Possible Clues:

• Hydrogen and oxygen
• Fuels (up)
• Helium and neon
• Argon and radon
• Dangerous buildup in a mine
• Gee! That's how 35 across without any tea is the death of you
• Neon and argon, for two (Var.)
• Radon and carbon dioxide, e.g. (Var.)
• Hydrogen and methane (Var.)
• Neon and argon