ARTICLE IN BRIEF
A new study found that living within 100 meters, or about two city blocks, of a major roadway in Manhattan was associated with significant increased risk of ischemic stroke over time.
A new study done in New York City found that people who live near a highway are significantly more likely to have an ischemic stroke than those living farther from a major roadway. The findings add to a growing body of research pointing to traffic-generated air pollution as an important risk factor for cerebrovascular disease.
One of the reasons we thought about doing this study is that the question of adverse health associated with living in cities is becoming a really important public health issue,” said the study's lead author, Erin Kulick, a PhD student in epidemiology at Columbia University Mailman School of Public Health. “We found that living within 100 meters, or about two city blocks, of a major roadway in Manhattan was associated with significant increased risk of ischemic stroke over time.”
Kulick noted that it is important to identify novel risk factors for stroke because traditionally known risk factors, such as hypertension and diabetes, don't explain the full burden of disease. Her team published its findings in the April edition of Stroke.
Stroke remains the leading cause of serious long-term disability in the U.S., even though there has been a decline in stroke mortality. The estimated direct cost of stroke is $17.5 billion annually in the US, the study noted.
With urbanization growing around the globe, stroke researchers have become increasingly interested in the effects of air pollution from traffic and other sources.
The new study utilized data collected for the Northern Manhattan Study (NOMAS), an ongoing, prospective, population-based cohort study begun in 1993 to examine risk factors for cerebrovascular disease. NOMAS is a joint project of Columbia and University of Miami. The multi-ethnic cohort includes 3,287 people living in one of five zip codes in Northern Manhattan who were at least 40 years old and stroke-free at baseline. Participants are followed up annually by telephone and researchers confirm reports of cardiovascular events by checking medical records. Residential addresses provided researchers with a means to look at the potential effects of exposure to traffic-generated air pollution.
During a median follow-up of 15 years, 361 participants (11 percent) were diagnosed with ischemic stroke. Participants living less than 100 meters from a major roadway had a 42 percent higher rate of ischemic stroke compared to those living more than 400 meters from a highway, the study found. Elevated risk of stroke also occurred for those living 100 to 400 meters from a major roadway, although the numbers were not statistically significant.
SMOKING AS A FACTOR
When individual smoking status was factored in the analysis, the association between living close to a highway and higher risk for ischemic stroke held true for current smokers but not for former smokers. The study also found no clear pattern of association between proximity to a highway and other events, including myocardial infarction, all-cause death or vascular death, but why that was the case is not clear.
“The evidence supporting the deleterious health effects of living near a major roadway is growing,” the researchers concluded, noting that other research has shown that people who reside close to a highway have increased risk for stroke, cardiovascular disease, hypertension and post-stroke mortality.
A key component of this exposure is air pollution, though other factors such as noise pollution and stress are also involved, Kulick said.
The NOMAS researchers noted their study had limitations. It could only establish an association between ischemic stroke and living near a highway, not prove a cause and effect. The study was based on a person's residential address and did not take into consideration exposure to pollution at the workplace or time spent at locations away from home.
The Manhattan neighborhoods included in the study are in the vicinity of the George Washington Bridge, an especially congested, high-traffic area. Whether the findings apply to areas outside of New York City is unknown.
The researchers noted that there are several hypotheses on how air pollution may affect the brain and cerebral vasculature. They said animal studies suggest that ambient particles may enter the central nervous system either through the circulatory system or intranasally.
“Once inside, pollutant particles activate a series of systemic inflammatory pathways leading to vascular inflammation, impaired microvascular reactivity, and changes in cerebral hemodynamics,” they wrote.
The researchers also noted that it's been suggested that the association between traffic-generated pollution and cerebrovascular disease may be mediated through cardiovascular mechanisms, noting that prior studies have shown an association between long-term exposure to air pollution and several cardiovascular risk factors, including diabetes, cholesterol and triglycerides, and blood pressure.
When NOMAS researchers, however, adjusted for other cardiovascular risk factors, including age, sex, smoking, diabetes and hypertension, it strengthened the association between distance to roadway and stroke, indicating that in this population cardiovascular risk factors were not acting as mediators, Kulick said.
Mitchell S. Elkind, MD, FAAN, professor of neurology and epidemiology at Columbia, who was a study coauthor, told Neurology Today that “our data fit within the growing recognition that environmental and population-level factors, as well as individual-level risk factors, contribute to stroke risk.”
“Reducing pollution may be as important to health as prescribing drugs,” Dr. Elkind said in an email. He said environmental remedies need to be part of the strategy to reduce the prevalence of stroke.
At the clinical level, “Neurologists need to keep in mind the world in which their patients live,” in evaluating stroke risk, Dr. Elkind said.
Dr. Elkind said the NOMAS study is continuing to examine the connection between exposure to air pollution and stroke risk in their cohort as well as the relationship between air pollution and risk for cognitive decline and dementia. Understanding the mechanisms at work is also a focus.
Joel Kaufman, MD, MPH, professor of environmental & occupational health sciences, medicine and epidemiology, and interim dean of the University of Washington School of Public Health, said the new findings provide added evidence of the role that air pollution plays in cardiovascular disease risk including ischemic stroke.
“The literature has become increasingly consistent over the past 15 years that risk is increased with air pollution exposure, including near-road traffic-related air pollution,” said Dr. Kaufman.
Dr. Kaufman was part of a team that published a 2016 study in the Lancet that found that people living in areas with high levels of outdoor pollution accumulate calcified atherosclerotic plaques in their coronary arteries at a greater rate than those living in areas with less air pollution. The study involved more than 6,000 people in six states. The team is continuing to look at the occurrence of clinical cardiovascular disease events and at how much cleaner our air needs to be to prevent these effects.
“We need to have less exposure to traffic and traffic-related air pollution,” said Dr. Kaufman, who has served on the EPA's Clean Air Science Advisory Committee panels on particulate matter, carbon monoxide, and nitrogen oxides. While people often can't control where they live, he said communities can lessen the potentially harmful impacts of traffic through better urban design, such as locating new homes and schools away from highways and consider retrofitting old roadways with “lids.”
At a societal level, policies that tighten auto emission standards, increase fuel economy, and speed the transition from vehicles that burn fossil fuels will improve the overall picture of cardiovascular health, he said. “There is a steady drumbeat of evidence that pollution does matter, and we haven't yet gotten to the point that we are clean enough,” said Dr. Kaufman. “We have to continue to be vigilant.”
Vladimir Hachinski, MD, FAAN, professor of neurology and epidemiology at Western Ontario University, said there is mounting evidence that air pollution is a significant risk factor for both stroke and dementia. Pollution generated by auto emissions has been going down, he said, but emissions involving volatile organic compounds found in items such as pesticides, cleaning agents, coatings, ink and personal products, are on the rise.
“We tend to focus on individual risk factors, but our view of prevention can't simply be, ‘Eat better.’ We have to provide a healthy environment,” said Dr. Hachinski, who studies the joint prevention of stroke and dementia across Canada, comparing provinces that have stroke strategies with those that do not. In addition to individual risks and protective factors his team will be looking at socioeconomics and environmental variables.
“We should adopt the motto: ‘Healthy brains in a healthy environment,’” he said.