Natasha Salmon cannot be in the same room as a bowl of peanuts without getting wheezy. The last time she ate satay sauce, she was struggling to breathe after only a mouthful.

“It was five or six years ago and I was in a Thai restaurant,” she recalls. “I accidentally ate some satay [made from peanuts]. Within seconds my face, lips and airways were completely swollen. I used the adrenalin pen that I always carry with me in case of a severe allergic reaction. But my friends had to call an ambulance and I was ill for about a week.”

Natasha’s allergy to peanuts first developed when she was just five, and her symptoms have grown worse over the years. The 21-year-old journalism student from Farnham in Surrey developed an allergy to raw fruit and vegetables at 15, and recently has suffered swelling of her lips, face and throat after eating salad foods such as lettuce, celery and cucumber.

“I can’t begin to describe how much I miss a fresh, crisp Golden Delicious,” she says. “In the winter my allergies are less severe and I will treat myself and eat just the skin of an apple. I’ll manage it by taking an antihistamine and putting Vaseline on my lips. But there is no doubt that my allergies are getting worse. My immunologist can’t explain why, though it’s thought to be related to my hay fever. I just have to deal with them and hope they subside.”

Food allergies such as Natasha’s have risen sharply over the past 20 years, with 1-2 per cent of adults and 4-6 per cent of children thought to be affected. The number of children admitted to hospital for food-related anaphylaxis (a life-threatening allergic reaction) has risen seven-fold since 1990 – but no one quite knows why.

The latest theory, published in the Annals of Allergy, Asthma and Immunology last week, suggests that chemicals called dichlorophenols could be to blame. US researchers who looked at food allergies in more than 2,000 people found that those with the highest levels of dichlorophenols in their urine had an 80 per cent higher risk of having a food allergy.

Their theory is that dichlorophenols, which they say are found in purified tap water and in pesticides and disinfectants, have anti-bacterial properties that could affect the microflora in the gut that are thought to protect against food allergies.

So should we stick to organic fruit and veg and substitute tap water for the bottled kind? Not just yet, according to Jonathan Brostoff, professor of allergy and environmental health at King’s College London. He says the study has several flaws, not least the fact that it classified people as having a food allergy by using a blood test that is only 50 per cent reliable. Food allergy diagnosis is a complex area, he says, and hard to diagnose without a clinical interaction with the patient.

“Another problem is that the study didn’t publish its results on how many people with the lowest levels of dichlorophenols tested positive for food allergy,” he adds. “It might be possible that these chemicals affect the balance of the immune function and the risk of food allergy, but we would need far more data to show cause and effect.”

Even if an association was shown, UK tap water is perfectly safe, says Prof Jeni Colbourne, head of the UK’s Drinking Water Inspectorate, and probably safer than in the US, where controls are not so strict and higher doses of chlorine are used. Dichlorophenols, she explains, are not used to treat water, as has been implied in some reports. They are by-products that only occur if the chlorination process reacts with industrial pollutants, called phenols, present in untreated water.

“In the UK there are strict controls requiring water companies to monitor water for the presence of phenols and to minimise the presence of such disinfectant by-products,” she says. “Water tastes foul when dichlorophenols are present, even at exceptionally low concentrations, so taste and odour monitoring is the other safeguard. Even if present in trace amounts in tap water, consumers would be reporting it in very large numbers.”

She also points out that dichlorophenols do not accumulate in the body but are rapidly excreted in urine. “So the finding in the study of the substance in the urine of some participants is actually indicative of normal healthy body function.”

Dichlorophenols are also by-products of a common antibacterial and antifungal agent called triclosan (used in many consumer products, including toothpaste) and in the UK are more likely to be found in household cleaning products, kitchen utensils and containers and pesticide residues than in water. Although little research has been done on UK exposure levels, the Chemicals Regulation Directorate says dichlorophenols “have not been identified as a cause of concern”.

The reason behind the rise in food and other allergies remains a mystery. One well-known theory is the hygiene hypothesis – that as we become increasingly obsessed with cleanliness, our children are not exposed to the bugs that help the immune system develop properly. Another unproven theory is that the rise in allergies may be due to basic changes in Western diets, with processed foods becoming more common and fresh fruit and veg less so.

“No one has an answer, although the hygiene hypothesis is still favoured,” says Kristian Bravin, a dietician in Leicester. “Increased awareness and improved diagnosis also play a role.” He adds that up to 20 per cent of the population believe they have a food allergy or intolerance, but in fact far fewer are affected.

Some experts now think that government guidelines introduced in the Nineties may have contributed to the explosion in food allergies. “Parents were advised to avoid giving peanuts to young infants on the grounds that early consumption of potential allergens could affect underdeveloped immune systems, resulting in allergy – when the opposite seems to be the case,” says Prof Brostoff.

The guidelines were quietly withdrawn in 2009 after a large study the previous year showed that Jewish children living in the UK were almost 10 times more likely to develop a peanut allergy than those living in Israel, where peanut protein – often in the form of Bamba, a popular snack – is commonly given to infants in the first year of life.

“At the moment there is confusion about what to advise,” says Bravin. Several key studies now taking place will hopefully help decide the best strategy for preventing food allergy. They include a joint US/UK study of 640 children, led by Prof Gideon Lack, a paediatric immunologist at Guy’s and St Thomas’ and King’s College hospitals and co-author of the study of Jewish children, looking at whether regularly feeding peanuts for the first three years of life reduces the risk of allergy. Another study at King’s is looking at whether introducing six allergenic foods from three months on might also be beneficial.

Prof Brostoff says the best thing parents can do to reduce the risk of food allergy in their children is to gradually introduce potential allergens – not just peanuts, but cow’s milk, eggs and fish – at an early age.

“The lining of the gut membrane is the most powerful immune organ in the body,” he says. “The advice I always give is that it is better to introduce foods when the gut is more plastic and able to adapt. Giving a food early in a baby’s life can help make the gut more tolerant, and numerically this effect is more important than the presence in the environment of any chemicals.”

Meanwhile, Natasha Salmon manages to enjoy food despite her restricted diet. “Luckily I love carbohydrates,” she says. “Spanish and Italian foods tend not to contain nuts, so my favourite foods are paella and pasta.”