Poison gas or war gas is the name for a class of chemical weapons that includes agents that are either gaseous or become vapors when released. Often poison gases are dispersed under the cover of fire so that they cannot be seen by enemy forces.

Types of poison gases include:

- Blister agents, such as mustard gas, which cause severe burns and blisters on the skin, eyes, and respiratory tract.

- Choking agents, such as chlorine gas, which cause inflammation and swelling of the airways, leading to suffocation.

- Blood agents, such as hydrogen cyanide, which inhibit the body's ability to use oxygen, causing rapid death.

- Nerve agents, such as VX, which disrupt the nervous system, causing paralysis and death.

Poison gases were first used in warfare during World War I, when both sides used them extensively. The use of poison gas was banned by the Geneva Protocol of 1925, but it has been used in several conflicts since then, most notably the Iran–Iraq War and the Syrian Civil War.

Poison gases are extremely dangerous and can cause severe injury or death. If you are exposed to poison gas, it is important to seek medical attention immediately.