Indian authorities said on Tuesday they found traces of lead and nickel particles in blood samples, after hundreds of people were hospitalised due to an unknown illness 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 the death of one person and hospitalisation of more than 400 in the past few days.

Andhra Pradesh’s state government said on Tuesday the AIIMS team had found traces of lead and nickel in their samples, while a second hospital is also running similar tests. The illness has infected more than 300 children, with most of them suffering from dizziness, fainting spells, headaches and vomiting. They have tested negative for COVID-19. High levels of lead in the bloodstream can impair development of brains, nervous systems and vital organs such as heart and lungs.

Earlier on Tuesday, federal lawmaker GVL Narasimha Rao, who is from the state, said on Twitter that he had spoken with 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, adding 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. However, 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, though they are found in DDT applied 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, U.S. health authorities say.