The leak the Apple data centre in North Carolina has landed five people in hospital.
Chlorine can be stored as a liquid when cooled, but will quickly turn into a gas if released into the air. The gas attacks people's respiratory systems: it is much denser than air, and dissolves the mucous membrane, which can cause fluid to build up. If that fluid is inhaled, it can cause someone to suffocate, similar to drowning. Even low levels can cause shortness of breath.
It is believed that the chlorine was a key component in the facility's water-cleaning facility, perhaps for its water-cooled components.
The Maiden facility is Apple's largest data centre on the East Coast, and at 505,000 square feet it is one of the largest in the world.
The site has been used by Apple to highlight its green credentials: it was the first large-scale data centre to be given the green LEED Platinum certification.
Most of the energy used at the facility comes from solar arrays in a vast 40MW, 200-acre installation on the same site. The energy shortfall is made up from fuel cells that will use biogas from nearby landfills to generate electricity and purchasing renewable energy from local sources, according to Apple.