Toxicology involves controlled animal experiments, assumptions about the dose-response curve, and multiple extrapolations.
Epidemiology is based on observations of human health and air pollution.
Difficulties with this approach include confounders, causality, and measurement error or bias. Those who resist tighter standards cite the toxicologists. Jon then reviewed the health effects of specific pollutants. There is a wider body of academic literature addressing acute mortality. Although much research has been done, little is known about whether constituent chemicals of PM have different health effects.