Sometimes unhelpful (or just plain wrong) health advice sticks in our brains. For example, you don't necessarily need to drink eight glasses of water every day, and an apple a day may not keep the doctor away if you have fructose intolerance. But what about overly sanitized homes ruining our immune systems?
Although it's been debunked time and time again, this incorrect interpretation of the 'hygiene hypothesis' has stuck around in our collective consciousness. Now, researchers in the UK have published a paper systematically rejecting the idea that we're just too clean for our own good.
"For more than 20 years there has been a public narrative that hand and domestic hygiene practices, that are essential for stopping exposure to disease-causing pathogens, are also blocking exposure to the beneficial organisms," said Graham Rook, lead author and a University College London microbiologist.
"In this paper, we set out to reconcile the apparent conflict between the need for cleaning and hygiene to keep us free of pathogens, and the need for microbial inputs to populate our guts and set up our immune and metabolic systems."
The researchers stress that microbes are incredibly important to us. Our whole bodies, including our guts, skin and lungs need them to keep us effectively running.
The hygiene hypothesis is specifically about early childhood exposure to particular microbes which have co-evolved with humans to help develop a robust immune system – particularly in regards to allergies and other immune disorders.
Doing things like not washing your hands before eating is not going to help you develop a better immune system, it just means you're more likely to give yourself gastroenteritis. (However, it's important to note that your choice of washing products can breed hardier microbes, so it's best to stick to plain soap.)
In the new paper – which is a review of previous literature – the team lays out four particular nails in the coffin for the 'clean home is bad for immunity' adage.
Firstly, babies and children develop their own little microbial system, seeded first by their mother, and then mostly by family members and their environment. Microbes from individuals shed and intermix, creating a specific household microbiome mostly shared by those living together (including pets).
"Exposure to our mothers, family members, the natural environment, and vaccines can provide all the microbial inputs that we need," Rook said.
"These exposures are not in conflict with intelligently targeted hygiene or cleaning."
Secondly, vaccines are actually surprisingly good at priming our immune system for other things too, in the same way a potentially deadly disease is.
"In the 1980s, it began to be reported that vaccination with a live measles vaccine in Africa reduced overall childhood mortality to a degree that could not be explained by the incidence of measles itself," the team wrote.
"The nonspecific effects of vaccines are similar to the nonspecific survival benefits seen after recovery from the corresponding infections."
Thirdly, we are now well aware that exposure to the outdoors is important for helping us develop robust immune systems. But no one is cleaning the great outdoors, and the team notes that the bacterial profiles of nature are completely different to the ones you find indoors anyway.
"Exposing children to biodiversity from the natural environment in their school playgrounds resulted in increases in peripheral blood biomarkers of immunoregulation," the team wrote in their paper.
"So evolutionary and epidemiologic considerations point to the view that children need exposure to the microbiota of the natural environment rather than to the unnatural microbiota of modern buildings."
Finally, although this last point is not yet definitive, the team suspects that when there are health issues related to a clean environment, it might not be the removal of organisms that are causing the health problems, so much as the harsh cleaning products used.
The team suggests that targeting our cleaning might help limit these kinds of issues, as well as our exposure to the sort of microbes that might make us sick.
"So cleaning the home is good, and personal cleanliness is good, but, as explained in some detail in the paper, to prevent spread of infection it needs to be targeted to hands and surfaces most often involved in infection transmission," Rook said.
"By targeting our cleaning practices, we also limit direct exposure of children to cleaning agents."
The review has been published in The Journal of Allergy and Clinical Immunology.