Atmospheric pollution is an increasing problem in many countries as the number of motor vehicles and industries increases. The chief pollutants are various oxides of nitrogen and sulfur, ozone, carbon monoxide, various hydrocarbons, and particulate matter. Of these, nitrogen oxides, hydrocarbons, and CO are produced in large quantities by the internal combustion engine, the sulfur oxides mainly come from fossil fuel power stations, and ozone is chiefly formed in the atmosphere by the action of sunlight on nitrogen oxides and hydrocarbons. The concentration of atmospheric pollutants is greatly increased by a temperature inversion which prevents the normal escape of the warm surface air to the upper atmosphere.
Nitrogen oxides cause inflammation of the upper respiratory tract and eye irritation, and they are responsible for the yellow h;ize of smog. Sulfur oxides and ozone also cause bronchial inflammation, and ozone in high concentrations can produce pulmonary edema. The danger of CO is its propensity to tie up hemoglobin (see p. 80), and cyclic hydrocarbons are potentially carcinogenic. Both these pollutants exist in tobacco smoke, which is inhaled in tar higher concentrations than any other atmospheric pollutant. There is evidence that some pollutants act synergistically, that is, their combined actions exceed the sum of their individual actions, but more work is required in rhis area.
Many pollutants exist as aerosols, that is, very small particles that remain suspended in the air. When an aerosol is inhaled, its fate depends on the size of the particles. Large particles are removed by impaction in the nose and pharynx. This
: For a more detailed account, see JB West: Pulmonary Pathophysiology—The Essentials, ed 6. Baltimore, Lippincott Willi;ims & Wilkins, 2003.
means that the particles are unable to turn the corners rapidly because of their inertia, and they impinge on the wet mucosa and are trapped. Medium-sized particles deposit in small airways and elsewhere because of their weight. This is called sedimentation and occurs especially where the flow velocity is suddenly reduced because of the enormous increase in combined airway cross section (Figure 1-5). For this reason, deposition is heavy in the terminal and respiratory bronchioles, and this region of a coal miner's lung shows a large dust concentration. The smallest particles (less than 0.1 micron in diameter) reach the alveoli, where some deposition occurs through diffusion to the walls. Many small particles are not deposited at all but are exhaled with the next breath.
Once deposited, most of the particles are removed by various clearance mechanisms. Particles that deposit on bronchial walls are swept up the moving staircase of mucus which is propelled by cilia, and they are either swallowed or expectorated. However, the ciliary action can be paralyzed by inhaled irritants. Particles deposited in the alveoli are chiefly engulfed by macrophages that leave via the blood or lymphatics.
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