The Occupational Health and Safety Regulation provides that, except as otherwise determined by WorkSafeBC, an employer must ensure no worker is exposed to a substance exceeding the Threshold Limit Values (TLVs) prescribed by the American Conference of Governmental Industrial Hygienists (ACGIH). Twice a year, the ACGIH publishes a list of substances for which they have set new or revised TLVs. When WorkSafeBC adopts the new or revised ACGIH TLVs as regulatory exposure limits for chemical substances, these exposure limits are referred to as B.C. Exposure Limits (ELs). An EL is the maximum allowed airborne concentration for a chemical substance for which it is believed that nearly all workers may be exposed over a working lifetime and experience no adverse health effects. ELs may be set out as an 8-hour time-weighted average concentration, a 15-minute short-term exposure limit, or a ceiling limit.