Occupational Asthma

Occupational asthma is asthma caused by exposure to a specific agent at work. It accounts for 9 to 15% of adult-onset asthma. Work-exacerbated asthma is pre-existing asthma made worse by work, and it is more frequent than the true occupational form.

Two main types

• Sensitizer-induced (over 90%): a latency period of weeks to years precedes symptoms. High molecular weight agents (flour, enzymes, latex, animal and grain proteins) act through IgE. Low molecular weight agents (isocyanates, acid anhydrides, persulfate salts, colophony, western red cedar dust, platinum salts, glutaraldehyde) act mainly by non-IgE mechanisms.
• Irritant-induced: onset follows a single large exposure or repeated high exposures, with no latency.

High-risk jobs are bakers, spray painters, healthcare workers, cleaners, hairdressers, farmers, woodworkers and laboratory animal handlers. Atopy and smoking raise the risk for high molecular weight agents.

The outlook depends on how long exposure continues after symptoms begin. Early removal (within 1 year of onset of symptoms) gives the best chance of recovery, but about 70% still have asthma years after leaving the job, and continued exposure leads to fixed airflow obstruction. Complications: persistent asthma, severe exacerbations, job loss and loss of income.

• Cough, wheeze, chest tightness and dyspnea that begin or worsen at work, and improve on days off and holidays
• Delayed pattern with symptoms at night after the shift
• Rhinoconjunctivitis that precedes asthma by months (especially with high molecular weight agents)
• Chest examination often normal between episodes
• Red flags: nocturnal worsening, symptoms in coworkers, frequent exacerbations, and a patient who works with a known sensitizer

1. Confirm asthma: spirometry with bronchodilator reversibility (rise in FEV1 of 12% and 200 mL or more), or methacholine challenge if spirometry is normal.
2. Serial peak expiratory flow: at least 4 readings a day for 3 weeks, covering at least 2 weeks at work and 1 week off (sensitivity about 80%, specificity about 90% when analyzed by computer-assisted programs). Readings should show a fall at work and recovery off work.
3. Immunological tests for high molecular weight agents: skin prick test or specific IgE (a negative test makes IgE-mediated disease unlikely).
4. Specific inhalation challenge in a controlled laboratory, the reference standard: a fall in FEV1 of 15% or more after exposure. It is used when the diagnosis is uncertain and the decision carries major consequences.
5. Supporting tests: sputum eosinophil count before and after work exposure, fractional exhaled nitric oxide (a rise of about 17 ppb after exposure), and methacholine hyperresponsiveness at work versus off work.
6. Workplace assessment: the safety data sheets of agents handled and the process involved.

Differentials: non-occupational asthma, work-exacerbated asthma, irritant-induced asthma, hypersensitivity pneumonitis, chronic obstructive pulmonary disease, vocal cord dysfunction and gastro-esophageal reflux.

1. Removal from exposure is the central treatment. For sensitizer-induced disease, complete avoidance gives the best outcome. Reduction of exposure with respirators or ventilation helps less and risks progression.
2. Inhaled therapy as for asthma of any cause:
   • Budesonide-formoterol 200/6 micrograms, 1 inhalation twice daily as maintenance, with 1 to 2 additional inhalations as needed (maximum 12 a day).
   • Step up to budesonide-formoterol 400/12 micrograms twice daily for poor control, adding tiotropium 5 micrograms once daily.
   • Salbutamol 100 micrograms, 2 puffs by spacer as reliever.
3. Exacerbation: prednisolone 40 mg orally once daily for 5 days (children 1 mg/kg, maximum 40 mg), and salbutamol 2.5 to 5 mg nebulized every 20 minutes for 3 doses.
4. Associated rhinitis: fluticasone furoate nasal spray 27.5 micrograms, 2 sprays in each nostril once daily.
5. Severe eosinophilic or allergic asthma despite exposure removal: omalizumab 150 to 375 mg SC every 2 to 4 weeks (dose by weight and IgE), or mepolizumab 100 mg SC every 4 weeks.
6. Workplace controls: substitution of the agent, enclosure, local exhaust ventilation, and respirators only as a last resort. Medical surveillance with questionnaire and spirometry at hiring, at 3 to 6 months and then yearly for exposed workers.
7. Notification and compensation: report the case to the occupational health authority, and complete the necessary documentation. Retraining and redeployment support.
8. Pregnancy: budesonide is the preferred inhaled steroid, and asthma control should be maintained.

Follow-up at 1 and 3 months, then every 6 to 12 months with spirometry. Counsel on smoking cessation and on the importance of leaving the exposure early.