Human health effects associated with breathing or otherwise consuming small amounts of chlorine over long periods of time are not known. They are currently under investigation. Some studies show that workers develop adverse effects from repeat inhalation exposure to chlorine, but others do not. Laboratory studies show that repeat exposure to chlorine in air can adversely affect the immune system, the blood, the heart, and the respiratory system of animals. Chlorine causes environmental harm at low levels.
Chlorine is especially harmful to organisms living in water and in soil.