Sulfur dioxide (SO2) is a highly reactive gas common in air pollution. SO2 is emitted primarily from fossil fuel combustion at power plants and other industrial facilities. SO2 is also emitted from industrial processes such metal extraction from ore, and burning high-sulfur containing fuels by locomotives, and large ships. Information source: U.S. EPA.
Short-term exposure to SO2 of less than one day can cause bronchioconstriction and increased asthma symptoms. Long-term health effects are unknown. Note that SO2 (and other gaseous sulfur oxides) can react with compounds in the atmosphere forming small particulates, which have additional adverse health effects. Information source: U.S. EPA.