Occupational exposure limits
- TWA (Time-Weighted Average). The average concentration a worker may be exposed to over a normal 8-hour workday, 40-hour week. Set by OSHA (PEL), NIOSH (REL), ACGIH (TLV-TWA) in the US; by SCOEL / IOELV in the EU.
- STEL (Short-Term Exposure Limit). A 15-minute average that must never be exceeded, even if the 8-hour TWA is fine.
- Ceiling (C). A concentration that must never be exceeded at any moment.
- IDLH (Immediately Dangerous to Life or Health). A NIOSH concept: the maximum concentration from which a worker could escape within 30 minutes without irreversible health effects.
ppm vs mg/m³
Vapours and gases are usually expressed in ppm (parts per million by volume). Dusts, mists and fumes are expressed in mg/m³. For a gas at 25 °C and 1 atm the conversion is mg/m³ = ppm × (molar mass / 24.45). Our ppm converter does this automatically.
PPE selection logic
- Engineering controls first (fume hood, local exhaust, closed system).
- Administrative controls (rotation, training, restricted access).
- PPE last. Never the first line of defence.
For gloves, Section 8 should specify a glove material (nitrile, butyl, Viton, PVA...) and a breakthrough time. A generic "wear gloves" line is not compliant.
Respiratory protection
Section 8 lists a cartridge or filter type (A, B, E, K, P for the EU EN 14387 series; OV, AG, HE etc. for NIOSH). Assigned Protection Factor (APF) determines whether a half-mask, full-face or supplied-air respirator is required.