An emergency shower is a fairly basic apparatus, drenching victims with substantial quantities of water. But the equipment is rarely used in its "simplest terms."
Victims are often exposed to hazardous chemicals such as corrosives (acids and bases), oxidizers, and solvents. The object is to both remove the hazardous materials to halt further injury, and to mitigate the effects of the exposure to that point. Since contact with most hazardous materials requires at least 15 minutes of immediate and constant flushing, having the correct volume of water and a minimum water pressure available are critical.