Occupational Asthma — Protecting Trades from Respiratory Sensitisers (2026)

What Occupational Asthma Is — and Why It Matters

Occupational asthma develops when the airways become sensitised to a substance breathed in at work. After a period of exposure — sometimes weeks, sometimes years — the immune system overreacts and the airways become inflamed. From that point on, even tiny amounts of the substance can trigger an attack: wheezing, breathlessness, chest tightness and coughing. It is one of the most common reportable occupational lung diseases in the UK.

The critical point for any employer or self-employed tradesperson to understand is that sensitisation is usually irreversible. There is no "safe level" for a sensitised worker — exposures far below the legal limit can still cause a severe reaction. Many people who develop occupational asthma have to leave the trade entirely. That is why prevention, not management, is the only sensible strategy.

Occupational asthma is also RIDDOR-reportable. Under the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013, when a worker is diagnosed with occupational asthma and a doctor confirms it is linked to their work, the employer must report it to the HSE. A pattern of reports — or a single serious case — can trigger an HSE inspection of your control measures.

The Main Respiratory Sensitisers Trades Meet

Different trades face different sensitisers. Knowing which ones apply to your work is the first step in controlling them.

Isocyanates — the biggest single cause

Isocyanates are the most common cause of occupational asthma in Great Britain. They are found in two-pack (2K) spray paints and lacquers, some adhesives, and polyurethane (PU) foams. The highest-risk group is vehicle and spray painters using 2K products — every time a 2K paint or lacquer is sprayed, an isocyanate mist is released into the air. Even brief, low-level exposure during spraying, mixing or cleaning can sensitise a worker.

Wood dust

Wood dust is a sensitiser as well as a known carcinogen. Hardwood dust and dust from manufactured boards such as MDF are particular concerns for joiners and carpenters. Sanding, sawing, routing and machining all generate fine dust that lodges deep in the airways. Repeated exposure can lead to both occupational asthma and, over the long term, nasal cancer.

Flux fumes from soldering (colophony / rosin)

Rosin-based solder flux — colophony — gives off fume when heated. Electricians, plumbers and anyone doing electronics assembly or pipework soldering can be exposed. Colophony fume is a well-established cause of occupational asthma, and the fine plume rises straight into the breathing zone of the person bent over the work.

Resins, epoxies and flour

Some two-part resins and epoxy hardeners are respiratory sensitisers, relevant to floor layers, fibreglass and resin-flooring installers. Flour dust is another recognised cause, mainly affecting bakery work but worth noting where dusty powdered materials are handled.

Welding fume

Welding fume is now treated by the HSE as a substance that can cause cancer and respiratory disease, and it can trigger or worsen asthma. Since the HSE strengthened its enforcement position, suitable exposure control is expected for all welding indoors and outdoors. Stainless steel and other alloys produce particularly hazardous fume.

The common thread is sensitisation: once a worker reacts to a substance, even trace exposure triggers attacks. That is why prevention is essential and why "just wear a mask" is never a complete answer.

Your COSHH Duties

All of these substances are covered by the Control of Substances Hazardous to Health Regulations 2002 (COSHH). COSHH requires you to assess the risk and then control exposure — and it sets out the order in which controls must be applied. You cannot jump straight to handing out masks.

1. Assess the risk

Start with a COSHH assessment for each hazardous substance and process. Identify who is exposed, how, and for how long. Use the safety data sheet for every product — it will tell you whether a substance is a respiratory sensitiser. Keep these assessments current and review them when products or methods change.

2. Eliminate or substitute

The most effective control is to remove the hazard. Where you can, substitute a safer product: water-based products instead of solvent or 2K systems, lower-isocyanate or isocyanate-free coatings, no-clean or low-colophony flux, and alternative materials where the job allows. Substitution removes the risk at source for everyone, not just the person wearing protection.

3. Engineering controls

Where you cannot eliminate the substance, control it at source with engineering controls:

• Local exhaust ventilation (LEV) on tools and benches to capture dust and fume before it reaches the breathing zone
• Spray booths with proper extraction and, for 2K spraying, breathing-air systems
• On-tool dust extraction for woodworking machines, sanders and saws
• Fume extraction at the point of soldering and welding

4. Safe systems of work

Back up engineering controls with safe working practices: restricting who enters spray areas during and immediately after spraying, allowing clearance times before re-entry, and keeping non-essential workers away from the process.

5. Suitable RPE — the last line

Respiratory protective equipment (RPE) is the last line of defence, not the first. When it is needed, it must be the correct grade for the hazard: air-fed (abrasive-blast style) breathing apparatus for 2K spraying, and FFP3 masks for dust. All tight-fitting RPE must be face-fit tested to the individual wearer, and clean-shaven faces are required for a proper seal. RPE only works if it is the right type, fitted correctly and maintained.

LEV Examination and Testing (TExT)

Local exhaust ventilation is only useful if it works — and the law requires you to prove it does. LEV systems must undergo a thorough examination and test (TExT) at least every 14 months, carried out by a competent person. The test checks that the system is capturing contaminant effectively and that air flow rates meet the design specification. Keep the test records: an HSE inspector will ask for them, and a failed or missing TExT is a common enforcement finding.

Workplace Exposure Limits and Housekeeping

Many of these substances have a workplace exposure limit (WEL) — the maximum concentration of an airborne substance, averaged over a reference period, that a worker can be exposed to. For sensitisers, though, keeping below the WEL is not enough on its own, because a sensitised worker can react well below the limit. The duty is to reduce exposure to as low as is reasonably practicable.

Good housekeeping is part of control. The single most important rule for wood dust is: never dry sweep. Dry sweeping and using compressed air to blow down throw fine dust back into the air, where it is breathed in. Use on-tool extraction and an industrial vacuum (Class M or H) to clean up instead. The same discipline — capture at source, vacuum rather than disturb — applies across dusty and fume-generating work.

Health Surveillance — Catching It Early

Where workers are exposed to respiratory sensitisers, COSHH requires health surveillance. The purpose is to catch early signs of sensitisation before they develop into permanent, disabling asthma. For respiratory sensitisers this typically involves:

• A respiratory questionnaire on starting work and at regular intervals
• Lung-function testing (spirometry) for exposed workers
• Referral to an occupational health professional if symptoms appear

Health surveillance also gives you a record that controls are working — and an early warning if they are not. A cluster of symptoms across a team is a sign that exposure is not being controlled and that you need to revisit the hierarchy.

Training, Information and Acting Early

Workers need to know which substances they handle are sensitisers, what the symptoms of occupational asthma are, and how to use the controls and RPE provided. Training and information are COSHH duties in their own right.

Act early on symptoms. The classic warning sign of occupational asthma is wheezing, breathlessness or chest tightness that gets worse on workdays and improves on days off or holidays. If a worker reports this pattern, treat it seriously: review their exposure, refer them to occupational health, and do not wait for it to get worse. Catching sensitisation early — and removing the person from exposure — can be the difference between a manageable issue and a permanent, career-ending condition.

Quick Reference: Sensitisers, Sources and Controls