Atmospheric scientist says Carterton schoolchildren might have been exposed to harmful gas

Carterton schoolchildren who were made ill from a mystery substance on Friday may have been exposed to a harmful gas, an atmospheric scientist suggests.

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.

Emergency services flooded the school on Friday afternoon with more than 50 children suffering headaches, vomiting and skin irritation at South End School.

Police tracked down a plane originally believed to be involved in the incident and are "90 per cent sure" it didn't drop anything.

Barclay also ruled out the likelihood of a plane being involved, as it would be unlikely it could drop enough concentrated gas to the area.

She also quashed theories of fertiliser on school grounds being a potential source with it being dispersed in large areas.

"It's quite likely the source of something nearby and close to the ground than having something fly over.

"If you had a source that was close to the school it wouldn't necessarily need to be a very big source in order for the kids to be impacted by it," Barclay said.

The origin of the mystery substance could be tracked backwards if the meteorologist conditions were known at the time of the incident, she added.