Four effects given have to be differentiated in the action of forest belts and forest fringes on the distribution of waste gases and smoke damage: Reduced wind velocity; increased turbulence which remains active even beyond the leeward fringe of a forest; the actual filtering by the plants, which is usually minor; and the physiological relief of the plants as a result of a wind screen. The concentration of smoke gases can be reduced by forest belts only in very rare cases, i.e. when it decreases markedly with increasing height above the ground, e.g. at sites which are above the original level of smoke distribution. The fourth effect (protection from wind) is of major practical importance.