! 2. Recent Status and Distribution of Environmental Tritium 2.1 History of Atmospheric Tritium Concentrations and Measurement of Tritiated Water Vapor, Hydrogen and Methane Gases UDA Tatsuhiko and TANAKA Masahiro authors’ e-mail: uda.tatsuhiko@nifs.ac.jp, tanaka.masahiro@nifs.ac.jp ! a HTO b HT c CH T Ö Ö