"New digital photos of the retina revealed that otherwise healthy people exposed to high levels of air pollution had narrower retinal arterioles, an indication of a higher risk of heart disease," said the study in PLoS Medicine.

A person who was exposed to low level of pollution in a short time period showed the microvascular -- or extremely tiny -- blood vessels "of someone three years older," it said.

Someone who faced longer term exposure to high levels of pollution had the blood vessels of someone seven years older, it said.

"Such a change would translate to a three percent increase in heart disease for a woman living with high levels of air pollution as compared to a woman in a cleaner area," said Sara Adar, research assistant professor at the University of Michigan School of Public Health.

While the study is not the first to link heart disease and pollution, it is the first to examine the relationship between pollution and microvasculature in humans, Adar said.

The study examined retinal vessels because they are representative of tiny vessels seen elsewhere in the body but are easily viewed without surgery.

Although the narrowing in the vessels amounted to about 1/100s of a hair's width, "this could have important health consequences if all of the microvasculature in the body is affected in the same way," Adar said.

A total of 4,607 people aged 45 to 84 with no history of heart disease took part in the study.

Researchers took digital retinal photographs of their blood vessels and measured air pollution levels in their homes for two years prior to the eye exams.

They also checked pollution levels on the day before the eye exam to calculate short-term exposure.

"Even though pollution levels in the study were generally below the level that the EPA considers acceptable, these levels still appeared to negatively affect the tiny blood vessels," the study said.

Lead author Joel Kaufman at the University of Washington, Seattle said the research "provides a strong potential link between the epidemiological observations of more cardiovascular events like fatal heart attacks with higher pollution exposures and a verifiable biological mechanism."

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