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Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find one of our health articles more useful.

Pneumonitis is a general term for inflammation of lung tissue. Chronic inflammation of lung tissue can lead to irreversible scarring (pulmonary fibrosis). Pneumonitis is not a specific disease but a sign of an underlying problem.

Acute chemical pneumonitis causes swelling of the lung tissue, movement of fluid into the air spaces in the lung and reduced ability to absorb oxygen and remove carbon dioxide. In severe cases, death may result from hypoxia.

Chronic pneumonitis may follow low levels of exposure to the irritant over long periods of time, causing inflammation which may lead to fibrosis, resulting in decreased gas exchange and stiffening of the lung, and ultimately leading to respiratory failure and death.

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Causes of pneumonitis

Causes of pneumonitis include:

  • Pneumonia.

  • Inhalation of foreign matter, usually of stomach contents when vomiting (aspiration pneumonitis).

  • Pertussis.

  • Exposure to an inhaled allergen (hypersensitivity pneumonitis) - eg, humidifier lung, farmer's lung, bird fancier's lung.

  • Connective tissue diseases1.

  • Adverse reaction to a drug or toxic chemical; many household and industrial chemicals can cause acute and chronic pneumonitis:

    • Exposure to dangerous levels of chlorine gas may occur at home when using cleaning materials, in industrial accidents or when near to swimming pools.

    • Inhalation of dangerous substances can occur during smelting, welding or other metalwork, in the production or use of solvents or pesticides, during fires and when handling grain.

    • Medication: a variety of medications can cause interstitial pneumonitis - eg, interferon therapy, amiodarone, nitrofurantoin.

  • Radiation therapy2.

  • Sepsis: the body's inflammatory response to infection.


  • It is common, if all causes of pneumonitis are considered.

  • Between 2,000 and 4,000 new patients are diagnosed with interstitial lung disease in England each year with the majority having either sarcoidosis or idiopathic pulmonary fibrosis3.

  • Hypersensitivity pneumonitis accounts for between 4-15% of all reported cases of interstitial lung disease. The incidence is variable and depends on a variety of factors, including disease definition, geographical variation and farming practices4.

  • The risk of developing side-effects after radiotherapy is not only dependent on radiation dose but may also be affected by patient-related risk factors - eg, older age and the presence of comorbidity5.

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  • History of exposure to a precipitating cause - eg, birds, radiotherapy, dusts, drugs, chemicals.

  • Clinical features will depend on the severity and underlying cause and may include:

    • Shortness of breath.

    • Cough.

    • A burning sensation in the chest.

    • Symptoms which chronic pneumonitis may lead to: fatigue, weight loss, exercise intolerance, cyanosis and finger clubbing.

Differential diagnosis

Chronic pneumonitis may mimic other chronic lung diseases, such as idiopathic pulmonary fibrosis6. Any other cause of acute, subacute or chronic respiratory distress or cough should be considered, including infection (eg, legionellosis, Q fever, tuberculosis), sarcoidosis and lung cancer.

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  • Blood tests: FBC (may be raised neutrophilia, lymphocytosis, eosinophilia), raised ESR and CRP.

  • Blood gases: hypoxaemia.

  • Sputum or culture of lung secretions with bronchoscopy.

  • Serum precipitating antibodies against offending antigens for suspected hypersensitivity pneumonitis7.

  • CXR: may be normal or show micronodular or reticular opacities.

  • CT scan: may also be normal but far more sensitive. May show diffuse, patchy ground-glass attenuation and small, poorly defined centrilobular nodules; patchy areas of air-trapping; evidence of pulmonary fibrosis and honeycombing may be seen in chronic and advanced disease8.

  • Pulmonary function tests: spirometry usually shows restrictive changes but may be a mixed obstructive/restrictive picture. Lung gas transfer is often reduced.

  • Lung biopsy: this is occasionally required if other tests fail to establish the diagnosis.


  • Avoidance of any established precipitating cause.

  • Treatment of pneumonitis depends on the underlying cause and may include medications such as:

    • Systemic corticosteroid therapy, which can speed resolution of hypersensitivity pneumonitis.

    • Antibiotics for infection.

    • Anti-fibrotic drugs (showing promising results in the treatment of idiopathic pulmonary fibrosis)9.



  • Prognosis is good with early diagnosis and management.

  • Late-diagnosed chronic pneumonitis may lead to progressive, irreversible lung disease.


Avoid exposure to the cause - eg, control of occupational hazards, routine maintenance of heating, ventilation and air-conditioning equipment.

Further reading and references

  • Zhong L, Altan M, Shannon VR, et al; Immune-Related Adverse Events: Pneumonitis. Adv Exp Med Biol. 2020;1244:255-269. doi: 10.1007/978-3-030-41008-7_13.
  • Jain V, Berman AT; Radiation Pneumonitis: Old Problem, New Tricks. Cancers (Basel). 2018 Jul 3;10(7). pii: cancers10070222. doi: 10.3390/cancers10070222.
  1. Vivero M, Padera RF; Histopathology of lung disease in the connective tissue diseases. Rheum Dis Clin North Am. 2015 May;41(2):197-211. doi: 10.1016/j.rdc.2014.12.002. Epub 2015 Feb 27.
  2. Wright CL, Werner JD, Tran JM, et al; Radiation pneumonitis following yttrium-90 radioembolization: case report and literature review. J Vasc Interv Radiol. 2012 May;23(5):669-74. doi: 10.1016/j.jvir.2012.01.059.
  3. Service specification; NHS England, 2018
  4. Riario Sforza GG, Marinou A; Hypersensitivity pneumonitis: a complex lung disease. Clin Mol Allergy. 2017 Mar 7;15:6. doi: 10.1186/s12948-017-0062-7. eCollection 2017.
  5. Vogelius IR, Bentzen SM; A literature-based meta-analysis of clinical risk factors for development of radiation induced pneumonitis. Acta Oncol. 2012 Nov;51(8):975-83. doi: 10.3109/0284186X.2012.718093. Epub 2012 Sep 5.
  6. Costabel U, Bonella F, Guzman J; Chronic hypersensitivity pneumonitis. Clin Chest Med. 2012 Mar;33(1):151-63. doi: 10.1016/j.ccm.2011.12.004. Epub 2012 Jan 24.
  7. Lacasse Y, Girard M, Cormier Y; Recent advances in hypersensitivity pneumonitis. Chest. 2012 Jul;142(1):208-17. doi: 10.1378/chest.11-2479.
  8. Torres PP, Moreira MA, Silva DG, et al; High-resolution computed tomography and histopathological findings in hypersensitivity pneumonitis: a pictorial essay. Radiol Bras. 2016 Mar-Apr;49(2):112-6. doi: 10.1590/0100-3984.2014.0062.
  9. Sgalla G, Iovene B, Calvello M, et al; Idiopathic pulmonary fibrosis: pathogenesis and management. Respir Res. 2018 Feb 22;19(1):32. doi: 10.1186/s12931-018-0730-2.

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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