STUDY: Sunlit Rooms Have Half as Much Bacteria as Dark Rooms, and No Respiratory-Disease-Triggering Bacteria

Open the curtains! Or maybe build more windows! Study finds sunlight is the best disinfectant!

Rooms bathed in sunlight have about half as many viable bacteria as dark rooms, and almost no respiratory-disease-triggering bacteria, a study published in Microbiome this month found.

Folk wisdom has told us for centuries that sunshine helps keep us healthy, but new research helps explain why.

Researchers at the University of Oregon set up several dusty, dollhouse-sized rooms, some with regular UV blocking glass, some with glass that let UV rays in, and some kept dark.

They “inoculated” each of the rooms with bacteria-laden dust collected from actual homes in Portland and put them outside, while keeping the insides at a normal room temperature.

After 90 days, they found 12 percent of the bacteria was still alive and viable (able to reproduce) in the the dark rooms. The rooms exposed to daylight through UV blocking glass had only 6.8 percent viable dust bacteria remaining, and the rooms that let UV light in had only 6.1 percent of viable bacteria surviving.

Dust kept in the dark contained species of bacteria closely associated with respiratory diseases, which were largely absent in the rooms exposed to daylight.

The bacteria that did survive in the sunlit rooms were mostly the same ones that would be found in outdoor air, suggesting sunlight causes the microbiome of indoor dust to more strongly resemble bacterial communities found outdoors.

“Until now, daylighting [illuminating a building with natural light] has been about visual comfort or broad health,” study co-author and co-director of the Biology and the Built Environment Center Kevin Van Den Wymelenberg told NPR. “But now we can say daylighting influences air quality.”

“Humans spend most of their time indoors, where exposure to dust particles that carry a variety of bacteria, including pathogens that can make us sick, is unavoidable,” said lead author of the study  Ashkaan Fahimipour in a press release.

In future studies, the researchers said they’d like to determine exactly how much light is necessary to kill microbes so architects can begin to design buildings with that in mind.

Fahimipour warned against household cleaners claiming to kill 99.99% of bacteria, as completely eradicating microbes is impossible, and undesirable even if it were possible.

“Sanitizing isn’t the best approach,” he told NPR. “And some microbes are actually good for us, like the ones in yogurt. It may be better to enrich an indoor setting with microbes that are not harmful, or even beneficial.”

UPDATE: Since reading the book The Contagion Myth, I no longer view any bacteria as “bad” or “pathogenic”… I view them as agents of detoxification… but a build up of bacteria in your house may be a sign of a build up of toxins!