Five pupils were exposed to levels of radon almost eight times the legal limit, while boarding at Kingswood School in Bath, Somerset, in 2019.
Two children of staff who lived on site were exposed to levels almost 14 times the limit, while their parents were also reportedly exposed to high levels.
Radon gas is said to be the second-largest cause of lung cancer in the UK, resulting in more than 1,000 fatal cancers each year according to the Health and Safety Executive (HSE).
The colourless, odourless substance is found in rocks and soils, as well as some building materials and water.
It is known to seep out of the ground, and can build up in houses and indoor spaces such as schools. Laws are in place to limit exposure to radiation in the workplace.
An HSE investigation found Kingswood School - where boarding pupils pay annual fees of up to £40,000 - “knew [it] had a radon problem as far back as 2007”, when the school carried out monitoring and introduced measures to reduce radon levels.
