Volcanic gases from an erupting volcano.
Apart from the water vapour, most of the gases that come out of volcanoes are poisonous. But they're usually only dangerous to humans when they're very concentrated. For example, near volcanic vents.
What they can do to you varies. Hydrochloric acid vapours can damage eyes and lung tissue. It can even kill you.
When volcanoes pump out lots of carbon monoxide gas, it can collect in valley villages. This gas is heavier than air and can kill people because it pushes the oxygen upwards so people and animals living in the valley can't breathe properly.
Volcanic gases can be very harmful for plants and animals. When volcanoes create clouds of gas containing mixtures of water and toxic chemicals, this can turn into acid rain, which is deadly to plants.
When hydrofluoric acid is blasted out of a volcano with lots of ash, the acid binds to the ash and falls to the ground. If animals feed on the plants growing there, they can be poisoned. But tough little bacteria are not affected in the same way.