Hazard recognition and toxicology
Toxicology items on this exam are practical: given a substance, a route, a concentration and a duration, can you decide whether exposure is acceptable and what control it forces? Expect ppm-to-mg/m³ conversion, oxygen-deficiency recognition, and questions that hinge on the gap between an enforceable limit and a guideline.
Routes and dose
Inhalation reaches the blood fastest; dermal absorption is the sleeper hazard for solvents, pesticides, and amines, which is why glove selection matters as much as respirator selection. Ingestion usually rides in on hands, food, or cigarettes. Dose is concentration times time, so a short high exposure and a long low one can produce entirely different effects: acute (narcosis, burns, pulmonary edema) versus chronic (cancer, neuropathy, organ damage).
Limit vocabulary
- PEL - OSHA permissible exposure limit, enforceable, an 8-hour time-weighted average unless stated otherwise.
- STEL - 15-minute average, not to be repeated more than four times a day.
- Ceiling - a level never to be exceeded, even briefly.
- TLV and REL - ACGIH and NIOSH values: authoritative guidance, not citations.
- IDLH - conditions that pose an immediate threat to life or health or would impair escape.
Hazard classes worth memorizing
Simple asphyxiants (nitrogen, methane, carbon dioxide) displace oxygen; chemical asphyxiants (carbon monoxide, hydrogen sulfide, cyanide) interfere with oxygen transport or use. Flammability depends on flash point, vapor density, and the LEL/UEL band. Vapors heavier than air pool in trenches and basements.
Worked example
A drum label lists benzene at 10 mg/m³. Convert at 25 °C: ppm = (mg/m³ × 24.45) ÷ molecular weight = (10 × 24.45) ÷ 78.11 ≈ 3.1 ppm. Benzene's 8-hour PEL is 1 ppm, so the reading is about three times the limit. The decision is not simply 'add a respirator': benzene is a confirmed human carcinogen and is absorbed through skin, so engineering controls, dermal protection, and exposure monitoring all apply.
Traps
Quoting a TLV as a legal limit, or treating a PEL as a 15-minute value when it is an 8-hour TWA. Relying on odor as a warning - hydrogen sulfide deadens the sense of smell at the concentrations that kill. Reading '10% LEL' as 10% gas in air: for methane (LEL 5% by volume) it is 0.5% methane.