A rise in allergies is not caused by being too clean but by losing touch with mostly harmless organisms which scientists call "old friends", a study has found.
The report, from the International Scientific Forum on Home Hygiene, sets out to dismantle the myth that the epidemic rise in allergies in recent years has occurred because people live in increasingly sterile home environments - and overdo the hygiene.
The report, called The Hygiene Hypothesis and its Implications For Home Hygiene, Lifestyle and Public Health, concludes that losing touch with microbial "old friends" may be a fundamental factor underlying rises in allergies.
As well as allergies, numerous chronic inflammatory diseases (CIDs) such as Type 1 diabetes and multiple sclerosis seem to stem from impaired regulation of our immune systems, the report says.
One of its authors, Dr Rosalind Stanwell-Smith, from the London School of Hygiene and Tropical Medicine, said allergies and CIDs were serious and complex health problems.
"It's time we recognised that simplistically talking about home and personal cleanliness as the cause of the problem is ill-advised, because it's diverting attention from finding workable solutions and the true, probably much more complex, causes," Dr Stanwell-Smith said.
"If worrying about 'being too clean' results in people needlessly exposing themselves and their children to pathogens that can make them ill, this would clearly be dangerous."
The "old friends" are described as a variety of mainly harmless species of organisms which inhabit indoor and outdoor environments as well as the skin, gut and respiratory tract of both people and animals.
They also include some potentially harmful organisms such as worms, which can establish chronic, long-term infections or infestations.
Professor Graham Rook, co-author of the report, developed the "old friends" version of the hypothesis.
"The rise in allergies and inflammatory diseases seems at least partly due to gradually losing contact with the range of microbes our immune systems evolved with, way back in the Stone Age," Professor Rook said.
"Only now are we seeing the consequences of this, doubtless also driven by genetic predisposition and a range of factors in our modern lifestyle - from different diets and pollution to stress and inactivity.
"It seems that some people now have inadequately regulated immune systems that are less able to cope with these other factors."
Lead author Professor Sally Bloomfield said scientists were yet to come up with answers on how to reverse the trend in allergies and CIDs, but that relaxing hygiene practices would cause more problems.
"One important thing we can do is to stop talking about 'being too clean' and get people thinking about how we can safely reconnect with the right kind of dirt," Professor Bloomfield said.