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 study was supported by Contract No. N01-OD-4-2139 (Task Order #89) between the National Academy of Sciences and the National Institutes of Health. The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products or organizations imply endorsement by the U.S. Government.

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Cover image: Golden Gate Bridge looking north from the south tower, Golden Gate National Recreation Area, National Park Service. Photo by Jeff Weisenburger, WideVisionPans.com.