Plants are sensitive to sulfur dioxide and they are affected by it both directly and indirectly. The direct effects may be acute or chronic, depending on the duration and intensity of the exposure. Sulfur dioxide inhibits photosynthesis by disrupting the photosynthetic mechanism. The opening of the stomata is promoted by sulfur dioxide, resulting in an excessive loss of water. The cumulative effect of sulferous pollution is to reduce the quantity and quality of plant yield. Generally, its impact is more severe when in combination with other pollutants such as oxides of nitrogen, fluorides, and ozone. At the ecosystem level, sulfur dioxide affects species composition by eliminating more sensitive species. This reduces primary productivity and alters trophic relationships which have far‐reaching implications for the animal and microbial populations in the community. Another indirect effect results from the acid rain which leaches out nutrients from plant canopy and soil. The acidic run‐off changes the pH of the receiving waters and adds large quantities of nutrients which disturb the equilibrium of aquatic communities. Plants vary widely in their tolerance to sulfer dioxide. Lichens and bryophytes are among the most sensitive and have been successfully used as indicators of sulfur dioxide pollution.