The substance that made children sick and put a Wairarapa school into lockdown on Friday was sulphur coming from compost at a neighbouring property, police have confirmed.
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Ten children were hospitalised and dozens were treated at the scene as a result of a "very strong smell" discovered at South End school in Carterton.
"Just after 1pm on Friday one of the neighbouring properties next to the school had a truckload of fertiliser, compost really, delivered to their address," police said.
It was fresh and hot compost, which was emitting a strong sulphur smell. The first children starting feeling sick at about 1:20pm.
School principal Clare Crawford was notified about the smell and called police, triggering an emergency response operation.
Ambulances, fire engines and a rescue helicopter were all dispatched to the scene, and parents were asked to bring clean clothes for their children to put on after they went through a decontamination process.
Over 100 people went through a decontamination process with the Fire and Emergency Hazmat (hazardous materials) unit.
The children who were hospitalised had low-level symptoms and any long term effects are extremely unlikely, police said.
In an initial Facebook post the school said a low-flying aircraft was seen by a student who believed it had dropped a substance over the school. That has now been disproven.
Ms Crawford says a full school assembly was held on Monday morning to make sure all the students were okay, and to thank them for following instructions during the lockdown period.
She thanked emergency services and members of the community who brought food and blankets to the school on Friday evening.
"You can't ask for more than that, it's just amazing. So we as a school say thank you so much to all of those people who supported us."