What makes Chlorine hazardous?

Chlorine is a toxic gas that attacks the respiratory system, eyes, and skin. Because it is denser than air, it tends to accumulate at the bottom of poorly ventilated spaces. Chlorine gas is a strong oxidizer, which may react with flammable materials.

Chlorine is detectable with measuring devices in concentrations as low as 0.2 milligram/litre (mg/l), and by smell at 3 mg/l. Coughing and vomiting may occur at 30 mg/l and lung damage at 60 mg/l. About 1000 mg/l can be fatal after a few deep breaths of the gas. The IDLH (immediately dangerous to life and health) concentration is 10 mg/l. Breathing lower concentrations can aggravate the respiratory system and exposure to the gas can irritate the eyes. The toxicity of chlorine comes from its oxidizing power. When chlorine is inhaled at concentrations greater than 30 mg/l, it reacts with water and cellular fluid, producing hydrochloric acid (HCl) and hypochlorous acid (HClO).

When used at specified levels for water disinfection, the reaction of chlorine with water is not a major concern for human health. Other materials present in the water may generate disinfection by-products that are associated with negative effects on human health.

In South Africa, the Occupational Safety and Health Act (OSHAct) has set the permissible exposure limit for elemental chlorine at 1 ppm, or 3 mg/L. The National Institute for Occupational Safety and Health has designated a recommended exposure limit of 0.5 mg/l over 15 minutes.