Limits are not the hierarchy.

PEL vs TLV vs REL

PEL is legal. TLV is ACGIH advisory. REL is NIOSH advisory. Meeting a PEL does not close a still-open substitute.

ppm vs mg/m³

mg/m³ = (ppm × MW) / 24.45 at 25 °C. Dusts and fibers are not ppm.

TWA / STEL / C

TWA is the shift. STEL is a short peak. C is a ceiling. Do not average a ceiling.

Dose vs exposure

Exposure is what is in the air. Dose is what the body takes. Controls sit on exposure first.