People could be forgiven for being confused over whether a bit of dirt in the diet is good or bad for health.

People could be forgiven for being confused over whether a bit of dirt in the diet is good or bad for health.

Food scientists warn against unhygienic practices yet respiratory doctors have suggested that coming into regular contact with a bit of dirt may ultimately protect children against asthma and other allergic conditions.

Food poisoning and asthma are both increasing for different but related reasons. Changes in lifestyle have created new opportunities for food-poisoning microbes as well as the factors that are thought to lead to asthma.

Kitchen practices learnt a generation ago - such as keeping separate knives for raw and cooked meat - are often unknown to younger people brought up on a diet of microwaved meals.

Modern homes are poorly ventilated, with fitted carpets and central heating providing perfect breeding grounds for house-dust mites, which are known to induce the allergic reactions that lead to asthma.

Scientists believe that the immune system of children is designed to be "primed" early in life to fend off attacks by potentially lethal infections, such as intestinal parasites. If the environment is too clean, the immune system locks on to more innocuous foreign bodies, such as house-dust mites, and switches the attack from the gut to the lungs - causing asthma.

Asthma specialists point out that children raised on farms are less likely to develop asthma than children who are brought up without pets. Children brought up in poorer, less hygienic homes are also less prone to the respiratory disease than wealthier children. Some scientists, such as John Warner of Southampton University, suggest that it may be possible to inoculate very young children against developing asthma by introducing certain bacteria into their diet soon after birth, when the immune system is being primed.

However, food poisoning specialists, such as Hugh Pennington of Aberdeen University, argue that the potential danger of being infected with something as dangerous as E.coli 0157 outweigh the possible benefits. He has warned against taking children under five to visit working farms because of the risk of them being infected with the deadly E.coli.