Chemical risk & exposure
From exposure data to decisions: chemical risk characterisation
Risk characterisation is the step where hazard and exposure data become a decision: carry on, add controls, or stop work.
Risk characterisation is the final, decision-making stage of a chemical risk assessment. It answers one question: given what we know about the hazard and the exposure, how much risk do workers, the public or the environment actually face?
A good characterisation delivers five things
- Acceptability: is the exposure acceptable, tolerable with monitoring, or unacceptable?
- Magnitude: is the risk low, moderate or high?
- Who is at risk: which workers or communities, in which tasks, under what conditions?
- Uncertainty: what data are missing, and what was assumed?
- Recommended actions: the controls, training, monitoring or policy changes that follow.
The Probability × Severity matrix. Most operators rate risk on a matrix:
- Severity of consequence comes from the GHS hazard classification. A Category 1A carcinogen or a corrosive that can cause permanent injury scores high; an irritant scores lower.
- Probability comes from the exposure assessment: route, duration, frequency and how well current controls work.
- The risk rating is read where the two meet. It shows whether the risk is acceptable, needs more controls, or means work must stop until controls are in place.
Example ratings
| Task | Hazard | Severity | Probability | Risk |
|---|---|---|---|---|
| Mixing drilling mud powders | Respirable silica (Carc. 1A) | High | High: daily, about 4 h a shift | High |
| Pipe painting | Solvents, isocyanates, isophorone (Carc. 2) | Moderate | Moderate: outdoors, variable ventilation | Moderate to high |
| Warehouse handling | Mixed chemicals in closed containers | Low to moderate | Low: short durations | Low |
Write a clear statement. Every characterisation should end in one plain conclusion. For example: "The activity presents a high occupational health risk. Exposure is unacceptable and requires immediate engineering and administrative controls."
Be honest about uncertainty. Statements such as "no recent air monitoring" or "PPE compliance varies between shifts" are findings in their own right, not footnotes. They often justify a monitoring programme.
Risk changes when conditions change
- Good ventilation, a closed system, job rotation or substitution all lower the risk.
- Missing PPE or a move indoors raises it.
- Reassess whenever the task, location or controls change.
What to do
- Use your organisation's approved matrix and rating definitions.
- After controls are added, record the residual risk, not just the initial rating.
- Make the final step explicit: proceed, proceed with conditions, or stop.
How INRC can help
INRC’s occupational health and safety risk assessments end in a clear conclusion and a residual risk rating, backed by exposure data from our environmental monitoring service where it is needed. Contact us to discuss your site.
This article is general guidance, not legal advice. Check current legislation and regulations for your specific project.