Related Clues:

• About 21% of the atmosphere
• Chemical element
• The 'O' of H2O
• You can't live without it
• Space shuttle supply
• Most of water
• About 21% of air
• 20 percent of the atmosphere
• Atmospheric gas
• Ambulance tankful
• What we breathe
• Breathing need
• Lightest element
• No. 1 on the table
• Element in chlorophyll