A woman was taken to hospital and 160 people were evacuated to safety from a chlorine gas leak in Highbury Leisure Centre.
The London Fire Brigade (LFB) said the mass evacuation was prompted after the mixing of chemicals at the swimming pool produced a toxic chlorine gas at 12.06pm on Sunday.
Firefighters carried out a sweep of the building in Highbury Crescent and ventilated the property to make the scene safe.
An LFB spokesman a woman was taken to hospital as a precaution by London Ambulance Service crews.
He added: “The leisure centre was evacuated as a precaution after chemicals were accidentally mixed producing a chlorine gas.”
The member of staff has now been released from hospital, a spokesperson for GLL, who runs the leisure centre, said.
Highbury Leisure Centre Manager Mital Karia said: “We’d like to apologise to customers for any inconvenience on Sunday afternoon when Highbury Leisure Centre was temporarily evacuated as a precaution following a minor incident involving the accidental mixing of pool chemicals.
“We take the safety of staff and customers seriously and have started an investigation into the exact circumstances.
“We’d like to thank staff and authorities for their assistance.
“The centre remains open for customers.”
It came after paramedics took 29 people to hospital from the London Aquatics Centre in Stratford when a “high quantity of chlorine gas was released” in July last year.
Dozens of people were treated by London Ambulance Service for breathing difficulties at the Queen Elizabeth Olympic Park in Stratford.
A total of 48 people were checked over after more than 200 people were evacuated from the swimming facility.
