Thirteen people had to be treated by paramedics following reports of a gas leak at a hotel swimming pool.
Residents had to be evacuated from the four-star Ashford International Hotel in Kent at about 15:15 GMT after guests complained of breathing problems.
Firefighters were called to make the building safe while paramedics and an ambulance hazard response team treated patients.
One woman was taken to hospital for further precautionary checks.
Twelve other hotel guests were treated at the scene, the South East Coast Ambulance Service said.
Kent Fire and Rescue Service, Public Health England and Ashford Borough Council are investigating what happened.
The hotel, run by the QHotels Group, was open on Sunday evening.
A hotel spokeswoman said it would not be commenting on the incident at this time.