A chemical reaction forced the evacuation of an aquatic centre north of Wellington on Monday.
Firefighters in full protective gear washed off the remnants of a gas which forced the evacuation of Levin Aquatic Centre just before midday.
A lifeguard was using two pool chemicals to monitor the water levels, when they mixed due to a faulty pump.
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Incident Controller Chris Kennedy says the effect was immediate.
"There was a smell and straight away respiratory issues. For safety, we evacuated the area and told anyone nearby to keep windows shut," he says.
The 33 people inside the aquatic centre at the time had to be evacuated. They were taken just over the road to Levin's fire station, where a triage centre was set up. There, paramedics treated them for coughing and difficulty breathing.
One person was taken to hospital, and Brent Harvey from Horowhenua Council says the rest were given the all-clear.
"I believe all have been seen and checked and have gone home."
It's the second time the pool has been evacuated this month - two weeks ago, a faulty chemical sensor triggered a similar response from fire and emergency.
Harvey hopes the pool will be back open tomorrow, once the building has been ventilated and given the all-clear.