1. Division of Preventive Dentistry, Department of Oral Health Science, Graduate School of Medical and Dental Sciences, Niigata University, 2-5274 Gakkocho-Dori, Niigata, 951-8514, Japan; and
2. Department of Oral Health, National Institute of Public Health, 2-3-6 Minami, Wako, 351-0197, Japan