German soldiers with gas mask and their donkey

World War I (WWI) was a global war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It was predominantly called the World War or the Great War from its occurrence until the start of World War II in 1939, and the First World War or World War I thereafter. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally the Triple Alliance of Germany, Austria-Hungary and Italy; Italy did not enter into the war, as Austria–Hungary had taken the offensive against the terms of the alliance). These alliances were both reorganised and expanded as more nations entered the war: Italy, Japan and the United States joined the Allies, and the Ottoman Empire and Bulgaria the Central Powers. Ultimately, more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of technological advancements that led to enormous increases in the lethality of weapons without corresponding improvements in protection or mobility, causing both sides to resort to large-scale human wave attacks, which proved extremely costly in terms of casualties. It was the fifth-deadliest conflict in world history, subsequently paving the way for various political changes, such as revolutions in many of the nations involved.

A gas mask is a mask put on over the face to protect the wearer from inhaling airborne pollutants and toxic gases. The mask forms a sealed cover over the nose and mouth, but may also cover the eyes and other vulnerable soft tissues of the face. Some gas masks are also respirators, though the word gas mask is often used to refer to military equipment (e.g. Field Protective Mask). The user of the gas mask is not protected from gas that the skin can absorb.

Airborne toxic materials may be gaseous (for example the chlorine gas used in World War I) or particulate (such as many biological agents developed for weapons such as bacteria, viruses and toxins). Many gas masks include protection from both types. Gas masks are used in construction to protect against welding fumes, in deconstruction to protect against asbestos or other hazardous particles, and in the chemical industry when handling hazardous materials, as in making repairs to leaking equipment or cleaning after spills; workers are usually issued gas masks as a precaution against leaks.

During riots where tear gas or CS-gas is employed by riot police, gas masks are commonly used by police and rioters alike. Aside from serving their functional purposes, gas masks are also used as emblems in industrial music, by graffiti taggers because the mask protects them from the graffiti canister's toxic fumes, and by Urban Explorers venturing into environments where hazardous materials, such as asbestos, may be present.