Elizabeth McCormick, assistant professor of architecture and building technology at the University of North Carolina, at Charlotte School of Architecture, argues that the tendency to kill all germs means that beneficial bacteria in homes and buildings are also eradicated. In her role as editor of the new book Inside OUT: Human Health and the Air-Conditioning Era, she suggests that researchers should focus on finding ways to introduce healthy germs into homes.

According to a report from the US Environmental Protection Agency, Americans spend 90 per cent of their time indoors, but McCormick points to an emerging body of research asserting that often these spaces have become ‘too clean’.

“Advertisements for hand soaps and detergents, for example, underscored the fear of germs, while advertisements for prescription drugs also emphasised visual representations of germs,” she explains. “Ultimately, the realisation that diseases could be spread through human transmission, not environmental conditions, induced a culture of excessive cleanliness and urban germaphobia.”

This theory, known as the ‘hygiene hypothesis’, is one explanation for a jump in childhood illnesses and increased microbial resistance to antibiotics.

“In designing ‘healthy’ buildings, the discourse has revolved around the total elimination of microbial communities, which eliminates organisms that are both non-pathogenic and necessary for healthy and robust immune function,” she reasons. “This microbial network is an inevitable and essential component of both human and non-human life; however, buildings often neglect this notion, instead opting for sterile, antiseptic interior spaces.”

Healthy germs

Microorganisms are everywhere: outside, in soil, on plants, in the food we eat, in animals, in and on our bodies, on surfaces, and in our homes. While microorganisms are sometimes seen as harmful contaminants, they serve numerous beneficial purposes in our everyday lives, from contributing to the human immune system to maintaining soil health.

McCormick points to a growing body of research that proposes a ‘probiotic approach’ to architecture. According to the ‘hygiene hypothesis’, overly clean environments fail to provide necessary exposures to microorganisms to educate our immune systems so that our bodies can respond to infectious organisms.

“Exposure to a diverse set of microorganisms at a young age has been associated with decreased asthma risk,” she notes. “However, it’s not clear if only diversity is needed or whether a specific microbial community made up of a beneficial mixture is required. For example, studies have shown how childhood exposure to dogs is associated with a decreased risk of allergies and asthma.”

Researchers are now exploring whether targeted hygiene is more effective to manage microbial diversity, in place of indiscriminate sterilisation procedures.

Indoor air quality

As well as issues around eradicating healthy bacteria with over-cleanliness, McCormick also discusses how indoor pollutants can cause more damage to the human body than outdoor air pollution.

“It’s possible, and even likely, that the air in the middle of a busy intersection might actually be cleaner than the air in your living room right now,” she explains. “However, most Americans still perceive the risks of outdoor air pollution as being substantially higher than the threat of indoor air, even though dangers posed by long-term exposure to unhealthy indoor air have become more apparent in recent years, particularly for people who suffer from allergies and asthma, as well as children and the elderly. Allergies are increasing across the globe but more so in the developed world due to chemical exposures in low-quality indoor air.”

A meta-analysis of more than 40 studies published between 1977 and 2013 found that children living in homes with gas stoves had a 42-per-cent increased risk of having childhood asthma and a 24-per-cent increased risk of lifetime asthma. Despite these findings, however, national accounting of asthma triggers is incomplete, so it’s still difficult to compare against the health threats of other indoor pollutants.

Researchers have determined that both the ventilation source (dispersal of outside organisms indoors) and the conditions within the building (temperature, relative humidity, floor type, and space type) contribute to the indoor microbiome.

Potential solutions

While there are some changes people can make at a household level, including regularly ventilating spaces and not over-cleaning, the new book’s authors say much more could be done. McCormick points out that although ambient air pollution has been federally regulated in the United States for more than 50 years, there are still no federal laws to protect indoor air.

An obvious solution would be to improve ventilation in buildings, but she accepts that increasing ventilation rates to the levels necessary to create a healthy building would have tremendous impacts on energy consumption.

“While many technologies can improve indoor air quality, a techno-centric approach will not challenge our relationship with isolated conditioned environments,” McCormick explains. “Mechanical conditioning could lead to excessive energy consumption, carbon emissions, and expansive ecological degradation.”

Rather, McCormick and her co-authors hope to promote design changes that increase biodiversity and encourage more creative solutions that don’t rely exclusively on technology. “The human relationship with dirt, germs, and cleanliness is actually a relatively new construct that stems from social theories of health and hygiene,” she says. “Because of the close associations between sociocultural constructs and technological development, truly healthy buildings must embrace both technology and human behavior concurrently.”

She posits that one such creative solution in the future could be to “seed” indoor microbiomes with probiotics (beneficial microorganisms) to enhance overall human health. One vision involves seeding materials in indoor spaces with these beneficial organisms and positively changing the overall microbiome.

In the immediate term, McCormick says much more work is needed to determine the right balance of too much and too little indoor microbial exposures, given evidence that certain microbial exposures have beneficial health implications.

For more details and to purchase the book, Inside OUT: Human Health and the Air-Conditioning Era, edited by Elizabeth McCormick, (Routledge 2024) click here.