Argentina is one of the world's largest exporters of genetically-modified soya. It's big business, but some local residents fear herbicides used by the industry could be making them sick.
Horacio Brignone lives in the village of María Juana in the Argentine flatlands, or pampas. From his window he can see fields of soya.
His 20-year old son has suffered from asthma since he was three years old, he says, but when he recently moved to a city the condition disappeared.
"He hasn't had an attack for two years since he went to study in Córdoba," says Horacio. "But when he came home for two months recently, he began to cough again."
He blames the weedkiller sprayed on the soya fields.
"We are 50m from the fields," Mr Brignone says. "We can't be certain, but respiratory problems and skin complaints are very common round here among those who live very close to the fields."
Mr Brignone's wife, Rosalía Ramonda, has suffered from rashes on her skin for years.
She tells me: "It goes red and is itchy. For years this happened and I just used to say: 'It's something I ate.'"
In recent decades, soybean production has increased sharply in Argentina. GM crops were supposed to need less herbicide, but, as weeds have grown resistant, higher quantities of herbicide have been used.
While the area of cultivated land grew by 45% between 1994 and 2010, figures from the UN's Food and Agriculture Organisation show that the amount of herbicide used in Argentina increased from 19,376 tonnes of active ingredients to 227,185 tonnes - a rise of more than 1,000%.
Stronger concentrations are also being sold more frequently, according to data from the Argentine agro-chemical industry body CASAFE.
The most commonly used weedkiller in Argentina is glyphosate.
The Argentine Federation of Medical Professionals has called for glyphosate to be banned, but the suggestion that it could be harmful is strongly contested.
The European Union recently decided it was safe enough to relicense the herbicide in Europe for 18 months, while it continues to assess the evidence.
Dr Damián Verzeñassi and his team at the faculty of medical sciences in the University of Rosario have been studying the health of more than 80,000 people in 25 communities in the Argentine pampas since 2010.
It's an epidemiological study based on surveys rather than medical examinations. It measures the incidence of illnesses in the local population and it is not possible to draw from it a conclusive link between pesticides and human health.
No hard proof
The study found that in these rural areas, 40% of people suffer from respiratory problems - a higher rate than in the rest of Argentina.
"We have more respiratory problems in the countryside than in the city," he says. "And we used to think that if you lived in the countryside you breathed and lived more healthily."
The study also found a high incidence of children with skin rashes, particularly after the spraying of herbicides had taken place nearby.
The cancer rate in these areas appears to be significantly higher than the national average too.
"Between 75% and 95% of the communities in these areas live within 1,000 metres of the fields where they spray herbicides," says Dr Verzeñassi.
"And all of these communities, except for one, for 20 years have been in areas where transgenic crops are produced, which rely on agro-toxic chemicals, fundamentally glyphosate.... so we cannot rule out that the possibility that the problems we are finding are related to these chemicals."
Last year, the World Health Organization's International Agency for Research on Cancer said glyphosate was "probably carcinogenic".
Monsanto, one of the world's leading manufacturers of glyphosate, disputes the findings of the WHO's cancer agency, which it says hasn't considered all the evidence and has "mis-characterised the hazard".
It says that other independent regulators have concluded that glycophosate "can be safely used" and "at realistic exposure levels there is not a cancer risk".
Mark Buckingham of Monsanto said: "Glyphosate has a lower toxicity than most other pesticides.
"In terms of acute toxicity [exposure during one day] glyphosate is less toxic than vinegar, table salt, paracetamol and caffeine."
Marcelo García, of the Argentine soya industry association ACSOJA, said that many other countries, including the US, are debating the health implications of herbicides, but so far there is no proof that glyphosate causes cancer.
More than 30,000 people in the Argentinean province of Santa Fe have signed a petition calling for herbicide spraying to be banned within 800 metres of homes and 1,000 metres of schools.
Sonia Lobos lives in Maria Juana. She is a mother of a 13-year-old boy who was diagnosed with leukaemia on Christmas Eve last year.
"One day he said: 'Mummy, look I've got a bruise and I don't remember being hit.' I touched it and felt a hard lump," she says.
Mrs Lobos lives closes to the fields and says lorries carrying chemicals to spray the crops pass her house regularly. "I don't know if this is really what caused my son's illness, but it is the most likely thing I can think of," she said.
The Argentine ministry of agro-industry did not want to comment. But the pro-government congressman Eduardo Amadeo dismissed any suggestion that glyphosate or herbicides can be linked to illnesses.
He said: "There is not one single case that can be scientifically proven to be related to glyphosate. Thanks to herbicides like glyphosate, agricultural productivity in the world has tripled.
"Without them the world would suffer hunger."