How to Conduct a COSHH Risk Assessment

In almost every workplace, employees may encounter substances that pose a risk to health, whether it's cleaning chemicals, dust from manufacturing, fumes from welding, or biological agents. Understanding and managing these risks is not just good practice; it’s a legal requirement under the Control of Substances Hazardous to Health (COSHH) Regulations 2002 and the accompanied L5 ACoP.

COSHH stands for the Control of Substances Hazardous to Health. It is a set of regulations enforced in the UK that requires employers to control exposure to hazardous substances in order to prevent ill health. Under COSHH, "hazardous substances" include:

• Chemicals, fumes, dusts, vapours, mists, and gases
• Nanomaterials
• Biological agents (e.g., bacteria, viruses)
• Substances with a Workplace Exposure Limit (WEL)
• Mixtures of substances with harmful effects

It does not cover asbestos, lead, or radioactive substances, as these are regulated separately.

Exposure to hazardous substances can cause short-term effects like skin irritation or headaches, or more serious long-term conditions such as asthma, cancer, and neurological damage. COSHH helps ensure:

• A safe working environment
• Protection of vulnerable groups (children, older adults, pregnant workers)
• Compliance with legal duties
• Prevention of costly incidents, sick leave, and legal action

The assessor should have a basic understanding of the regulations, be able to systematically gather relevant exposure information and specify suitable types of control measures. The substances likely to be present and their hazardous nature should be identified from supplier data, labels, official sources, previous experience/knowledge. Consideration must be made of how the substances might get into the body and cause harm. What harm can the substances cause? Are the effects serious? Delayed? Who might be exposed and how?

• Identify all hazardous substances in use
• Safety Data Sheets (SDS)
• Product labels
• Work processes (e.g., cutting, welding, spraying)
• Understand physical forms (vapours, gas, mist, dust, fibres, liquid)
• Note quantities, concentrations, and frequency of use
• Determine who is exposed and how (special considerations for pregnant or nursing immunocompromised, older or younger than average, asthmatic or allergic)
• Assess how substances can enter the body (inhalation, absorption, ingestion, injection)

This can usually be done for groups of employees with similar work patterns rather than each individual. The factors in Step 1 should be considered, and the likelihood of exposure can be determined by considering how closely someone works with the substance. The frequency of exposure can be judged from knowledge of the type of work.  The level and duration of exposure can be determined from knowledge of the work and observation/experience of those doing the work. Sample measurements may need to be taken to estimate levels, and in some cases, detailed measurements may be needed. Does potential exposure present a significant risk to health or is further action needed?

• Assess the severity and type of health risks (acute vs chronic)
• Consider exposure duration and frequency
• Measure or estimate exposure levels (using monitoring or sampling if needed)
• Take special note of carcinogens, asthmagens, or mutagens

A control hierarchy should be used, and all involved should know how to use them and maintain them. There may be a need for routine monitoring of exposure and health surveillance; make sure that employees have sufficient information, instruction and training

Use the hierarchy of control:

• Elimination – Can the substance be removed altogether?
• Substitution – Can a less harmful alternative be used?
• Engineering controls – e.g., local exhaust ventilation
• Administrative controls – e.g., rotating staff to limit exposure time
• Personal protective equipment (PPE) – as a last resort

Ensure control measures are:

• Suitable and sufficient
• Maintained regularly
• Known and understood by staff

For employers with five or more employees, recording the risk assessment is a legal requirement. Still, it's good practice for all employers to document:

• Identified hazards
• Risks evaluated
• Controls in place
• Monitoring and training plans

Reassess if:

• Work processes or substances change
• New health data or SDS updates emerge
• Exposure monitoring shows unacceptable levels
• An incident occurs

Measuring the concentration of contaminants in the air is crucial for assessing exposure and ensuring it remains within safe limits. Concentrations help compare actual exposure levels against health-based standards.

• For gases and vapours, concentration is typically measured in parts per million (ppm) or parts per billion (ppb).
  • 1 ppm means 1 part of gas per 1 million parts of air—roughly equivalent to one party balloon's worth of gas in 50 three-bedroom houses.
• For solids like dust or particles, concentration is measured in milligrams per cubic metre (mg/m³).
  • 1 mg/m³ is about the same as one teaspoon of dust spread over a football field one metre deep.

