Chlorine is a reactive gas used by humanity for over two centuries. Exposure to chlorine has occurred in a number of situations, including as a chemical warfare agent, in industrial and domestic exposures, and as a result of accidents and spills. The toxicology of chlorine is related almost entirely to effects in the respiratory system. A consistent symptomology occurs in both animals and humans. This ranges from sensory irritation, to irritation and bronchospasm, to cellular changes to bronchioles and alveoli, to development of pulmonary disease. While full recovery from such injuries remains the most likely outcome, there is little doubt that permanent loss of function is possible in severe cases. In all industrial applications of chlorine, occupational exposures to chlorine should be controlled to at least the recommended exposure standard. However, a focus of activity on ensuring that excursions (such as leaks or "gassing" incidents) above these values do not occur is likely to be more beneficial. Treatment of chlorine exposure is essentially symptomatic, with the efficacy of some treatments (such as corticosteroid therapy) still not well established.