Sulfur trioxide irritates the mucous membranes of the respiratory tract. A concentration of 1 volume of SO3 in a million volumes of air (one part per million or 1 ppm) is enough to cause coughing and choking. Sulfur trioxide dissolves in water to form sulfuric acid, which is a strong acid capable of corroding or destroying many materials. Sulfur trioxide can absorb moisture from the atmosphere to form very fine droplets of sulfuric acid. Inhalation of these droplets can harm the respiratory system. Chronic exposure leads to a much greater likelihood of suffering from bronchitis. Sulfur trioxide can also dissolve readily in rain drops, and fall to.
