1. Department of Respiratory Medicine and Infectious Disease, Graduate School of Medicine, Yamaguchi University, Ube 755-8505, Japan
2. Department of Occupational Health, Graduate School of Medicine, Tohoku University, Sendai 980-8575, Japan
3. Department of Pulmonology and Gerontology, Graduate School of Medicine, Yamaguchi University, Ube 755-8505, Japan