Some 300 people were evacuated from the thermal baths in the southern French spa town of Digne-les-Bains in a precautionary measure after some reported feeling unwell.
Thirteen people were brought to hospital for check-ups, the district authorities said.
The spa was closed for a technical checkup after the incident that occurred on Saturday morning.
The incident was likely due to a chlorine gas leak, according to La Provence newspaper.
The spa guests who were taken to hospital had headaches but none were seriously injured, the newspaper reported.
