Chlorine is a severe nose, throat and upper respiratory tract irritant. People exposed to chlorine, even for short periods of time, can develop a tolerance to its odour and irritating properties. In general, volunteers have experienced irritation of the nose, a weak cough, and increased dryness of the throat at concentrations ranging from 0.02 to 2 ppm. Concentrations of 1 to 2 ppm produce significant irritation and coughing, minor difficulty breathing and headache. Concentrations of 1 to 4 ppm are considered unbearable. Severe respiratory tract damage including bronchitis and pulmonary edema (a potentially fatal accumulation of fluid in the lungs) has been observed after even relatively low, brief exposures (estimates range from 15 to 60 ppm). The development of pulmonary edema may occur immediately or can be delayed up to 48 hours after exposure. Numerous cases of chlorine exposure have been reported, but actual exposure levels have not been well documented. Symptoms observed in non-fatal cases include difficulty breathing, cough, spitting up blood, tightness in the chest, a blue discolouration of the skin, severe headache, nausea, vomiting and fainting. Even with severe exposures, complete recovery usually occurs within one week to a month, depending on the extent of injury to the respiratory tract and lungs. However, long-term respiratory system and lung disorders have been observed following severe short-term exposures to chlorine.

One of the disorders observed is called Reactive Airways Dysfunction Syndrome. With this condition, asthma-like symptoms and increased reactivity of the airways is experienced. In some other cases, permanently reduced lung function has been observed. It is not possible to draw firm conclusions from one limited study which showed neurological effects (e.g. impaired balance and verbal recall) in 7 people with very high, short-term chlorine exposures.

Individuals with pre-existing lung disorders (for example, asthma) are more sensitive to the irritating effects of chlorine.