Zheng Gumei thought she was down with a cold until the doctor told her to wait outside the room so he could talk to her son alone.

"I knew then that I must have a serious illness," the 47-year-old farmer recalled, wiping away the tears and then staring into the distance. "I'm having treatment now. See, my hair has fallen out," she said, taking off her hat to show the side-effects of chemotherapy.

Like many other residents of Xinglong, a small rural community next to an industrial park in China's Yunnan province, she had little doubt about the source of her cancer. "The pollution in this village is bad, people get sick."

Such stories have become much more common in China in recent years as breakneck economic growth increasingly takes its toll on the nation's health.

Since last year, there has been an explosion of lead poisoning cases close to smelting plants. Studies have shown that communities that recycle electronic waste are exposed to cadmium, mercury and brominated flame retardants. Elsewhere, there have been protests against chemical factories that are blamed for carcinogens that enter water supplies and the food chain.

Nationwide, cancer rates have surged since the 1990s to become the nation's biggest killer. In 2007, the disease was responsible for one in five deaths, up 80% since the start of economic reforms 30 years earlier.

While the government insists it is cleaning up pollution far faster than other nations at a similar dirty stage of development, many toxic industries have simply been relocated to impoverished, poorly regulated rural areas.

Chinese farmers are almost four times more likely to die of liver cancer and twice as likely to die of stomach cancer than the global average, according to study commissioned by the World Bank. The domestic media is increasingly filled with reports of "cancer villages" - clusters of the disease near dirty factories.

There have been few epidemiological studies to validate such claims, but the scale of such reports highlights the growing fear of pollution. Last year, investigative journalist Deng Fei, posted a widely circulated Google map showing more than 100 "cancer villages". More recent reports suggest the number could be over 400.

The vast majority are on the wealthy eastern seaboard, the first area in China to accept "outsourced" dirty industries from overseas. But as these regions have moved up the value chain and tightened regulations, there are signs that the pollution and cancer belt may be moving inland to areas that are either less aware of the dangers or too poor to turn away business.

Deep in the scorched dry countryside of northeast Yunnan, the residents of Xinglong fear they may soon join the list of sick villages. An acrid stench assails the senses near the Luliang City Industrial Park, the thicket of polluting factories that locals blame for an outbreak of deadly tumours.

Cui Xiaoliang says he lost his aunt and father to cancer after the village streams changed colour. Pointing to the lurid red discharge from the Yinhe paper mill and a yellow trickle below the Peace Technology chemical factory, he said health had declined along with the environment.

"Before the factories were built, there was no cancer. We were free of strange diseases," he said, grimacing at the nauseating fumes. "Now, we hear every year that this person or that person has cancer, especially lung and liver cancer. My aunt never drank alcohol or smoked. Her cancer was completely caused by pollution."

At the village clinic, doctor Zhang Jianyou said he has noticed an increase in cancer cases among the 3,000 residents. "The pollution has definitely has an impact," he said. "I have been here 43 years. In the past, cancer was not obvious, but in recent years it has become a very evident problem. Last year alone, we had five cancer cases."

When locals tried to protest, Zhang said they were blocked by the authorities because the chemical factories contribute to the local economy.

Everyone the Guardian spoke to at the village knew of someone who had died of cancer and most blamed the toxins that flowed from the chemical factories into the nearby Nanpan river and ground water supply.

Farmers said they have no other source of water for their crops and animals. Goat herders said a tenth of their animals had died.

The impact may well have spread into the human food chain. Wang Qingdi, a peach farmer who lives next to the chemical factory, said her crops were ruined by contaminated water and air, but she still sold them at the market because she had no other source of income.

"When the wind blows in this direction, a thick layer of soot settles on my peach trees," he said. "Lots of fruit turn black and fall to the ground, I dare not eat the rice I plant and harvest because the pollution is so bad. I sell it on the street."

The county environment department said it was monitoring the industrial park and paying particular attention to three companies: Longhai Chemical, Yunnan Luliang Peace Technology and the Yinhe paper mill. But inspectors lack the authority and the resources to keep close tabs and impose harsh punitive measures on any factories that break the rules.

"It is like police trying to catch a thief. It's not easy," said Song Bin of the Luliang Environmental Protection Department. "Some factories secretly discharge pollution. Some shut down treatment devices when electricity is in short supply. Others turn off their systems at night when they know we are not checking."

He was cautious, however, about the health implications. "It is hard to say whether there is relationship between cancer and the factories because the workers do not have unusually high rates of the disease," he said. "Many officials have suggested we invite experts to do a systematic study, but we haven't done this yet because of budget and other reasons."

The Guardian requested data on factory emissions and water quality. Under the government's information transparency law, such information is supposed to be publicly available, but officials insisted their monitoring results were for internal reference only.

Yinhe paper mill refused to comment. The chemical factory - Yunnan Luliang Peace Technology - said the pollution problems dated back to previous owners and were now being rectified.

"The cancer situation in the village has nothing to do with us," said Candy Xu, foreign sales manager. "The pollution accumulated over 10 years. It can't be solved immediately but we deal with it year by year. Within three-to-five years I believe we can clear it up. The previous company was irresponsible to the local residents and it is not fair to blame us for their mistakes."

The new owners from the rich coastal province of Zhejiang have invested in new equipment and are trying to shift production towards cleaner, high-end nutritional supplements and feed additives, but their website still lists sodium dichromate – a highly carcinogenic chemical – among its products.

In a recent study of "cancer villages", Lee Liu of the University of Central Missouri said the problem was exacerbated by the government's tendency to focus on urban development at the cost of rural areas. This – and a lack of independent oversight by NGOs and journalists – have mixed into a toxic cocktail.

"China appears to have produced more cancer clusters in a few decades than the rest of the world ever had," he notes.

Whether the village of Xinglong will join the list cannot be confirmed without a full study. But rising cancer rates and appalling pollution levels leave locals in little doubt.

For Zheng, her breast cancer does not just threaten her life, but the financial well-being of her daughter. She has had to borrow 20,000 yuan (£2,000) for two courses of chemotherapy and estimates it will cost another 80,000 yuan to cure the disease. She knows that is far from certain.

"My brother-in law had cancer like me. He is dead already," she said as her infant daughter pulled at her shirt. "I want to tell the factories they make too much pollution. Because of them Xinglong village is sick."



Additional reporting by Chen Shi