What is humidifier lung, also known as hypersensitivity pneumonitis (HP)?

From the Guidelines

Humidifier lung, also known as hypersensitivity pneumonitis caused by contaminated humidifiers, is an inflammatory lung condition that requires immediate removal of the contaminated humidifier and treatment with oral corticosteroids in moderate to severe cases. The condition occurs when microorganisms like bacteria, fungi, or amoebae grow in the standing water of humidifiers, and their particles become aerosolized, leading to symptoms such as cough, shortness of breath, fever, and malaise that develop 4-12 hours after exposure 1.

Key Considerations

• The diagnosis of hypersensitivity pneumonitis (HP) relies on a thorough clinical history of exposures, and the identification of a compelling exposure is crucial in reducing diagnostic uncertainty 1.
• The presence or absence of lung fibrosis provides important prognostic information, and a time-based classification scheme is inferior to the identification of the presence or absence of fibrosis as a prognostic marker 1.
• High-resolution computed tomography (HRCT) of the chest is essential in classifying the disease as fibrotic or nonfibrotic, with HRCT findings indicative of lung fibrosis including reticular abnormality, ground-glass opacity, traction bronchiectasis, honeycombing, and loss of lobar volume 1.

Treatment and Prevention

• Treatment involves immediate removal of the contaminated humidifier and, in moderate to severe cases, oral corticosteroids such as prednisone at 0.5-1 mg/kg/day for 1-2 weeks, followed by a gradual taper over 2-4 weeks 1.
• Prevention is crucial and involves regular cleaning of humidifiers with vinegar or hydrogen peroxide solutions every 3 days, complete drying between uses, and using distilled water rather than tap water 1.
• Most patients recover completely with proper treatment and removal of the exposure source, though some may develop chronic lung disease if exposure continues or treatment is delayed 1.

From the Research

Humidifier Lung Characteristics

• Humidifier lung is a rare phenotype of hypersensitive pneumonitis that often occurs during dry winter when the use of humidifiers increases 2
• The elevation of serum KL-6 levels in humidifier lung patients is mild, and granulomas are not apparent on histological examination 3
• The mechanism driven by the high concentration of endotoxin could be one of the main causes of humidifier lung 3

Diagnosis and Symptoms

• Diagnosis of humidifier lung can be established through detailed medical history taking, including the use of contaminated humidifiers 2
• Symptoms of humidifier lung include fever, cough, hypoxemia, and late inspiratory crackles in both lungs on physical examination 3
• Chest computed tomography may show diffuse ground-glass opacities, and histological examination of a transbronchial lung biopsy may show alveolitis without granulomas 3

Prognostic Factors

• Clinical and radiologic findings are related to the risks of lung transplantation and mortality of humidifier disinfectant-associated lung injury 4
• Young age, peripartum, and low O2 saturation are factors associated with ICU admission 4
• Consolidation, pneumomediastinum, and pulmonary interstitial emphysema are short-term prognostic CT findings 4

Causative Antigens

• Humidifier water can be contaminated by various bacteria and fungi, as well as Mycobacterium gordonae and a high concentration of endotoxin 3
• Microbiological studies have revealed a variety of fungi and bacteria in the water supplies of humidifiers and air conditioners 5
• Patients with humidifier lung may demonstrate high concentrations of IgG antibodies against extracts prepared from water of patients' humidifier systems 5