Indian authorities said on Tuesday they found traces of lead and nickel particles in blood samples, after hundreds of people with an unknown illness were admitted to hospital in the southern state of Andhra Pradesh.
Teams of doctors, including those from New Delhi's premier All-India Institute of Medical Sciences (AIIMS), are investigating.
Andhra Pradesh's state government said on Tuesday the AIIMS team had found traces of lead and nickel in their samples. A second hospital is running similar tests.
The illness infected more than 300 children, with most of them suffering from dizziness, fainting spells, headaches and vomiting. They tested negative for Covid-19.
High levels of lead in the bloodstream can impair the development of the brain, nervous system and vital organs such as heart and lungs.
Earlier on Tuesday, federal politician GVL Narasimha Rao, who is from the state, said on Twitter that he had spoken to government medical experts and that the "most likely cause is poisonous organochlorine substances".
"It is one of the possibilities," said Geeta Prasadini, a public health director in Andhra Pradesh state, who said they were awaiting test reports to ascertain the cause.
She said no new serious cases have come to light in the past 24 hours. A 45-year-old man died over the weekend.
Organochlorines are banned or restricted in many countries after research linked them to cancer and other potential health risks. But some of the pollutants remain in the environment for years and build up in animal and human body fat.
It was not immediately clear how extensively the chemicals are used in India, although they are found in DDT used for mosquito control.
Exposure to organochlorine pesticides over a short period may produce convulsions, headache, dizziness, nausea, vomiting, tremors, confusion, muscle weakness, slurred speech, salivation and sweating, US health authorities said.
Health officials and experts were baffled by the mysterious illness in Andhra Pradesh.
It was first detected on Saturday evening in Eluru, an ancient city famous for its hand-woven products. People started convulsing without warning, said Dr Prasadini.
Since then, symptoms ranging from nausea and anxiety to loss of consciousness were reported in 546 patients admitted to hospitals. Many have recovered and returned home, while 148 are still being treated, said Dasari Nagarjuna, a government spokeswoman.
Teams of experts arrived in the city from India’s top scientific institutes. Different theories were suggested and tested.
What confounded experts is that there appeared to be no common link between the hundreds of people who fell sick.
All of the patients tested negative for Covid-19 and other viral diseases such as dengue, chikungunya or herpes. They are not related to each other. They do not all live in the same area. They are from different age groups, including about 70 children, but very few are elderly.
Initially, contaminated water was suspected. But the chief minister’s office confirmed that people who do not use the municipal water supply also fell ill, and that initial tests of water samples did not reveal any harmful chemicals.
Opposition leader N Chandrababu Naidu on Twitter called for an "impartial, full-fledged inquiry into the incident".
Andhra Pradesh state is among those worst hit by Covid-19, with more than 800,000 detected cases. The health system in the state, like the rest of India, has been stretched by the virus.
