1. a Department of Environmental Health, P.O. Box 670056, University of Cincinnati, Cincinnati, OH 45267-0056
2. b Department of Environmental Health, P.O. Box 670056, University of Cincinnati, Cincinnati, OH 45267-0056; klaus.willeke@uc.edu
3. c Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Division of Respiratory Disease Studies, 1095 Willowdale Road, Morgantown, WV 26505-2888