Hyderabad: 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.

The number of people reporting symptoms went up to 556 on Tuesday, 458 people were discharged and only 98 patients are still in hospital undergoing tests.

More than 45 children below the age of 12 were among the patients. All complained of convulsions, nausea and fainting. One person, a 45-year-old, died on December 5.

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".

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, US health authorities say.