Is Your Home Too Clean for Your Child's Health?

We spend the great majority of our time indoors, and what we encounter inside, like flu virus on a doorknob, can have a significant impact on our health.

But in our haste to disinfect our homes, are we killing the good germs with the bad? Just as research is expanding our understanding of the microorganisms we harbor in and on our bodies -- the human microbiome -- some experts are now looking more closely at the mix of microbes inside homes, offices and other buildings. The indoor microbiome is the ecosystem of microbes inside the built environment: "That's the bacteria, fungi and viruses that live and breed in these indoor spaces," says Jack Gilbert, a professor of surgery and faculty director of the Microbiome Center at the University of Chicago. "The exposures to microbes that we get from that environment are likely crucial to our health and wellness. Our immune systems are trained as children based on the microbes we are exposed to."

At least in the developed world, we spend about 90 percent of our time indoors, says Jordan Peccia, a professor of chemical and environmental engineering at Yale University. So most exposures to chemicals or microbes occur inside, and that's where infectious diseases are typically spread, he says.

[See: How to Disinfect Germ Hotspots.]

"Importantly, there are more than a trillion species of bacteria on this planet, and only around 100 to 200 are actually dangerous pathogens," says Gilbert, who co-wrote the new book "Dirt Is Good: The Advantage of Germs for Your Child's Developing Immune System." "The rest are mostly unable to harm us. However, they do still stimulate our immune system when we come into contact with them."

While much of the focus on health in the home has centered around reducing potential hazards, such as dampness that can cause mold growth and keeping surfaces clean to reduce the spread of infectious disease, scientists are increasingly trying to understand how the indoor microbiome might positively impact health.

Research published last year in the New England Journal of Medicine found that despite similar genetic ancestries and lifestyles Amish school children in Indiana -- who grew up in an environment that was rich in microbial exposures -- had much lower asthma and allergy rates than Hutterite school children in South Dakota. Dust samples taken from Amish households that practice traditional farming -- where it's common to come into direct contact with farm animals -- had median levels of endotoxins, a toxin present in bacterial cells, that were 6.8 times as high as those found in the homes of Hutterite families, who lived on large, industrialized, communal farms.

"The environment (including the microbes in it) has a huge impact on risk for most common complex diseases, especially asthma and allergy in children," says Dr. Donata Vercelli, a professor of cellular and molecular medicine and associate director Asthma and Airway Disease Research Center at the University of Arizona. Vercelli and Gilbert were co-authors on the study. "Growing up on a traditional farm, that is, in a place rich in microbial exposures, protects from asthma and allergy almost completely," she says.

Not that children necessarily need to grow up on a farm to benefit. Other research has found that having pets, and particularly a dog, in increases the diversity of microbes in the indoor environment. "Based on the best data that we have right now ... microbial exposures seem to be most important in the first year of life, and maybe even earlier than that -- maybe the first six or first three months of life," Peccia says. A mother's exposures to germs, while pregnant, also seem to be important to the child, he adds.

What's not well understood is which microorganisms might be most beneficial to have indoors. Microorganisms that belong to the genus lactobacillus are among those studied. These bacteria show up in breast milk, are in the vaginas of many women -- and so they're among the first bacteria a child would encounter with a natural birth -- they're common in cattle and they've been shown to be on dogs as well, Peccia says. They're also sometimes found in probiotics, like yogurt. However, rather than there being one "magic bullet," based on emerging but still relatively limited research on the indoor microbiome, experts say it appears that -- as with the human microbiome -- having a rich diversity of germs is what matters most. And that's what many argue we're losing in more sterilized, modern indoor environments -- and with less exposure to things like farm animals and other germs we might encounter outdoors.

[See: Green Exercise: 10 Ways to Incorporate Nature Into Your Workout Routine.]

"It's almost as if some attempts to keep everything super clean -- and all these wipes that are marketed to rub on our kitchen counters -- are in some ways the opposite of what we should be doing," says Joan W. Bennett, a fungal geneticist and professor of plant biology and pathology at Rutgers University-New Brunswick. Bennett is chairing a National Academies of Sciences, Engineering, and Medicine committee that is reviewing credible scientific literature on the indoor microbiome. "To develop a robust human microbiome, we have to be surrounded by an appropriate microbiome in our environment." That's not to mention the health hazards some harsh cleaning chemicals pose, from triggering allergic reactions to raising cancer risk.

Bennett says obviously you wouldn't want to leave rotting food out and you would want to flush toilets, as well as fix leaks that could lead to mold growth, or address standing water that could raise the risk of Legionnaire's disease, a type of pneumonia caused by bacteria. "But maybe a pacifier that falls on the floor does not have to be sterilized before it's returned to the baby's mouth, and maybe it's OK to let your kid crawl around in the dirt, not just on some artificial turf," she says. "We, in some ways, have super-sterilized our immediate environments, and it's not good."

Admittedly though, Bennett and others say much remains unknown about the indoor microbiome and how to balance the good and bad of germ exposure inside. What's more, this may be a moot discussion for some who are impoverished and live in rundown homes -- with everything from plumbing problems to standing water -- conditions well-known to be potentially hazardous to health, she says.

"I'm not convinced that a clean a house is unhealthy for people, certainly not past the first year of life," Peccia says. "I've seen a lot of data that dirty homes -- that homes that have a lot of microbes and fungi in them -- are hazardous to human health. I would say moreso right now the weight of evidence is on keep your home clean. You can still have a dog."

Harnessing the positive effects of the indoor microbiome is still largely a futuristic work in progress, as scientists are still seeking to unravel the mysteries of how the indoor environment might affect the human microbiome. Still, some in the building industry are already looking at adding probiotics to construction materials to make indoor spaces healthier, says Brent Stephens, an associate professor of architectural engineering at the Illinois Institute of Technology in Chicago. However, he doesn't see the expanding understanding of the indoor microbiome changing the industry's focus -- which is still primarily on reducing potential building hazards -- any time soon. And he thinks more efficient, simpler measures, like eating yogurt with probiotics, are probably a better approach to properly balancing microorganism exposures.

[See: 9 Ways to Boost Your Immune System.]

Peccia agrees. That includes bringing outside air in. "The more you ventilate a home, the healthier typically it will be. I can't think of any negative effects of ventilation as long as you live in an environment or in a city or in a country where the outdoor air isn't really bad," he says. "If you live in Beijing, China, ventilation is not going to help you much because the outdoor air is probably worse than what's indoor. But if you lived in most places in the United States, bringing more outdoor air into a building is usually a good thing."

Michael Schroeder is a health editor at U.S. News. He covers a wide array of topics ranging from cancer to depression and prevention to overtreatment. He's been reporting on health since 2005. You can follow him on Twitter or email him at mschroeder@usnews.com.