I85PREP
⌘ K
COSHEM study materials
COSHEM study resources

COSHEM: Chemical and biological hazards

COSHEM · Work through your exam, one topic at a time.

All guide sections

Chemical and biological hazards

What this domain tests

This domain - chemical and biological hazards - tests exposure routes, occupational exposure limits, control of substances and biological agents, and the use of SDS and monitoring. Questions often require a TWA calculation or identification of the weakest control.

Core rules

  • Routes. Inhalation, skin absorption, ingestion and injection. Skin and ingestion are easily missed when a substance has a low vapour pressure but is toxic by contact.
  • Exposure limits. An 8-hour time-weighted average (TWA) controls chronic exposure; a 15-minute short-term exposure limit (STEL) controls peaks; a ceiling limit must never be exceeded. Limits differ between countries and are not interchangeable.
  • Safety data sheets. Sixteen sections cover identification, hazards, composition, first aid, firefighting, accidental release, handling, exposure controls, stability, toxicology and disposal. The SDS supports assessment but does not replace it.
  • Controls. Eliminate or substitute the substance first, then enclose or use local exhaust ventilation, then administrative controls and RPE. LEV needs commissioning, airflow checks and maintenance.
  • Biological agents. Bloodborne viruses, bacteria, fungi and prions require containment, safe sharps, disinfection, vaccination where justified, and exposure incident protocols. Biosafety levels reflect agent risk and intended work.
  • Health surveillance. Use where exposure can cause identifiable disease and valid techniques exist, for example solvent-related neurotoxicity or sensitisation.

Worked example

A worker is exposed to solvent X, TWA limit 100 ppm, for 4 hours at 40 ppm, 2 hours at 80 ppm and 2 hours at background 0 ppm. The 8-hour TWA is (4x40 + 2x80 + 2x0) / 8 = (160 + 160) / 8 = 40 ppm, which is 40 percent of the limit. That looks acceptable, but the assessor must still check the 15-minute STEL, skin absorption and peak tasks such as decanting. If a 15-minute sample reaches 150 ppm against a STEL of 100 ppm, the task fails even though the TWA passes.

Traps that cost marks

  • Averaging concentrations without weighting by time.
  • Treating a STEL as an 8-hour limit or ignoring ceiling values.
  • Mixing ppm and mg/m3 without conversion.
  • Relying on odour as a warning property.
  • Putting RPE first and leaving the source uncontrolled.
  • Missing skin absorption and ingestion routes.