New Delhi/London - The pesticide that killed 23 Indian schoolchildren last week is a nerve poison banned by many countries because of what the World Health Organisation (WHO) describes as its "high acute toxicity".
As early as 2009, the United Nations health agency urged India to consider a ban on the pesticide monocrotophos - the substance said by a magistrate investigating the deaths to be the cause of the poisoning.
It had also warned that in India - against strong international health warnings - many pesticide containers are not thrown away after use, but recycled and used for storing water, food and other consumables.
In last week's case in the Indian state of Bihar, the children fell ill within minutes of eating a meal of rice and potato curry in their one-room school.
They were vomiting and convulsing with stomach cramps - symptoms that experts say would be common in poisoning with such a toxic chemical.
The lunch was part of India's Mid-Day Meal Scheme, which aims to tackle malnutrition and encourage 120 million poor children to attend school. It had already drawn widespread complaints over food safety.
An initial forensic investigation found the Bihar children's meal had been prepared with cooking oil that contained monocrotophos - a substance that belongs to a family of chemicals called organophosphates that share a common mechanism of toxic action.
"Basically they are nerve poisons," said David Coggon, a professor of occupational and environmental medicine at Britain's University of Southampton.
"They interfere with transmission between one nerve and another, or with transmission between nerves and muscle cells."
Banned in many countries
According to WHO, swallowing just 120 milligrams of monocrotophos - the weight of about five grains of rice - can be fatal to humans. Initial symptoms can include sweating, nausea, vomiting, blurred vision and hyper-salivation, or foaming at the mouth.
Monocrotophos controls a range of pests from aphids to caterpillars, mites, moths, stem borers and locusts on various crops such as cotton, rice and sugarcane.
According to a detailed 2009 WHO report on the health risks of monocrotophos in India, the countries and regions that have banned its use include Australia, Cambodia, China, the European Union, Indonesia, Laos, Philippines, Sri Lanka, Thailand, Vietnam and the United States.
Its import is illegal in at least 46 countries.
Yet in India, monocrotophos "is widely used and easily available", and is frequently linked to fatal poisoning, both accidental and intentional.
"Its low cost and many possible applications have kept up demand in India despite growing evidence of its negative impact on human health," the WHO report said.
And although both the WHO and the UN Food and Agriculture Organisation recommend puncturing and crushing pesticide containers to prevent people using them for anything else, in India "the reality is different", the WHO said.
"Many pesticide containers, because of their sturdiness and look, are often later used to store objects, food grains and water, and sometimes even medicines."
Coggon said that, while a ban on monocrotophos would doubtless help reduce its risks in India, using it more safely could also help to minimise reduce the threat.
"It's about trying to develop a safety culture," he said. "It's about developing systems that will ensure these things are handled as safely as possible - having the right sorts of containers, the size, the formulation... and educating people about the use of chemicals in general."