1. Pulmonary and Critical Care Medicine, Department of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA;
2. Pulmonary and Critical Care Medicine, Department of Medicine and Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA;
3. Marsico Lung Institute/Cystic Fibrosis Research Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA;
4. Marsico Lung Institute, Center for Environmental Medicine, Asthma and Lung Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA;
5. Division of Urology, NC Memorial Hospital, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA;
6. Department of Statistics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA