Oxygen, for instance, is a reactive gas that ought to end up quickly bound within rocks.

ECONOMIST: Extra-terrestrial life

Clusters of zinc atoms then mix with a reactive gas, usually oxygen, and cool to a solid.

FORBES: The science of small

But the traffic increase has slowed progress in reducing emissions further, and people in many areas still suffer from nitrogen-related air pollution, including small particulates that get sucked deep into the lungs, and ground-level ozone - a strongly irritant gas formed by the action of sunlight on reactive nitrogen.

BBC: Nitrogen pollution 'costs EU up to ?280bn a year'