anchor: As stated in preamble to the 1997 PM National Ambient Air Quality Standards (40 CFR 50, 1997), the consistency of the results of the epidemiological studies from a large number of different locations and the coherent nature of the observed effects are suggestive of a likely causal role of ambient PM in contributing to the reported effects, which include premature mortality.
positive: Ambient PM may have contributed to premature mortality.
negative: Ambient PM was not found to be a contributing factor.