Humidifier Lung

Humidifier lung is a form of hypersensitivity pneumonitis caused by inhaling aerosols from contaminated humidifiers, air conditioners, ventilation systems and heating systems. It differs from other forms of the disease through its source: stagnant water in reservoirs supports growth of microorganisms that become airborne in the mist.

Antigens include:

• thermophilic actinomycetes (Thermoactinomyces vulgaris)
• fungi (Aspergillus, Penicillium, Cladosporium)
• Gram-negative bacteria
• amoebae
• endotoxin

Disease occurs in office workers, printing workers, textile workers and home users.

Presentation depends on the pattern of exposure

• Acute: influenza-like illness with cough and dyspnea 4 to 8 hours after exposure, resolving in 12 to 48 hours
• Subacute: dyspnea and cough over weeks
• Chronic: insidious dyspnea, weight loss and lung fibrosis

A separate and serious entity, humidifier disinfectant lung injury, arose when household humidifier disinfectants containing polyhexamethylene guanidine and chloromethylisothiazolinone were inhaled. It causes rapidly progressive fibrosis in children, young women and pregnant women, and carries high mortality. Humidifier fever is a milder syndrome of fever, malaise and myalgia without radiological change, and it settles within 24 hours.

Outcome is good if exposure ends early. Continued exposure leads to fibrosis, which is irreversible. Complications: pulmonary fibrosis, respiratory failure and pulmonary hypertension.

• Fever, chills, malaise, cough, chest tightness and dyspnea, starting hours after exposure, and improving on weekends away from work
• Bibasal inspiratory crackles and sometimes wheeze
• Weight loss, fatigue and clubbing in chronic disease
• Symptoms in other occupants of the same building
• Red flags: progressive dyspnea, hypoxemia, weight loss, clubbing and exposure to disinfectant additives

1. Exposure history: humidifier or air conditioning type, water maintenance, and any additives. Inspect the equipment and culture the water and aerosol for bacteria, fungi and amoebae.
2. High-resolution CT of the chest: ground-glass opacities, poorly defined centrilobular nodules, mosaic attenuation with air trapping on expiratory images, and fibrosis (traction bronchiectasis and honeycombing) in chronic disease.
3. Pulmonary function tests: restrictive pattern with reduced diffusing capacity for carbon monoxide, and desaturation on exercise.
4. Serum precipitating IgG to the suspected antigens (supports exposure, not diagnostic alone).
5. Bronchoalveolar lavage: lymphocytosis of over 30 to 40% with a low CD4 to CD8 ratio.
6. Surgical or transbronchial cryobiopsy when the diagnosis is uncertain: poorly formed non-necrotizing granulomas, cellular bronchiolitis and interstitial infiltrates.
7. Full blood count and inflammatory markers, and workplace investigation.

Differentials: other hypersensitivity pneumonitis, infection, organic dust toxic syndrome, sarcoidosis, drug-induced lung disease and idiopathic pulmonary fibrosis.

1. Eliminate exposure: drain, disinfect or discard the humidifier, replace ultrasonic and cool-mist units with steam units or avoid humidifiers, clean air conditioning systems, use distilled water and change water daily, and never add disinfectant products to a humidifier.
2. Corticosteroid for symptomatic or progressive disease: prednisolone 0.5 mg/kg orally once daily (usually 30 to 40 mg) for 1 to 2 weeks, then tapering over 4 to 6 weeks, with a longer course (3 to 6 months) for subacute or chronic disease. Give omeprazole 20 mg once daily, calcium and vitamin D.
3. Prophylaxis during prolonged steroid treatment: trimethoprim-sulfamethoxazole 480 mg orally once daily.
4. Fibrotic or progressive disease:
   • Mycophenolate mofetil 500 mg orally twice daily, increasing to 1 g twice daily, or azathioprine 1 mg/kg once daily, increasing to 2 mg/kg (check thiopurine methyltransferase).
   • Nintedanib 150 mg orally twice daily with food for progressive pulmonary fibrosis (monitor liver tests and diarrhea).
   • Monitoring of blood count and liver tests every 2 weeks at first, then monthly.
5. Oxygen therapy for resting or exertional hypoxemia, and pulmonary rehabilitation.
6. Humidifier disinfectant lung injury: treat with high-dose steroids (methylprednisolone 1 g IV daily for 3 days, then prednisolone 1 mg/kg orally), immunosuppression, and lung transplantation for end-stage disease.
7. Humidifier fever: paracetamol 1 g orally every 6 hours, rest and exposure removal.
8. Vaccination against influenza and pneumococcus.

Follow-up with spirometry, diffusing capacity and imaging at 4 to 6 weeks, 3 months and 6 months. Counsel on equipment cleaning, and on early return of symptoms.