This message has been accepted without question by consumers, many of whom choose organic produce primarily for health reasons. Indeed, health is the main motivation for 70 per cent of organic food customers in Germany, and for 46 per cent in the UK.
The health argument is twofold. First, to avoid the negatives of conventional farming. Organic crops are grown without synthetic insecticides, fungicides, herbicides, molluscides, growth regulators and other chemicals that are used routinely on most farms, and they are stored without additional agro- toxins. Ten or more applications of chemicals over a growing season would not be unusual for many crops, followed by further dosing to prevent spoilage in store.
Concerns about chemical contamination have been given weight by the UK Pesticide Safety Directorate's annual review of pesticide residues in food, published last Thursday. Of the 2,500 samples taken in 1998, 25 per cent included detectable residues of pesticide and 1.4 per cent exceeded government-defined Minimum Residue Levels (MRLs).
Particularly bad were pears, many of which contained the unlicensed growth regulator chlormequat; and winter lettuce, which contained the unlicensed fungicide iprodione and the organophosphate (OP) insecticide malathion. One tin of corned beef contained the insecticide DDT, banned across the EU as a persistent bio-accumulative carcinogen. Lindane, another carcinogen, was found in a bar of dark chocolate.
Peter Beaumont, director of the Pesticides Trust, believes that the short- term dangers to health are small, as the actual amounts of pesticide involved are small. However, he adds, there can be wide variations among individual vegetables, leading to higher potential dosages. Of two carrots grown in the same field, one may contain 30 times more pesticide than another.
"The average level may be safe, but eat three `hot' carrots and you may get a bad tummy ache. Many pesticides such as the OPs work in the same way, and the toxic effects are additive."
Another concern is the widespread use of antibiotics in animal feeds to increase growth rates, as they encourage the spread of antibiotic-resistant bacteria which may go on to cause disease in humans. By contrast organic livestock may only be given antibiotics medicinally. Growth hormones are also used in conventional farms to make animals put on weight fast - and residues in food can potentially affect those who eat them. Again, such hormones are banned on organic farms.
Organic food is also seen as carrying a lower risk of BSE infection - a reasonable supposition since no organically bred and reared cattle in the UK have ever developed the disease. People are also going organic to avoid genetically modified material, which is strictly forbidden in organic foods. While there is no firm evidence that GM foods are a danger to health, nor is there any firm evidence that they are safe.
Rather harder to pin down are the positive perceptions of organic food: that it is richer in minerals and vitamins, and that it encapsulates a more vigorous "life force" which can be transferred to its eater. Neither the Ministry of Agriculture, the Department of Health, the Soil Association, nor the Henry Doubleday Research Association (HDRA), Britain's premier organic research body, could point to any specific evidence of the health- enhancing (or detracting) qualities of organic food.
However the Putney-based Institute for Optimum Nutrition had some information to offer. Its nutritionist Yara D'Avella quotes a study by Bob Smith (Journal of Applied Nutrition, 1993) of foods bought in Chicago shops, which showed that organic pears, apples, potatoes and wheat contained nearly double the nutritional minerals, and smaller amounts of toxic metals such as lead, mercury and cadmium.
Lawrence Woodward, director of the Elm Farm Research Centre near Newbury, cites a 1997 review of 150 investigations (1926-1994) on the subject carried out by Germany's Federal Institute for Health Protection of Consumers. The study concludes that organic produce contains less nitrate, especially in leaf vegetables; however, organic cereals contained less protein. Food selection experiments showed that animals "prefer organic produce".
A paper presented at an Elm Farm colloquium in 1989 confirms such findings. Ludwig Maurer, director of the Boltzman Institute for Biological Agriculture in Austria, showed that rabbits and hens preferred organic produce.
So, such evidence as exists does seem to indicate that organic food is better for health. But given the importance of the subject and the high level of public interest in it, the state of knowledge is patchy - indeed it is, says HDRA director Jackie Gear, a "public scandal".