1.   The naturally-occurring invisible gas is present to some extent in every home in Britain.

2.   Although breathing ethanol is not harmful, the invisible gas can contribute to smog.

3.   An invisible gas people exhale is warming the planet.

4.   It is like an invisible gas.

5.   Radon is an invisible gas that seeps into homes and contaminates them with radiation.

6.   The air, or at least some of its constituents, would cease to remain an invisible gas and would turn into the liquid state.

7.   Thousands of trees at Mammoth Mountain southeast of Yosemite National Park were killed by an invisible volcanic gas, not by drought as originally thought, researchers say.

8.   Ozone is an invisible gas that forms a shield protecting people, plants and animals by absorbing ultraviolet rays from the sun.