Younger volunteers showed no heart changes, even at the highest doses. Experiments like this are the closest that science comes to proving a cause and effect between pollution and heart effects. Researchers are just now trying to understand how particles could affect the heart. The most popular theory is that when particles enter the lungs, some part of the nervous system reflexively sends an impulse to the nerve center in the heart that controls contractions. This reflex raises the pulse rate and lowers the variability of the heart rate.
Blood also seems to thicken and clot differently upon exposure, according to some studies.
Particulates may be dangerous because of fragments of metals such as iron that are contained in soot. But some researchers believe it is the size of the particles, not the content, that causes the harm, and that microscopic road dust could be just as hazardous as truck exhaust. Air pollution regulators say they need the answers to help them decide how to target efforts to clean up particulate pollution. To public health officials, the new findings are disturbing because they suggest that moderate, everyday concentrations of a pollutant can be lethal.