More violent eruptions are possible only in unique circumstances, such as an eruption into a lake. Any future eruption is most likely to affect only a limited area downslope from the volcano. Poisonous substances, such as volcanic gas, includes a variety of substances. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissoclated gases in magma and lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action. The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide and hydrogen fluoride. Locally, sulfur dioxide gas can lead to acid rain and air pollution downwind from the volcano.