The cleaner the better? Not necessarily. Photo: Alija
It's called the hygiene hypothesis: the idea that although a cleaner environment has improved our health in some ways, it's also messed with our immune system, contributing to increased allergy and autoimmune disease.
But it's not as simple as just being 'too clean', explains immunologist Professor Dianne Campbell, Chair of Paediatric Allergy and Clinical Immunology at The Children's Hospital at Westmead.
'It's less about clean kitchens and bathrooms and more about changes to living conditions – like less crowded housing, better public health measures, less infectious disease and less contact with large animals."
"It's less about clean kitchens and bathrooms and more about changes to living conditions – like less crowded housing, better public health measures, less infectious disease and less contact with large animals. These have reduced our exposure to certain microbes in early childhood and this may have skewed our immune systems," she says.
The hygiene hypothesis sprang from 1980s research by British epidemiologist David Strachan who was trying to understand why, when so many other diseases had been beaten by better sanitation, the rates of asthma and allergy were increasing in western countries – yet less common in developing countries. One clue came from his study of 17,000 British children which found that those with a number of older siblings were less likely to have allergies. This raised interesting questions like could exposure to a broader range of microbes from mixing with other children in early childhood somehow protect against developing an allergy? Could it mean that young immune systems with more experience of dealing with a bigger variety of germs were better at telling the difference between a real threat and, say, a harmless speck of pollen?
Now a new twist to this theory has become a hot topic in science - the idea that changes to how we live have altered the kind of microbes inhabiting our gut, and it's this shift in our gut microbes that's affecting our immune systems.
"We all have millions of gut microbes and their makeup is influenced by where and how we live – it varies depending on which country you live in or whether you live on a farm, for instance," Campbell says.
"But the effects of these changes to gut microbes is broader than just allergy. It's about how the immune system has been programmed from early in life which can also influence the chances of developing an autoimmune disease such as Type 1 diabetes or coeliac disease. However, it's unlikely that changes to the immune system are the sole cause of allergy and autoimmune disease – it's more likely to be an interplay of multiple factors like genetic susceptibility, what viruses you're exposed to early on, as well as diet in infancy."
In a perfect world we'd know how to populate the human gut with the right mix of microbes to help promote a healthy immune system – but we're not there yet.
"We still don't know what the best population of microbes is and other factors like the use of probiotics are still being investigated," Campbell says.
But while squeaky clean kitchens may have little to do with the increase in allergy, there's another way in which our fixation with super-cleanliness could eventually do more harm than good.
"There's evidence that repeated exposure to products containing household disinfectants – but not hospital grade disinfectants - makes bacteria more resilient and less susceptible to killing by both disinfectants and antibiotics," says microbiologist Cheryl Power of the University of Melbourne's Department of Microbiology and Immunology.
"This has implications for treating possible infections with these microorganisms because they may be harder to kill both in the environment and in people who are infected," she points out.
"I wouldn't argue with using anti-bacterials in hand washes, especially if someone is sick with gastro or a cold, but I would argue that their inclusion in many other products, such as dishwashing sponges, is not only unnecessary, but potentially counterproductive. For cleaning the kitchen, hot water and soap are just as effective as anti bacterial products."