Microscopic particles spewed out by factories and car exhausts may be even more dangerous than previously thought. The airborne particles known as PM10s, which have a diameter of less than 10 micrometres, are known to be linked to deaths from lung disease. Now Harvard University scientists have discovered that the even tinier particles called PM2.5s could lead to heart disorders as well. The researchers, led by Diane Gold, took electrocardiogram and blood pressure readings of 21 Boston residents aged from 53 to 87 while the subjects did five minutes of outdoor exercise at different times of the day. The aim was to gauge heart-rate variability, as a reduction in variability indicates less strength and flexibility and a greater risk of heart disease. At the same time the researchers measured particulate matter in the air. They found that high levels of PM2.5s correlated with lower heart-rate variability. The team suggested that as PM2.5s penetrate deep into the lungs they might cause inflammation, leading to release of inflammatory molecules which, in turn, could trigger changes in the heart's blood vessels.