Children living in built-up cities are much more likely to have food allergies than those living in the country, according to a new study.

Youngsters who grow up in busy urban areas are more than twice as likely to have a peanut or shellfish intolerance than their rural counterparts, a US study has revealed.

The allergies could be triggered by exposure to pollutants at a young age, experts believe.

Conversely, those living in the country could develop immunities from being exposed to bacteria prevalent in nature.

Almost ten per cent of those born in densely populated areas have a food allergy, in comparison to just six per cent of those born in areas with a low population.

‘We have found for the first time that higher population density corresponds with a greater likelihood of food allergies in children,’ said lead author Ruchi Gupta, assistant professor of pediatrics at Northwestern University Feinberg School of Medicine.

‘This shows that environment has an impact on developing food allergies.

‘Similar trends have been seen for related conditions like asthma. The big question is – what in the environment is triggering them? A better understanding of environmental factors will help us with prevention efforts.’

The study included 38,465 children aged 18 and under from a range of backgrounds, whose food allergies were mapped by ZIP code.

Nearly 40 per cent of food-allergic children in the study had already experienced a severe, life-threatening reaction to food. Their reactions were equally severe regardless of where they lived, the study found.

Just 1.3 per cent of children from rural communities were allergic to peanuts, while 2.8 per cent had the allergy in urban areas.

Less than one per cent of children from the country had shellfish allergies, while 2.4 per cent from cities were allergic.

Past research has shown an increased prevalence of asthma, eczema, allergic rhinitis and conjunctivitis in urban areas versus rural ones.

‘Dr Gupta’s ongoing research on food allergy prevalence is providing critical information to help us address the growing public health issue of food allergies,’ said Mary Jane Marchisotto, executive director of the Food Allergy Initiative, which helped fund the study.

‘We are committed to finding a cure for food allergies and this study provides additional insight about why certain people have food allergies and others do not.’

Food allergy rates have risen sharply in the last 20 years. Symptoms include itching, a red skin rash, swelling of the face, and in the most serious cases, an anaphylactic shock, which can be life-threatening.

It is estimated that around one child in every 14 children under the age of three has one or more food allergies, according to NHS figures.

They are caused when the immune system mistakenly treats proteins found in food as a threat to the body, when in fact they should be harmless.

An estimated four out of five children with peanut allergies remain allergic to peanuts for the rest of their life.