Moise Adihou stands by a rough wooden bench beneath a mango tree, surrounded by a small crowd that has gathered to hear his story.

“We were in the field,” he says. “Abraham came to visit after school to tell us he came first in his class. We were happy, so we wanted to celebrate.”

Adihou is a neat, sombre man in his 50s, and what he is describing took place in the village of Gaohungagon in the Zou department of Benin, West Africa. Abraham was 13 and Adihou’s eldest child.

“My wife went to the mill to grind some maize but the mill broke down. So she asked to borrow some flour and came home and cooked maize porridge for the family. When we ate it, Abraham fell down crying in pain. Then my wife doubled over and I felt cramps in my stomach. I was conscious it might be the flour and my brother said, ‘Let’s give it to the dog.’ So we did, and the dog began to vomit. We managed to get to the hospital. After three days I regained consciousness and they told me my son was better. Only when I came home did I learn that Abraham had died.”

Celestine was pregnant and took longer to recover. She stands in silence as her husband talks, visibly frustrated with the baby she carries on her back, whom she taps with a twig to stop her crying.

“We can’t blame the family who lent us the flour because they became sick, too,” she says. “I blame myself because I cooked the maize… We still think of him.”

Abraham Adihou was poisoned by a cotton pesticide mistakenly sprayed on maize in the village store room. Such tragedies are not uncommon. Cotton is the world’s most important non-food crop. It is also highly susceptible to pests such as cotton bollworm, and more pesticides are used on it per unit than on any other crop. They are used liberally in some of the poorest countries in the world, where farmers lack basic safety equipment such as gloves and glasses, as well as training. Health problems linked to pesticides include birth defects and the acute poisoning that can result from accidental ingestion. It’s common for contamination to occur where there’s no separate storage space and families live and work in cramped conditions. In 1990, the World Health Organisation estimated 1m poisonings annually and 20,000 deaths from pesticide poisoning.

In Benin, one 2001 study discovered 65 deaths, including 10 children under 10, in two districts in just one season. Cotton in Benin is watered by rain, without irrigation. Farmers live in mud huts and cook on open fires with no electricity. In the north of the country cattle are used to pull ploughs. Elsewhere land is tilled by hand by the farmer, his wives (rural Beninese society is polygamous), children when they aren’t at school, and hired labourers. Yet, as in other developing countries, since the 1970s, cotton has mainly been farmed here using agrochemicals including, in some cases, the banned insecticide endosulfan, to raise yields.

Since the Rana Plaza factory collapse in Bangladesh last year, in which 1,130 people died, health and safety issues in the global garment industry have been under the spotlight. Reacting to pressure from NGOs, unions and politicians, some retailers are investing in improvements. But the farms from which high-street chains source their fabrics are only the first link in a supply chain that reaches across the world, and news of pesticide poisoning does not travel far.

Benin is a long strip of a country to the west of Nigeria. Independent from France since 1960 and a democracy since 1991, it is less than half the size of the UK, with a population of 10 million people who mostly depend on agriculture. Travelling with a team from a local NGO, I am here to meet cotton growers and visit a project that teaches them to eliminate pesticides by farming organically.

Setting out from the main coastal city, Cotonou, we drive for seven hours up the country’s biggest road before arriving in the fertile farming region around Aklampa in the commune of Glazoué. We are met at a turn-off by a farming agent on a motor scooter, and follow him in our four-wheel drive, bouncing up and down on the red-dust track as mopeds and the occasional car crammed full of people, with sacks of maize, yams and cassava tied to the roof, travel to market in the other direction.

Over three days here and in the Zou region farther south, we talk to more than 100 mostly organic farmers, some in fields, others gathered in village centres. It is the winter dry season and the cotton harvest is under way. Everywhere we stop to speak to people working on the land – their skin and heads uncovered in the fierce sun as they move down the rows, plucking the white cotton bolls from the twiggy plants – we hear reports of the skin, eye and stomach irritation, headaches and dizzy spells linked to the use of chemicals without correct safety precautions.

“The first thing is my health,” says Dieudonné Aifa, 50, whose 15-hectare farm near Loholohouedji, growing cotton, maize and cashews, was converted to organic farming eight years ago. “When I grew conventional cotton, I suffered from different kinds of sicknesses. My body was warm all over and stomach problems stopped me eating. Now I work very hard but at least I don’t get sick.”

Aifa has to work far longer hours now because organic farming relies on farmers rotating their crops, which takes time, and monitoring insects.

“The chemical approach is you go in, you spray, you kill everything,” says Keith Tyrell of Pesticide Action Network. “When you go organic you’ve got to use knowledge, be more hands-on. Farmers here are first taught what a pest is and which insects are their friends. Then they go around putting a stone in the left pocket for each pest they see, one in the right pocket for a beneficial insect. A lot of organic cotton fails because you can’t just take away pesticides and hope the crops will be OK. They won’t. You need intensive training.”

Such techniques don’t suit all farmers, especially those who lack land or labour. For crops to be certified organic, fields must be free of chemicals for three years during which farmers must put up with yields down by up to 50% without the benefit of the 20% premium paid for organic cotton. But with input costs reduced when they stop buying chemicals, in the areas where organic farming is successful, farmers’ incomes have increased.

