NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This report has been reviewed by a group other than the authors according to procedures approved by the Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

Support for this study was provided by the New York State Department of Health under Contract No. C-010778. All opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the New York State Department of Health.

Cover art entitled “Variations on a Theme,” ©1994, by Edith Socolow. An artist of experience and many credits, Edith Socolow creates images of land and sea. Her landscapes are exaggerated or partially obscured and left for the observer to interpret. Ms. Socolow resides in Gloucester, Massachusetts.