Children whose mothers swim during pregnancy may have an increased risk of developing eczema and asthma when they are older, scientists claim.
Airborne chemicals from chlorine, used to keep pools hygienic, may alter a child's immune system before it is even born, raising the chance of these conditions.
The same is true of everyday cleaning products and some cosmetics, researchers warned in an article for the British Journal of Dermatology.
Allergy risk? Scientists believe swimming while pregnant may expose unborn babies to chemicals which change their immune systems
The scientists, from the St John's Institute of Dermatology in London and the University of Manchester, have been investigating the increase in cases of asthma, eczema, and hayfever, which have risen fivefold over the past five decades.
The UK has one of the highest rates of asthma in Europe, with one in 11 children and one in 12 adults suffering from the condition. Meanwhile eczema is virtually a right of passage these days, affecting one in every five British children at some point.
Possible causes for the burgeoning number of atopic allergies, as these conditions are known, include getting less Vitamin D, and the fact that people wash themselves more often than 50 years ago, potentially reducing their immune resistance.
However, the scientists have reviewed existing evidence and concluded that airborne chemicals given off by cleaning or cosmetic potions are also a factor.
On the rise: Asthma affects one in 11 children in the UK, and one in 12 adults (posed by model)
Dr John McFadden, consultant dermatologist at St John's Institute of Dermatology, said more research is needed as the so-called 'hygiene hypothesis' remains unproven.
'We in the science world are still struggling to find the exact cause of this rise,' he said.
'We have not proved anything... but w do know we are using far more chemicals than we did 50 years ago, whether it is in personal care products or processed food, and we think this should be looked at and studied more.
Those who suffer from atopic allergies often have immune systems in which the element which combats parasites is stronger than that which tackles bacteria. This bias is the same as that which takes place in a woman's body during pregnancy, to protect against miscarriage and premature birth.
Normally, the balance in the baby's immune system would have righted itself within a year, but researchers suspect the chemical exposure babies receive in that time may be contributing to a permanent alteration in their immune systems.
At present the NHS encourages pregnant women to stay healthy by doing low-intensity exercise such as swimming.
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