The students were taken to hospitals in the neighbourhood. Barring three, who are still under observation in the Intensive Care Units of two hospitals, the rest have been discharged.
The police said a call was received at 7:35 AM about a suspected chemical leak in the customs area of the Tughlaqabad depot, which is located near the Rani Jhansi School and Government Girls Senior Secondary School, both run by the Delhi administration.
Soon, students started to complain of breathlessness and irritation in the eyes and were rushed to hospitals.
The National Disaster Response Force (NDRF) said a truck carrying 80 cans of chloromethyl pyridine had left the Tuglaqabad Depot on its way to Sonepat at 0330 hours.
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"After coming out of depot the driver parked the truck on the roadside near the Railway Colony in Tuglaqabad to have tea. During this halt, some chemical spilled on the side of the road. After having tea, the driver left for Sonepat," it said.
It is suspected that the chemical, which came from China, spilled when the containers were being loaded or unloaded.
The NDRF said a team had decontaminated the affected area, and disposed of the soil.
"Tughlaqabad area is safe now. All containers have been emptied and washed," NDRF Director General R K Pachnanda said.
Meanwhile, the affected students said they were taken to four hospitals -- the Hamdard Institute Of Medical Sciences and Research, Apollo, Batra and ESIC. One student was referred to Safdarjung Hospital.
"They complained of irritation in the eyes, breathlessness and pain in the abdomen. All of them were put on conservative treatment," said an official of the Hamdard Institute Of Medical Sciences and Research, where around 250 students, aged between 9 and 15, were admitted.
"It looks like the container started leaking early morning and the people nearby put mud and water to stop the leak at that time, but they could not anticipate its impact," said Narender Singh Rawat, a computer hardware serviceman, whose two daughters were hit by the fumes.
Another parent, who did not wish to identified, questioned the failure of the schools in not anticipating such disasters.
"It is common sense that a container depot would have all kinds of consignments. Permission should not be given to schools to function in the neighbourhood," he said.
The southeast district administration issued a show cause notice to the Customs Department and the Container Corporation Of India Ltd (CONCOR), the custodian of the depot.
"We will take strict action in the matter as hazardous chemicals were stored in the depot," B S Jaglan, district magistrate (southeast), said.
A CONCOR spokesperson said it was a cause of concern and they were looking into the leakage. "We will cooperate with the probe," he said.
"Therapeutic interventions as per clinical requirements was administered on them," Apollo said in a statement.
Union Health Minister J P Nadda had earlier instructed all Centre-run hospitals to be ready to help those affected. A team of doctors from AIIMS has been put on stand-by to cater to any emergency.
Delhi chief minister Arvind Kejriwal said strict action would be taken against those found guilty in the incident.
"We have asked the DM to take strict action against those found guilty of negligence," he told reporters after meeting the affected students in a hospital.