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The Body's Defenses Against Breathing Dusty Air

The respiratory tract is protected by mechanisms that work to keep our airways and lungs relatively clean

June 23, 2016
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Illustration: / Alila Medical Media

Illustration: / Alila Medical Media

Read more in the series about remodeling jobsite dust: Part 1, The Dirt on Dust

Under normal conditions, every cubic foot of air we breathe contains about 25,000 particles, most of which are too small to see (less than 40 microns in diameter). Fortunately, our respiratory tract is protected by mechanisms that work to keep our airways and lungs relatively clean.

Anatomy of the lungs. Illustration: / Alila Medical Media

Nose. When we breathe through the nose, air moves into the nasal cavity and sinuses where many airborne particles are trapped by a thin layer of mucus that lines the surface of the air passages. Fine hair-like structures called cilia continuously move the mucus (as many as four cups per day) into our throat, where most of it is swallowed. However, the quantity and viscosity of the mucus produced is affected by air quality. Allergens, for example, stimulate the immune system to send more blood to the nasal passages, which can cause swelling, inflammation, and increased mucus production. If the cilia can’t keep up, congestion or a runny nose may result.

Bronchial tubes. Particles that make it through the nasal cavity—or that bypass it when we breathe through our mouth because of congestion or exertion—enter the trachea, then the bronchial tubes, whose trunk-and-branch passageways grow smaller the deeper into the lung the air travels. Here, too, the wavelike motion of cilia transports particle-laden mucus upward into the throat, where it is either swallowed or spit out.

Alveoli. The exchange of oxygen and carbon dioxide takes place in the smallest, deepest branches of the lungs, called the alveoli. Here there are no cilia, and the defense against particulates is taken over by microphages. These special cells engulf and consume particles, then make their way to larger passageways, where cilia push them up and out of the lungs.

Size, Shape, and Number 

How the lungs react to the presence of particles depends on where the particles settle. The greatest danger comes from large quantities of small particles that reach the alveoli. Microphages that are overwhelmed may allow particles to collect in the lungs or may die before they are able to leave the lungs. In both cases, the result is the formation of scar tissue, or fibrosis. Over time, fibrosis can reduce the lungs’ ability to expand and contract, which affects the efficiency of oxygen exchange. Certain substances do more damage than others; silicosis, for example, is a condition named for the crystalline silica particles that cause the inflammation that can very quickly lead to fibrosis and impaired lung function.

Silica image: courtesy NIOSH / CDC



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About the Author

Amanda Voss, an author and policy analyst based in Denver, is managing editor of Energy Design Update. 

About the Author

Sal Alfano is executive editor for Professional, 202.365.9070

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