text: 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 .
  
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  The National Academy of Sciences reported on research priorities for PM , saying that they needed to study human health effects .