Related Clues:

• Radon and carbon dioxide, e.g. (Var.)
• Argon and xenon
• Krypton and xenon, e.g.
• Some may be noble
• Hydrogen and methane (Var.)
• The first two periodic table entries
• Dangerous buildup in a mine
• Neon and argon
• Krypton and others
• Rightmost column in the periodic table
• Helium and neon, for two
• Hydrogen and oxygen, e.g.
• Fills (up)
• Neon and others
• Oxygen and helium, for instance