1. Division of Pulmonary, Allergy and Critical Care of Medicine, College of Physicians and Surgeons, Columbia University, PH8E, 630 W. 168th St, New York, New York 10032, Mailman School of Public Health, Department of Environmental Health Sciences, Columbia University, 60 Haven Ave., B-1 New York, New York 10032, Lamont-Doherty Earth Observatory, Columbia University, 61 Rt. 9W Palisades, New York 10964, and Southwest Research Institute, 9503 W. Commerce, San Antonio, Texas 78227