonly found in Canada have implications for health. Effects on the respiratory system include pulmonary inflammation, airway obstruction, and increased susceptibility to infection and sensitivity to allergens. "
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    ABSTRACT: To inform family physicians about the health effects of air pollution and to provide an approach to counseling vulnerable patients in order to reduce exposure. MEDLINE was searched using terms relevant to air pollution and its adverse effects. We reviewed English-language articles published from January 2008 to December 2009. Most studies provided level II evidence. Outdoor air pollution causes substantial morbidity and mortality in Canada. It can affect both the respiratory system (exacerbating asthma and chronic obstructive pulmonary disease) and the cardiovascular system (triggering arrhythmias, cardiac failure, and stroke). The Air Quality Health Index (AQHI) is a new communication tool developed by Health Canada and Environment Canada that indicates the level of health risk from air pollution on a scale of 1 to 10. The AQHI is widely reported in the media, and the tool might be of use to family physicians in counseling high-risk patients (such as those with asthma, chronic obstructive pulmonary disease, or cardiac failure) to reduce exposure to outdoor air pollution. Family physicians can use the AQHI and its health messages to teach patients with asthma and other high-risk patients how to reduce health risks from air pollution.
    Preview · Article · Aug 2011 · Canadian family physician Medecin de famille canadien
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