# Air trapping

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Abnormal retention of air in the lungs

A CT image showing bronchiolitis obliterans with ground glass pattern, air trapping and bronchial thickening.

**Air trapping**, also called **gas trapping**, is an abnormal retention of air in the [lungs](/source/Lungs) where it is difficult to exhale completely.[1] It is observed in [obstructive lung diseases](/source/Obstructive_lung_disease) such as [asthma](/source/Asthma), bronchiolitis obliterans syndrome and [chronic obstructive pulmonary diseases](/source/Chronic_obstructive_pulmonary_disease) such as [emphysema](/source/Emphysema) and [chronic bronchitis](/source/Bronchitis#Chronic_bronchitis).

Air trapping is not a diagnosis but is a presentation of an illness, and can be a guide to the appropriate [differential diagnosis](/source/Differential_diagnosis).

## Imaging

Computed tomography (CT) is the most sensitive imaging modality for detecting air trapping. On inspiratory CT scans, air trapping may appear as areas of decreased attenuation (darker areas), often described as [mosaic attenuation](https://en.wikipedia.org/w/index.php?title=Mosaic_attenuation&action=edit&redlink=1). The diagnostic hallmark is persistence of these low-attenuation areas on expiratory CT, during which normal lung parenchyma increases in attenuation (or becomes whiter) while trapped air remains lucent.[2] Expiratory imaging is therefore essential for confirming air trapping and distinguishing it from vascular causes of mosaic attenuation.[3]

## Measurement and function

Exhaled volumes are measured by a pulmonary function test or simple [spirometry](/source/Spirometry), leading to an elevated [residual volume](/source/Lung_volumes) and a measurement of forced expiratory volume. Air trapping is often incidentally diagnosed on [computed tomography](/source/Computed_tomography) (CT) scanning. On expiratory films, retained hyperlucent gas will be visualised in cases of air trapping.[4]

Air trapping represents poorly aerated lung, but on its own is clinically benign. It is a common problem for smokers who dive. On diving the [lung volume](/source/Lung_volume) collapses and pushes air into the poorly aerated regions. On arising from a deep depth, these air-trapped areas of lung expand. This places great pressure on the lung tissue which can rupture.[5]

## References

1. **[^](#cite_ref-1)** U. Joseph Schoepf; ["6.4.9.5 Expiratory Hyperinflation – Air Trapping"](https://books.google.com/books?id=EGsjbGjSW9wC&pg=PA101), *Multidetector-Row CT of the Thorax* Birkhäuser, 2005, p.101

1. **[^](#cite_ref-2)** Miller, Wallace T.; Chatzkel, Jonathan; Hewitt, Michael G. (July 2014). ["Expiratory Air Trapping on Thoracic Computed Tomography. A Diagnostic Subclassification"](https://www.atsjournals.org/doi/10.1513/AnnalsATS.201311-390OC). *Annals of the American Thoracic Society*. **11** (6): 874–881. [doi](/source/Doi_(identifier)):[10.1513/AnnalsATS.201311-390OC](https://doi.org/10.1513%2FAnnalsATS.201311-390OC). [ISSN](/source/ISSN_(identifier)) [2329-6933](https://search.worldcat.org/issn/2329-6933). Retrieved 3 January 2026.

1. **[^](#cite_ref-3)** Tan, Samantha; Saffar, Bann; Wrobel, Jeremy; Laycock, Andrew; Melsom, Stephen (2023). ["Air trapping in small airway diseases: A review of imaging technique and findings with an overview of small airway diseases"](https://onlinelibrary.wiley.com/doi/10.1111/1754-9485.13540). *Journal of Medical Imaging and Radiation Oncology*. **67** (5): 499–508. [doi](/source/Doi_(identifier)):[10.1111/1754-9485.13540](https://doi.org/10.1111%2F1754-9485.13540). [ISSN](/source/ISSN_(identifier)) [1754-9485](https://search.worldcat.org/issn/1754-9485). Retrieved 3 January 2026.

1. **[^](#cite_ref-4)** Javier Luca ya; A. L. Baert; Janet L. Strife; ["4.6.5 Air Trapping"](https://books.google.com/books?id=gDfNK79GBLUC&pg=PA92), *Pediatric Chest Imaging: Chest Imaging in Infants and Children*, Springer, 2007, p. 92

1. **[^](#cite_ref-5)** Johny A. Verschakelen; W. De Weber; ["5.2.2 Air Trapping"](https://books.google.com/books?id=VJf5JSUbRsMC&pg=PA51), *Computed Tomography of the Lung: A Pattern Approach*, Springer, 2007, p. 51

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Adapted from the Wikipedia article [Air trapping](https://en.wikipedia.org/wiki/Air_trapping) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Air_trapping?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.