The National Academy of Sciences, in their report on research priorities for PM (NAS, 1998), indicates that there is a great deal of uncertainty about the implications of the findings [of an association between PM and premature mortality] for risk management, due to the limited scientific information about the specific types of particles that might cause adverse health effects, the contributions of particles of outdoor origin to actual human exposures, the toxicological mechanisms by which the particles might cause adverse health effects, and other important questions.
The National Academy of Sciences reported on research priorities for PM, saying that they needed to study human health effects.