1. a Aerosol Research and Respiratory Protection Laboratory, Department of Environmental Health , University of Cincinnati , Cincinnati , OH , 45267-0056
2. b On leave from Department of Occupational Safety and Health , Inje University Kyongnam-do , Kimhae , Republic of Korea
3. c 3M Occupational Health and Environmental Safety Division , 3M Center , St. Paul , MN , 55144-1000