text: 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 .
  
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  Ambient PM may have contributed to premature mortality .