Environmental, Occupational, and Industrial Toxicology
This domain tests exposure literacy: can you convert between exposure units, distinguish legally enforceable limits from advisory guidance, and recognise the sentinel case that signals a workplace or community problem? Questions are often quantitative and frequently hinge on whether a value is a time-weighted average, a ceiling, or an instantaneous danger threshold.
Exposure pathways and dose
Inhalation, ingestion, and dermal absorption are the main routes; for solvents, skin absorption can dominate and makes air monitoring falsely reassuring. Biomonitoring (blood, urine, exhaled breath) integrates all routes. Bioaccumulation and biomagnification explain why low environmental concentrations produce high body burdens in top predators.
Standards you must not confuse
- OSHA permissible exposure limits are legally enforceable; many are outdated and were adopted from 1968 guidance.
- ACGIH threshold limit values and NIOSH recommended exposure limits are advisory.
- IDLH is the concentration immediately dangerous to life or health, used for respirator selection, not for routine exposure assessment.
- Biological exposure indices correspond to the airborne exposure limit and, like TLVs, are guidance.
Converting concentrations
At 25 degrees Celsius: mg/m3 = ppm × molecular weight ÷ 24.45. A time-weighted average averages across the shift; a ceiling is never to be exceeded; short-term limits apply to 15 minutes.
Worked example. Toluene (MW 92) is measured at 150 ppm over an 8-hour shift. Mass concentration = 150 × 92 ÷ 24.45 ≈ 564 mg/m3. The OSHA 8-hour PEL is 200 ppm, so the time-weighted average is about 0.75 of the limit. That is not automatically acceptable: solvent mixtures are additive, task-based peaks may exceed short-term limits, and dermal uptake is not captured by air sampling. The correct answer combines the arithmetic with these caveats and recommends biological monitoring.
Sentinel events and surveillance
A cluster of young workers with unexplained pneumothorax suggests welding or silica exposure; bilateral sensorineural hearing loss with tinnitus suggests noise plus ototoxic solvent co-exposure; elevated blood lead in a family suggests take-home contamination of clothing.
Traps that cost marks
- Treating a TLV as a legal limit, or a PEL as a health-based value.
- Mixing ppm and mg/m3, or using 22.4 L/mol instead of 24.45 L/mol at ambient temperature.
- Ignoring dermal absorption, mixture additivity, and non-occupational exposure.
- Reassuring on blood lead: no threshold for lead's neurological effects has been established.