Workplace Exposure Limits (WELs) are legal limits on the concentration of hazardous substances in workplace air. They are defined in the HSE’s EH40 document and apply over two timeframes:

• LTEL (Long-Term Exposure Limit): Averaged over 8 hours
• STEL (Short-Term Exposure Limit): Averaged over 15 minutes

To assess exposure:

• Multiply the concentration by the time exposed (in hours)
• Divide by the reference period (8 hours for LTEL, 0.25 hours for STEL)

Example 1 – LTEL

• Exposure: 0.15 mg/m³ for 8 hours
• Calculation: (0.15 × 8) ÷ 8 = 0.15 mg/m³

Example 2 – Varying Exposure Time

• Exposure: 0.12 mg/m³ for 7h 20m
• Converted time: 7.33 hours
• Calculation: (0.12 × 7.33) ÷ 8 = 0.11 mg/m³

Example 3 – STEL

• Exposure: 0.68 mg/m³ for 15 minutes (0.25 hours)
• Calculation: (0.68 × 0.25) ÷ 0.25 = 0.68 mg/m³

If no STEL is listed for a substance, assume it is three times the LTEL.

These calculations help compare real exposure levels against legal limits to ensure worker safety

Chemical mixtures do not behave in a simple or predictable (linear) way. Their combined effects can fall into four main categories:

• Synergistic Effect – The combined effect is greater than the sum of the individual effects.
  Example: Smoking and asbestos together greatly increase the risk of lung cancer beyond their individual risks.
• Antagonistic Effect – One chemical reduces or counteracts the effect of another.
  Example: Caffeine (a stimulant) and alcohol (a depressant) may oppose each other’s effects on the nervous system.
• Additive Effect – The combined effect equals the sum of individual effects.
  Example: Some volatile organic compounds (VOCs) have additive narcotic effects.
• Potentiation Effect – One chemical with no effect on its own increases the effect of another chemical.
  Example: Isopropanol enhances the liver toxicity of carbon tetrachloride.

Always assess potential interactions.

• Acute vs Chronic Effects
  Acute effects are immediate, short-term reactions to a single high-level exposure (e.g., headaches from paint fumes) and are often reversible. Chronic effects result from long-term, repeated exposure and develop slowly over time (e.g., lung cancer from radon). These are often irreversible. 
• Asthmagens
  Substances that trigger or cause occupational asthma by irritating the airways, leading to inflammation, narrowing, and increased mucus production. Common triggers include dust, chemicals, and biological allergens.
• Mutagenic Effects
  Caused by substances (mutagens) that damage genetic material (DNA), potentially leading to heritable changes. Mutagens can also have carcinogenic or teratogenic effects.
• Carcinogenic Effects
  These substances can cause cancer after prolonged exposure. They are classified by their interaction with DNA (genotoxic) or by stimulating abnormal tissue growth (epigenetic). The UN classifies them as:
  • Category 1A: Known to be carcinogenic
  • Category 1B: Presumed to be carcinogenic
  • Category 2: Suspected to be carcinogenic

• Teratogenic Effects
  Substances that cause birth defects by affecting embryo or foetal development, even when the mother shows no signs of harm. Effects can occur at very low exposure levels.
• Skin Absorption (‘Sk’)
  Some chemicals can be absorbed through the skin, contributing to overall exposure. 'Sk' designation highlights the need to control skin contact, not just airborne levels.
• Sensitisation (‘Sen’)
  Some substances can cause individuals to develop an allergy (either respiratory or skin-based). Once sensitised, individuals may react severely to even very low levels of exposure.

COSHH is not just a compliance tick-box—it’s a proactive framework for safeguarding the health of workers across industries. By systematically identifying hazards, evaluating risks, and applying effective controls, you can create a safer, healthier work environment for everyone.

Start your COSHH risk assessment today by reviewing your inventory, updating your SDS library, and engaging employees in health and safety conversations.