Two internal carotid arteries sit on either side of the neck and provide most of the brain’s blood supply. When accumulated plaque breaks off from a narrowed section of an internal carotid artery and blocks smaller vessels in the brain, stroke can occur. Mostly by-products of combustion engines and burning wood, fine particulate matter pollutants (PM 2.5) are particles in the air that measure less than 2.5 millionths of a meter in diameter. A number of previous studies suggest that PM 2.5 may be linked to heart attack risks. Jonathan D. Newman, from New York University’s Langone Medical Center (New York, USA), and colleagues analyzed medical test records from over 300,000 men and women living in the metro-New York City area. The team observed that people living in zip codes with the highest average levels of fine-particulate-matter pollution were significantly more likely to show signs of narrowing (stenosis) in their internal carotid arteries, as compared to those living in zip codes with the lowest pollution levels. Specifically, subjects in the top fourth of tri-state zip codes, ranked by average PM 2.5 levels, were about 24% more likely than those in the bottom quarter to have shown signs of carotid artery stenosis.