In some Keralan villages half the families have a severely disabled child: the legacy, locals say, of pesticide spraying

The villagers see the dust rising from the track and know that Dr Mohammed Asheel is on his way.

In a small house set back from a main road surrounded by cashew nut plantations, Chandika Rai picks up her 11-year-old son, Kaushiq, and carries him into their front room.

For 30 minutes while the doctor, a nurse and a physiotherapist are there, their life is a little easier. Then the medical team is gone – on to a neighbouring house where another seriously disabled child is waiting – and Chandika is left alone again.

Asheel is in great demand in Kattuka, a huddle of cement-and-wood homes on a hillside of red earth and lush green trees in the far north of the Indian state of Kerala. In 50% of homes in the village, there is a child or an adult with severe disabilities.

The 29-year-old doctor, an official in the public health department, and the locals blame endosulfan, a pesticide developed in the 1950s and sprayed over the nearby cashew plantations in the 80s and 90s.

Campaigners say the plight of thousands of people such as Chandika and Kaushiq means endosulfan poisoning in southern India is one of the most serious such disasters in the world – and one of the least known.

In September, the Indian supreme court continued a ban on the use of endosulfan it had imposed earlier this year. Rejecting the arguments of dozens of producers, who say there is no link between the pesticide and the disabilities seen in Kerala and neighbouring Karnataka state, the judges maintained the moratorium on the use of the chemical in India.

They did, however, allow Indian companies to sell existing stocks overseas in the rare places where the product is still legal. The decision did little to allay the growing anger of people in places such as Kattuka.

"I was very happy when I heard about the ban," said Rai, 35, as she nursed her son. "We have suffered so much and I don't want anyone to suffer in the future like this. But why did it take so long? And why is no one punished?"

For 20 years, helicopters were a frequent sight in the sky over the districts of Kasagod in Kerala and Dakshina Kannada in Karnataka.

Two or three times a year, they swept low to spray chemicals over the cashew trees planted on recently deforested farmland. Most of the locals had never seen a helicopter before and had no idea what they were doing.

There were 60 communities around the cashew plantations, which grow nuts for the Indian market or for export, and after a few years the villagers got used to the spraying.

Then, towards the end of the 1980s, visiting doctors began to notice a variety of health problems, particularly among children, and began to suspect endosulfan was to blame.

Subsequent studies – contested by the industry – linked endosulfan to damage to the central nervous system and hormonal changes in mothers and babies.

When researchers compared children and teenagers from villages where endosulfan had been sprayed with populations from those where the chemical had not been used recently, they found the former suffering much higher levels of skin complaints, epilepsy and cerebral palsy.

Dr Ravindranath Shanbhag, 61, a pharmacologist who was one of the first to start seeing the problem and to demand an endosulfan ban, told the Guardian about 6,000 people have been affected in northern Kerala and southern Karnataka.

"The manufacturers say the health concerns are propaganda put about by western companies who want developing-world nations to switch to newer, more expensive products for which they still have the patent," said Shanbhag. "But I know the truth. I've been seeing it with my own eyes for many years."

There is much anecdotal evidence, too. Rai, like many other women, said she was exposed to heavy aerial spraying of endosulfan throughout her pregnancy with Kaushiq.

"I saw the helicopters, and we live almost in the plantation. But I never thought they could harm my baby," she said.

Tests last year found high levels of endosulfan still in the soil and Shanbhag worries that tens of thousands of unborn children may be harmed, and that more problems will come with future generations.

"There are hundreds of thousands of people now reaching adulthood who may have been exposed," he said. "What is going to happen to the children they plan to have? We simply don't know."

The financial burden the victims place on their families is significant. Many of the families are already poor, often manual labourers earning under £5 a day, when there is work.

Ramesh Kartagea, 28, is severely disabled and is cared for by his two sisters and mother. His father works in the fields; the women work as seamstresses. They were forced to sell their small home to pay medical bills.

Since earlier this year, they have received benefits from the Kerala state government worth about £25 a month. This has helped a little. They welcomed the ban – in part.

"It's a good thing but there are a lot of people already suffering," Shalini Kartagea, 30, said.

Asheel said the supreme court decision had countered a powerful lobbying effort by senior politicians with links to the pesticide industry.

One result has been investment in a series of new rehabilitation centres in Kasagod district. Though rudimentary, they are "better than nothing", the doctor said.

In one such centre, in the village of Perla Enkanakaje, the names of 27 patients are listed on a blackboard. "We identified 70 locals who need help," said Asheel. "Almost all were born between 1980 and 2002, the years of peak spraying in the district. We cannot cater to all of them but do what we can."

The physiotherapy Kaushiq Rai now receives helps at least "a little", his mother said. "He likes travelling. We take him to temples and he is happy," she said. "I think about all the suffering but, though I am angry, I am helpless."