A chemical compound of nitrogen and hydrogen with the formula NH3. Ammonia is a colourless gas with a characteristic pungent smell. It is generated through the decay of nitrogen containing biological waste such as urine and is therefore present in significant quantities in unhygienic stabling in which soiled bedding is not regularly removed. Due to its caustic nature, it can cause serious irritation of the respiratory system if it is inhaled, as well as facilitating the development of diseases of the hoof such as thrush or white line disease if the horse continually stands on wet bedding.