Modern hygiene has largely eliminated diseases such as cholera and dysentery – other than during war and after natural disasters. However, there is a common perception we may have gone too far, and that by cutting our exposure to mild pathogens we have unleashed a different epidemic: one of widespread allergies. Microbiologists are pushing back against this view, bringing together evidence that contradicts it.
“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,” Professor Graham Rook of University College London said in a statement. Rook is the first author of a paper in the Journal of Allergy and Clinical Immunology setting out four problems with this story.
The “hygiene hypothesis”, which blames the rise of allergies on lack of exposure to dirt and microorganisms, has some truth to it, Rook noted; “Exposure to microorganisms in early life is essential for the 'education' of the immune and metabolic systems […] Throughout life we need exposure to these beneficial microorganisms, derived mostly from our mothers, other family members and the natural environment.”
“The microbiota of the modern home is also enriched in microbiota of human origin,” the paper acknowledges. Nevertheless, the authors argue, the hygiene hypothesis ignores the fact the microorganisms we need most are seldom those found in our homes, cleaned or not. Instead, the way to make sure children (and adults) encounter the microbes they need is to spend more time in a natural environment – unwashed hands don’t qualify. To the extent that too much cleaning does harm, it is probably through exposing the lungs to cleaning products that trigger allergic reactions directly, rather than any local extinction of microorganisms.
More important factors in the loss of healthy microbiota are the decline in breastfeeding, the increasing rate of cesarean deliveries in industrialized countries, and possibly the overuse of antibiotics.
The paper’s other point will cause particular outrage among a subset of hygiene hypothesis advocates. “Any benefits of exposure to the infections of childhood […] are at least partly replaced by the recently revealed non-specific effects of vaccines.” In other words, the thing the best way to strengthen your immune system is not some concoction advertised on Youtube. Instead, vaccines – particularly against tuberculosis and measles – offer partial protection against a wide variety of diseases, not just the ones they were created to fight.
“Targeting hygiene practices at key risk moments and sites can maximize protection against infection whilst minimizing any impact on essential microbial exposures,” the paper continues. “Moreover this targeting must aim to reduce direct exposure of children to cleaning agents.”
Having children out of the house and in more natural environments when the cleaning is being done so they don’t breathe the products in would be helpful. Allowing indoor molds or bacteria to flourish, not so much.