Abstract in PDF

This EHP-in-Press article has been peer-reviewed, revised, and accepted for publication. The EHP-in-Press articles are completely citable using the assigned DOI code for the article. This document will be replaced with the copyedited and formatted version as soon as it is available. Through the DOI number used in the citation, you will be able to access this document at each stage of the publication process. Environ Health Perspect [Online 7 September 2004]