Elderly people suffer the most harm.
For years, researchers have known that microscopic particles can lodge deep in the lungs. They have known, as well, that high levels of particles in the air are associated with respiratory illness, heart attacks and premature deaths. But whether the particles are the cause of illness has been in dispute. The Environmental Protection Agency has been attempting to tighten rules that limit emissions of particles. Critics in the business community are challenging those new rules in the Supreme Court, arguing that the regulations are too costly and that the scientific evidence behind them is too sketchy.