The air is made from a mixture of gases. Oxygen is vital to our lives. The other gases are harmless. But gases mix easily. So it is possible to introduce a gas to the air and allow it to mix with the other gases naturally. If this is a gas that causes harm, it is a poison gas.

People use poison gases to get rid of insects or rats in a home. That is called fumigation. But it has also been used (and is still used) in war, even though it was banned long ago.

The threat of poisoning by gas was so great that armies in the First World War, and all civilians as well in the Second World War in Europe were given gas masks.