occupational asthma
Contents
Two distinct entities: sensitiser-induced (true occupational asthma — new disease caused by work) vs work-exacerbated (pre-existing asthma worsened by workplace irritants). The distinction has major implications for prognosis, management, and medicolegal outcome. Early removal from exposure is the most important intervention for sensitiser-induced disease.
classification
| Sensitiser-induced OA | Irritant-induced OA | Work-exacerbated asthma | |
|---|---|---|---|
| Mechanism | Immunological sensitisation (latency period) | High-level irritant exposure → airway injury (spectrum from classic RADS to chronic low-dose irritant exposure) | Pre-existing asthma worsened by workplace triggers |
| Latency | Weeks to years of exposure before symptom onset | Classic RADS: onset within minutes to hours of single massive exposure; may also occur after multiple/chronic irritant exposures | N/A |
| Pre-existing asthma | No (new disease) | No | Yes |
| Prognosis with removal | May improve or resolve; earlier removal → better outcomes | Often persistent (symptoms ≥ 3 months by definition in classic RADS) | Improves but underlying asthma remains |
| Compensation (Canada) | Compensable in all provinces | Compensable | Varies by province — may be partially compensable in some jurisdictions (e.g. Ontario WSIB) |
Classic RADS is the best-recognised form of irritant-induced OA, defined by onset within minutes to hours of a single massive irritant exposure with symptoms persisting ≥ 3 months. However, current evidence recognises a broader spectrum of irritant-induced asthma, including disease from repeated moderate exposures or chronic low-dose irritant inhalation. Tarlo, N Engl J Med. 2014 Cormier, Int J Tuberc Lung Dis. 2020
common sensitisers
| Agent | Occupation |
|---|---|
| Isocyanates (TDI, MDI, HDI) | Spray painters, foam manufacturing, insulation workers, auto body |
| Flour / grain dust | Bakers, millers |
| Wood dust (western red cedar) | Carpenters, sawmill workers |
| Latex | Healthcare workers |
| Laboratory animals | Research technicians |
| Enzymes (subtilisins) | Detergent manufacturing |
| Metals (platinum salts, chromium) | Welders, platers |
| Persulfates | Hairdressers |
The most common cause of occupational asthma in industrialised countries (> 400 agents identified overall). Exposure can occur even at very low concentrations. Ask about spray painting, foam manufacturing, and insulation work.
when to suspect
- Asthma onset as an adult with no prior history (especially if atopic) — 5–20% of new adult-onset asthma is attributable to occupational exposure
- Occupational rhinitis preceding asthma symptoms (may precede OA by up to a year — an early warning sign)
- Symptoms improve on weekends, holidays, or time away from work
- Symptoms worsen during or shortly after work shifts
- Multiple co-workers with similar symptoms
- Exposure to known high-risk agents
diagnosis
Step 1: Confirm asthma — objective evidence of variable airflow limitation (spirometry, methacholine challenge)
Step 2: Establish work-relatedness
| Test | Method | Notes |
|---|---|---|
| Serial PEF monitoring | ≥ 4 readings/day for ≥ 2 weeks at work and ≥ 2 weeks away | Gold standard for work-related variability; OASYS score ≥ 2.5 diagnostic; ABC score ≥ 15 L/min/h has 100% specificity, 72% sensitivity Moore, Chest. 2009; requires patient compliance |
| Methacholine challenge | At work vs after ≥ 2 weeks away | PC20 worsens at work, improves away; ≥ 3.2-fold improvement in PC20 off work supports OA |
| Specific IgE / skin prick | For high-molecular-weight agents (proteins, latex) | Supports sensitisation; does not confirm causation |
| Specific inhalation challenge (SIC) | Controlled exposure in specialised lab | Gold standard for confirming specific agent; limited availability |
Step 3: Identify the causative agent — occupational hygiene assessment, SDS (safety data sheet) review
management
sensitiser-induced OA
- Complete removal from exposure — the earlier the better. Continued exposure leads to progressive, irreversible decline. Complete removal is superior to reduction of exposure, particularly for low-molecular-weight agents Henneberger, Cochrane. 2019 Henneberger, Am J Ind Med. 2021
- Pharmacological treatment same as general asthma guidelines
- Workers’ compensation referral — document diagnosis thoroughly
- Monitor after removal: improvement may take months to years; up to 70% have persistent asthma despite removal, but outcomes are significantly better with earlier diagnosis and removal compared to continued exposure Tarlo, N Engl J Med. 2014
- Retraining / job modification if same workplace
work-exacerbated asthma
- Optimise asthma treatment per asthma guidelines
- Reduce workplace irritant exposure (engineering controls, PPE)
- Complete removal from work usually not necessary
- May benefit from pre-shift bronchodilator use in some cases
workers’ compensation in Canada
Workers’ compensation in Canada is administered provincially — each province has its own workers’ compensation board (e.g. WSIB in Ontario, WorkSafeBC, CNESST in Québec). Key points:
- Sensitiser-induced OA and irritant-induced OA (RADS) are recognised as compensable occupational diseases in all Canadian provinces
- Work-exacerbated asthma — compensability varies by jurisdiction. Some provinces (e.g. Ontario) may provide partial compensation for aggravation of a pre-existing condition; others may not
- Physician reporting of occupational disease is mandatory in most provinces
- Thorough documentation of exposure history, objective diagnostic evidence, and temporal relationship to work is essential for successful claims
- Specialist referral to an occupational medicine or respirology clinic with expertise in OA strengthens claims
what NOT to do
- Label all work-related symptoms as “occupational asthma” — work-exacerbated asthma is far more common and managed differently
- Diagnose OA without objective evidence (serial PEF, methacholine, or SIC)
- Delay removal from exposure in confirmed sensitiser-induced OA — prognosis worsens with continued exposure
- Forget to notify public health/workplace safety where mandated
- Assume normal spirometry excludes OA — spirometry may be normal between exposures; methacholine challenge or serial PEF is needed
- Assume reduction of exposure is equivalent to complete removal for sensitiser-induced OA — complete removal is superior, particularly for low-molecular-weight agents