For chemical agents, a measure of the maximum airborne concentration limits for toxic substances to which workers may be exposed without protection (e.g., respirators). Exposure limits are usually expressed in parts per million or milligrams per cubic meter. Material Safety Data Sheets (MSDSs) from the manufacturer or supplier of the material should list exposure limits.
API RP 2201, Safe Hot Tapping Practices in the Petroleum & Petrochemical Industries, Fifth Edition, July 2003 (Reaffirmed October 2010), Global Standards
API RP 2009, Safe Welding, Cutting, and Hot Work Practices in the Petroleum and Petrochemical Industries, Seventh Edition, February 2002 (Reaffirmed, March 2012), Global Standards