Authorities have confirmed the substance detected at an Ashburton school last week that led to four people being rushed to hospital.

Four people with minor symptoms were transported to Ashburton Hospital by ambulance on February 27.

The alarm was raised at St Joseph’s School in the Mid-Canterbury town before 9.40am after children began feeling unwell.

Fire and Emergency New Zealand said a “low reading” of a chemical was discovered.

Today, the agency confirmed the chemical was hydrogen cyanide.

The chemical is described as highly volatile, extremely toxic, and flammable.

It can be in the form of either a colourless liquid or gas and has a faint bitter almond odour.

It is widely used in plastics, pesticides, and mining.

“Crews detected extremely low amounts of hydrogen cyanide at the church, but it was such a low amount that it barely registered,” said a Fire and Emergency spokesperson.

“There was no further investigation.”

The source of the chemical has not been disclosed.

School principal Cath Blacklow sent a statement about the incident this afternoon.

“Last Friday, a number of pupils fell ill at St Joseph’s School and at our neighbouring church. Emergency personnel discovered very low levels of a chemical,” she said.

“Emergency personnel did not consider any additional investigations were necessary and thankfully, the pupils recovered quickly.

“As a precautionary measure, and given our high priority for student safety, the school asked that more testing be undertaken, including by professional fire crew and tradespeople.

“It was established that no chemicals were detected at the church or areas tested within the school that would be connected with the student illnesses on Friday.”

The Ministry of Education has been in contact with the school and offered “support if needed”.

A spokeswoman said that “hasn’t been required to date”.