Mustard agent is a persistent weapon that remains on the ground for weeks, and it continues to cause ill effects. If mustard agent contaminates a soldier's clothing and equipment while cold, then the other soldiers or nurses with whom they share an enclosed space could become poisoned as contaminated items warms up enough to become an airborne toxic agent, and an example of this was depicted in a British and Canadian documentary about life in the trenches particularly once the "sousterrain" (subways and berthing areas underground) were completed in Belgium and France. Towards the end of World War I, mustard agent was used in high concentrations as an area-denial weapon that forced troops to abandon heavily contaminated areas.