Effect of air pollutants on the pulmonary surfactant system

Air pollutants have been recognized to influence the structure and function of the surfactant system. Agents that have received the most attention include ozone, nitrogen dioxide, hyperoxia, diesel exhaust, tobacco smoke, silica and fibrous materials such as asbestos. The deleterious effects of air pollutants on the surfactant system depend on the size of the agent, on its solubility in aqueous solutions and chemical reactivity and on its concentration and the duration of exposure. Hereby the following general rules apply: the smaller the agent's size and the less water soluble the pollutant is, the greater the tendency to reach the alveoli during breathing. In addition, the reactivity also determines the depth of penetration into alveoli. Compounds with high reactivity such as O₃, which also fulfil the earlier rules, will react with the upper respiratory tract compared with compounds with slightly reduced reactivity, such as NO₂, which will penetrate the alveoli. The common consequence of exposure to air pollutants is an accumulation of surfactant phospholipids and surfactant-specific proteins in the bronchoalveolar lavage fluid. These components also are structurally altered, mainly by oxidant gases, resulting in impairment of their biological activity. Thus, for surfactant phospholipids, there is impaired adsorption to the air–liquid interface due to oxidation of their fatty acids. Also, surfactant protein A, regarded as a modulator of the surfactant system, shows impaired functions after exposure to oxidants. It is likely that in addition to the effects described in this review not all effects are known because the molecular effects of several key components (e.g SP-B and C) have not been well studied.