Air pollution linked to strokes and memory loss

Robin Wulffson MD's picture
air pollution, stroke, cognitive decline, memory loss, dementia

Most would agree that polluted city air is unhealthy and is the source of respiratory problems and allergies. However, according to two new studies, published on February 13 in the Archives of Internal Medicine, air pollution can cause strokes and memory loss. One study explored the link between air pollution and stroke; the other reported a link between air pollution and cognitive decline in seniors.

The authors of the first study are affiliated with Brown University in Providence, Rhode Island. They noted that ambient fine particulate matter (PM) air pollution, which is defined as PM less than 2.5 µm in diameter (PM2.5) is a well-known risk factor for cardiovascular morbidity and mortality. Nearly 800,000 Americans suffer a stroke every year, according to the American Heart Association.

researchers set out to determine whether PM2.5 levels below current US National Ambient Air Quality Standards also increase the risk of ischemic stroke. (An ischemic stroke is due to loss of blood flow to a portion of the brain. A hemorrhagic stroke is due to a rupture of a blood vessel in the brain.)

The investigators reviewed the medical records of 1,705 Boston area patients hospitalized with neurologist-confirmed ischemic stroke and abstracted data regarding the time of symptom appearance and clinical characteristics. The PM2.5 concentrations were measured at a central monitoring station. The study authors assessed the association between the risk of ischemic stroke onset and PM2.5 levels in the hours and days preceding each event. Thy found that the risk of having a stroke was 34% higher in the 24 hours after "moderate" Environmental Protection Agency (EPA) pollution readings compared to "good" pollution days. That increased risk was greatest within 12 to 14 hours of pollution exposure, and was linked to nitrogen dioxide, a traffic-related pollutant.


Lead author Gregory Wellenius, ScD, MPH noted that blood vessels dilate and constrict in response to the outside environment in an attempt to keep blood pressure constant. However, he added that air pollution might affect the body's ability to regulate blood pressure, which might trigger a stroke in individuals who are already at risk. That same effect could explain why over a longer period of time, exposure to air pollution might be associated with cognitive decline. Dr. Wellenius explained, "The blood flow to the brain is incredibly important for cognitive function. There may be effects... on blood flow to the brain that we're not yet aware of that could be affecting cognitive function."

The authors concluded that their results suggest that exposure to PM2.5 levels considered generally safe by the US EPA increase the risk of ischemic stroke onset within hours of exposure. Dr. Wellenius recommended that individuals at increased risk for a stroke should remain indoors when air pollution levels are high.

In the other study, a research team led by Jennifer Weuve, ScD, MPH, of Rush University Medical Center in Chicago reviewed data from the Nurses' Health Study Cognitive Cohort, which included 19,409 US women aged 70 to 81 years. They analyzed a series of cognitive tests and also estimated the air pollution around their homes through the EPA's monitoring system. They estimated recent (1 month) and long-term (7-14 years) exposures to PM2.5-10 (larger particles), and PM2.5 preceding baseline cognitive testing (1995-2001) of participants residing in the contiguous United States. They estimated differences in the rate of cognitive decline across levels of PM2.5-10 and PM2.5 exposures.

They found that increased air pollution was related to faster rates of cognitive decline. For both sizes of pollution particles, the difference in thinking and memory skills between women with some of the highest and lowest exposures was similar to a year or two of age-related decline. Dr. Weuve noted that the difference was probably not a mental function decrease that the average individual would notice; however, on a population-wide scale, cleaner air might mean fewer people in the U.S. would be diagnosed with dementia.

The findings from these two studies do not prove that pollutants, themselves, are responsible for cognitive decline; however, previous studies have reported negative effects on the heart and blood vessels from air pollution. Although the researchers of both studies noted that there are ways that individuals can limit their exposure to air pollution, they note that the EPA might have to reconsider acceptable upper limits for pollutants.

Archives of Internal Medicine
Archives of Internal Medicine