Aifa says he makes more money now than before, “that helped me pay to send my children to school and to build a house I can rent out. We’re semi-autonomous, we’ve been trained in many things we can do without the agent. Instead of burning the fields, now we plough them.”

At a public meeting in the newly opened local school, members of a women farmers’ group echo his enthusiasm. Kintadje Azonsode, also 50, stands up to show off twin boys, Luc and Lucien. Told she was infertile, she attributes her recent pregnancy to the switch to organic farming.

In Gaohungagon, where Abraham Adihou died, local musicians give a performance. A display of the ingredients of the natural food spray they now use, to attract beneficial insects to their crops, has been arranged on the ground: neem seeds, palm cake, papaya leaves, chilli, garlic, cow urine. The lyrics describe how to make the spray by leaving the seeds to soak in water for three days, pounding the leaves and so on. After the singing, young women dance and the visitors eat peanuts and hot cassava. In a mainly illiterate population, this is how recipes are taught and remembered, but the song is also a celebration. “To control pests you should keep them out manually and keep in the fields their natural enemies,” chants frontman Robert Tangni. “Organic cotton is wealth.”

It is 20 years since Beninese agronomist Professor Davo Simplice, now 62, carried out the first African study of farming practices that would reduce environmental damage. At the time, “nobody here believed it was possible”. He formed the Beninese Organisation for the Promotion of Organic Agriculture (Obepab). Since then, Obepab has trained 13,267 organic farmers and now oversees production of around 1% of Beninese cotton, which is fairly traded under the French Ecocert system. Last year President Yayi Boni suggested Benin might in future move to an all-organic cotton sector – the first country in the world to do so. For now, Simplice says his goal is 5% in 10 years.

It is a slow process. In villages such as Gaohungagon where there have been fatal accidents, some farmers will willingly give up chemicals and try organic farming. But more often the first step is to reduce pesticide use, which many farmers are keen to try because this will save them money. As a small organisation, Obepab found itself ill-equipped to store and sell its cotton to the commodities trading companies. Last year the government stepped in and now buys the whole crop. Soil in the area where the Adihous live is poor and following catastrophic floods in 2010, some farmers returned to conventional methods in search of higher yields. But in the two other regions where the project is based, organic farmers are thriving and independent research shows that while their margins are not always higher than those of conventional farmers, their outgoings are lower.

So how can we support projects like this one? Where can we buy the T-shirt? The answer is we can’t, or not directly. When British recycling charity Traid agreed to fund Obepab with the profits from its 11 secondhand clothes shops, it sought a retail partner to buy the crop, who could then track it through the supply chain and sell it as organic cotton from Benin. But the search came up empty. While most big retailers have sustainability policies, these generally stop short of full transparency; the only ones who do this are ethical clothing specialists such as People Tree and Pants to Poverty.

The industry acknowledges cotton production problems and over the past 10 years has launched several schemes. The biggest, Better Cotton Initiative, aims to reduce environmental impact, improve farmers’ lives and produce 30% of all cotton by 2020. Meanwhile, following falling sales of its cotton, the Fairtrade Foundation last year gave up on traceability in favour of a new sourcing programme that is easier to implement. Between 2008 and 2011, the financial crisis, a tightening up of certification in India (where 70% of organic cotton is grown) and war in Syria meant that global organic cotton production fell back.

The crop has yet to recover and problems with seed supply and cash flow persist (“The agricultural timetable simply doesn’t fit well with the fashion seasons,” says Liesl Truscott of Textile Exchange). Meanwhile organic clothes have disappeared from some British shops. Next, Tesco, Arcadia (owner of Topshop, BHS and Dorothy Perkins) and Primark have all stopped buying organic cotton, while Walmart/Asda, five years ago the biggest buyer, has reduced its organic stocks.

So are consumers not interested? Retailers take different views. Of high-street chains, H&M and C&A share the most ambitious sustainability targets for cotton (though Dutch chain C&A has no UK shops): 100% “more sustainable” by 2020 (M&S’s target is 50%). Both say research shows customers value organic clothes and want to buy them, but only if they cost the same as non-organic ones. Last year H&M overtook C&A to become the world’s biggest buyer of organic cotton, with Puma and Inditex (Zara) also in the top 10.

Among big retailers, C&A has done the most work in digging down into its supply chain. “Our industry is not the most transparent,” says Philip Chamberlain, the company’s British head of sustainable business. “What we have learned is that it is possible to trace the raw materials in your supply chain back to the earliest stages.”

Back in Benin, as we stop to buy fruit from hawkers and fill up the cars in market towns where illegal Nigerian petrol is on sale at the roadside, my eye is caught by the hairdressers’ painted signs in every row of shacks, and groups of girls and women braiding each other’s hair.

How do the women farmers trained by Obepab use the extra income from their new fields – called “afternoon fields” because they farm them after work on their husbands’ land is done? Does their hard-earned cash go on hairstylists or clothes?

“We like these things,” says Albertine Adjalian. “But the main thing is to send our children to school. The future is more important.”