'Hygiene hypothesis' connection: cleanliness and immune system


Dear Doctor: Our sister is a messy housekeeper. She calls it the “hygiene hypothesis,” and says it’s why her kids don’t have asthma or allergies and don’t catch colds often. Does avoiding antibacterial products and letting the dogs on the couch really keep her kids healthy? We can’t tell if she’s kidding.

Dear Reader: We admit that when your letter started out with a mention of housekeeping, we thought you’d written to the wrong columnists. But then you brought it around to an emerging area of study that examines our home environments and their potential effects on health and the immune system. Taken together, these are the basic components of the “hygiene hypothesis,” which your sister has been citing.

It’s a complex and intriguing collection of theories that began with a specific focus on asthma, but has, over the years, led to a series of broader discussions. In your sister’s case, whether or not she’s joking, she’s referring to questions about our modern-day standards of cleanliness -- that is, the ways in which the spaces where we live and work affect our health in general, and our immune systems in particular.

Let’s start with the original hygiene hypothesis, which dates back to the late 1980s. It focused on asthma, which is considered to be the most common chronic disease in the developed world. The thinking was that when infants are raised in the ultra-clean environments of the modern home, their developing immune systems fail to encounter the wide variety of microbes needed to properly educate their immune systems. This results in immune responses that go awry, leading to an array of childhood allergies and diseases, including the onset of asthma.

Elizabeth Ko, MD and Eve Glazier, MD

Newer sister theories suggest that being raised in germ-free environments may lead a child’s developing immune systems to become trigger-happy. As we continue to learn, an array of so-called “friendly” bacteria, fungi and viruses play a part in keeping our bodies functioning properly. The idea is that reduced exposure to bacteria, fungi and viruses can lead to an overreaction by the immune system when it encounters unfamiliar microbes, including those that don’t pose a threat.

It’s important to note that so far, these are all just theories. They are subjects of robust debate, and no specific mechanisms for or against these hypotheses have been identified as of yet. Still, as we learn more about the complexities of the human body’s connection to the world of microbes, it won’t be a surprise if the relationship turns out to be even closer and more complex than we originally suspected.

When it comes to the use of antibacterial household products, we urge caution. Yes, they can be effective at eliminating certain harmful germs. But in the process, they wipe out vast swaths of beneficial microbes and have the potential to play a role in antibiotic resistance. It’s not that adherents to the offshoots of the hygiene hypothesis are urging us to live in dirty homes. Rather, it’s about striking a reasonable balance between cleanliness and zero tolerance.

(Send your questions to [email protected], or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)