A mystery substance that made Carterton schoolchildren violently ill was probably compost, police have revealed.
Police told reporters today that compost had been identified as a probable cause of Friday's health scare at South End School.
While 10 children were taken to hospital and many others felt sick they were unlikely to face ongoing health problems, Wairarapa area commander Scott Miller said.
A neighbouring property had made fresh compost, which could have produced a strong sulphuric smell.
"We have identified that most of the children that fell ill were from the rear of the school near the compost manufacturers," Miller said.
"With the advice from the compost manufacturers - we have now spread the compost to a thin level and it no longer poses risks.
"They have been in the business for 50 years and haven't had combination of fresh compost and the heat."
There was nothing to indicate that the compost had been handled incorrectly," Miller said.
Emergency services flooded the school on Friday afternoon with more than 50 children suffering headaches, vomiting and skin irritation at South End School.
Ten children were taken to hospital for checks and were released that night.
Police said at the weekend that they had tracked down a plane at the centre of a mystery outbreak in the Wairarapa but say they're "90 per cent sure" it didn't drop anything that caused children to get sick.
Investigators were seeking a small plane, possibly a Cessna, in their bid to trace a sulphur-smelling substance that saw 10 Carterton children hospitalised and dozens more feeling ill on Friday.
Miller said speculation the plane was the source of the outbreak arose after a student said they thought it released a "white substance". However, police had talked to a number of adults and did not think the aircraft had dropped anything.
Instead, detectives believed the vapour had come from the nearby State Highway 2, or nearby houses.
"It was airborne and was blowing through the school," Miller said at the weekend.
"However, we don't have a source for that smell... at the moment to be quite honest it's a slight mystery."
Atmospheric Science Global director Jennifer Barclay told the Herald reports of the substance smelling like rotten eggs at South End school sounds suspiciously like hydrogen sulphide (H2S).
"My knowledge suggests that it's most likely a source that is close to the ground, typical examples are sewer, rotting vegetation, anaerobic composting," Barclay said.
"Because it smells like sulphur, the fact it smells like rotten eggs is an indicator it is a sulforaphane compound.
"For the children to react the way they have it would have to be a significant source or a very nearby source."
Exposure to H2S can cause nausea, tearing and redness of eyes, with high levels of exposure causing dizziness, loss of appetite, fatigue and vomiting.