As I walked past a compound in the Gambia, a man lifted a length of bright yellow cloth from an oil-drum vat steaming over a wood fire. Within seconds the colour changed to green and then to a rich navy blue. Babouka Faal was dyeing his batik indigo. His hands and the ground were blue, but Gambia is not the only place which has had its soil stained by indigo dye.

Traditionally dyers made indigo dye by fermenting the leaves of certain plants - woad (Isatis tinctoria) and Dyer’s Knotweed (Polygonum tinctorum) in temperate regions, and Indigofera species in the tropics. This process is little used now, yet there is an initiative to grow woad and other indigo-producing plants in the UK, Finland, Spain, Italy and Germany in efforts to try to make a natural indigo dye. However, it has been calculated that completely replacing synthetic dye with natural dye requires 6 million tonnes of the vegetable dye, derived from 462 million hectares of crop - 31 per cent of the world’s agricultural land.

Indigo was first produced as a chemical dye in 1897 by BASF. Current world production of the synthetic indigo employed in the manufacture of jeans, some 17,000 tons per year, uses thousands of tons of deadly toxins. The darker the blue chemical indigo dye is, the more toxic it is.

Eventually the cotton is spun into the right types of yarn for jeans. These yarns are dyed using water-soluble indigo created by adding a chemical reducing agent. The distinctive character of denim is achieved by weaving dyed warp thread with undyed weft. The fabric is heat-treated to stabilise it and the jeans are then made. After assembly, special finishes are achieved, such as “stone washing”, by an industrial laundering process using pumice or an enzyme treatment.

BASF has carried out an eco-efficiency analysis of indigo and the denim manufacturing process, and considered five different denim-manufacturing options, but its findings are short on detail. The most eco-friendly is achieved by using a 40 per cent indigo solution with an electrochemical dyeing process, eliminating the need for harmful chemical reducing agents. The least friendly is dye obtained from plants and used with the standard dyeing method. An intermediate process, still relying on toxic chemicals, is being launched to reduce the use of reducing agents. Meanwhile the electrochemical dyeing process is being developed.

The fashion for stone-washed jeans has led to the extraction of volcanic pumice from countries such as New Mexico and Turkey, causing environmental problems. Industrial stone-washing releases indigo into streams, cutting out light and as a result killing fish and plants.

Mining and refining the copper rivets and other fastenings some jeans carry as fashion accessories further damages the environment. The net effect of the chemical treatments and metal trimmings is to create garments that are less biodegradable and more difficult to recycle.

MM Margaret Mackintosh is a geographer and freelance writer