The UK occupational exposure limits used to protect the health of employees. The exposure limits are defined as the maximum concentration of an airborne hazardous substance averaged over a defined period of time to which workers may be exposed by inhalation and that should not cause adverse health effects. The short-term exposure limit (STEL) is averaged over fifteen minutes while the time weighted average (TWA) is averaged over eight hours. STELs are aimed to help prevent effects such as eye irritation that may occur following exposure for only a few minutes.