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Being Clean Doesn't Have to Be Bad for Our Immune System, Scientists Say

The "hygiene hypothesis," which says that exposure to dirt and germs strengthens our immune system, may be overstated.

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A handwashing station at the Exmoor Zoo in Barnstaple, England, in a photo taken June 11, 2020
A handwashing station at the Exmoor Zoo in Barnstaple, England, in a photo taken June 11, 2020
Photo: Harry Trump (Getty Images)

Staying clean might not be as bad for your immune system as is sometimes believed, researchers in the UK say. In a new paper this week, the authors argue that claims about the so-called hygiene hypothesis are overblown and people do benefit overall from keeping their home environment relatively clear of germs. But we do need regular contact with our microbial neighbors, they say—just in specific contexts, like being outside in nature.

The hygiene hypothesis, first put forth by epidemiologist David Strachan in the 1980s, argues that a relative lack of early childhood exposure to microbes can help cause certain immune-related conditions like allergies. The theory goes that our immune system needs these early encounters with bacteria, viruses, and other microbes to be well-calibrated, and when that doesn’t happen enough, we become more likely to overreact to substances that aren’t harmful, like pollen or other allergic triggers.


Studies throughout the years have shown that children living in “dirtier” environments, like farms or around lots of animals, do tend to develop allergies at a lower rate than similar groups of children living in more sterile environments. Other research has suggested that specific kinds of infections, such as those caused by parasitic worms, are similarly linked to a lower risk of asthma.

But researchers from University College London and the London School of Hygiene & Tropical Medicine say the link between childhood hygiene and the risk of developing immune problems is more complicated than the popular framing of the theory.


In their review of the evidence, published Monday in the Journal of Allergy and Clinical Immunology, they argue that good hygiene practices shouldn’t be tossed aside for the sake of maximizing exposure to microbes. Kids can still be plenty clean in their daily lives, the study authors say, but they also have to get exposed to the right sources of environmental microbes.

“Exposure to microorganisms in early life is essential for the ‘education’ of the immune and metabolic systems,” said lead author Graham Rook, emeritus professor of medical microbiology at UCL, in a statement released by the university. “But 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.”

Rook and his co-author point to evidence showing that so-called good bacteria are commonly introduced to people early on through their mothers and other family members, as well as the natural environment. Conversely, there’s less evidence suggesting that the many ubiquitous microbes found in a typical modern home are important to the development of a healthy immune system. There are also lots of nasty germs that can be kept from causing illness through simple steps like handwashing, without needing to worry about losing the ample supply of bacteria in and around our bodies.

And while kids may not be getting sick as much as they used to, they are still getting exposed to plenty of childhood germs—it’s just that these exposures are now often happening through the routine vaccination schedule common in the U.S. and other countries. Research has shown that vaccines do create their own non-specific immune responses, training the immune system in the same way as childhood infections of the past once did. (Contrary to rumors spread during the pandemic, there’s no evidence vaccines are somehow weakening our immune system.)


That said, excessive hygiene might indeed be bad for kids, but in a different way than commonly thought, the researchers argue. They cite research suggesting that greater exposure to chemicals found in many cleaning products may trigger the sort of immune dysfunction that leads to allergies and asthma.

The debate over the hygiene hypothesis isn’t likely to be settled by a single review article. But the authors hope their paper can convince people to think of hygiene in a more nuanced way. Rather than trying to sterilize our indoor environments all the time, they say it’s better to strive for targeted hygiene. If we’re about to eat something with our hands, for example, then we should wash them first; same if we just finished going to the bathroom. But there’s no need to clean every inch of our homes frequently, nor do we need the sort of “disinfection tunnels” in public spaces that became popular in some countries during the pandemic.


“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. By targeting our cleaning practices, we also limit direct exposure of children to cleaning agents,” Rook said. “Exposure to our mothers, family members, the natural environment, and vaccines can provide all the microbial inputs that we need. These exposures are not in conflict with intelligently targeted hygiene or cleaning.”