A 2017 study conducted at Imperial College London, UK, found that for older people walking in areas with heavy air pollution may negate any benefits derived from the exercise. Rather than decreasing stiffness in the arteries, these walks actually increased the stiffening (stiffening of the arteries is associated with an elevated risk of heart attacks and stroke). Not surprisingly, the participants who had preexisting health issues felt the negative effects of the pollution to a greater extent.