Time-Weighted Average (TWA)

An arc flash is the sudden release of thermal energy from an electrical arc fault — a short circuit that jumps through the air between energized parts. The arc can reach temperatures over 35,000°F (nearly four times the surface of the sun), causing severe burns, igniting clothing, and driving an explosive pressure wave known as the arc blast. OSHA sets no single “arc-flash standard”: the analysis method — the arc-flash boundary, incident energy, and arc-rated PPE — comes from the NFPA 70E consensus standard, which OSHA enforces through its electrical-PPE rules (29 CFR 1910.335, 1910.269) and the General Duty Clause.

An airborne concentration of a substance — typically half the PEL — at which certain provisions of an OSHA standard kick in (such as exposure monitoring, training, and medical surveillance), even though it is below the legal exposure limit. For respirable crystalline silica the action level is 25 µg/m³ as an 8-hour TWA, half the 50 µg/m³ PEL.

Changes to how and when work is done — job rotation, scheduling, safe work practices, housekeeping, training, and signage — that reduce a worker's exposure without altering the hazard itself. They are the fourth tier of the hierarchy of controls, below engineering controls and above PPE.

An ACGIH guidance value for interpreting biological monitoring results — the concentration of a chemical (or its metabolite) measured in a worker's blood, urine, or exhaled breath. A BEI represents the level most likely found in a healthy worker whose inhalation exposure was at the TLV, so it measures the dose the body actually absorbed, not just what's in the air.

California's Safe Drinking Water and Toxic Enforcement Act of 1986. It requires businesses to warn Californians before exposing them to any of the ~900+ listed chemicals known to the state to cause cancer, birth defects, or other reproductive harm, and prohibits discharging those chemicals into drinking-water sources. It's a California law, but it reaches nearly any product sold there.

A unique numerical identifier assigned by the Chemical Abstracts Service (a division of the American Chemical Society) to every distinct chemical substance. It has the format of up to seven digits, a dash, two digits, a dash, and a final check digit (e.g., 67-64-1 for acetone). Because chemical names vary, the CAS number is the unambiguous global key used on SDSs, inventories, and regulations.

An exposure limit that must not be exceeded at any instant during the workday — the strictest type of OSHA exposure limit. In OSHA's Z-tables it is marked with a “C” before the value. Unlike an 8-hour TWA (an average) or a 15-minute STEL, a ceiling applies to a single moment.

Whether two or more chemicals can be safely stored or used near each other without reacting dangerously — releasing heat, toxic gas, or fire. Incompatible chemicals must be physically separated (segregated) in storage. Each chemical's incompatibilities are listed in Section 10 of its SDS, and compatibility drives storage layout and spill response.