If you’ve ever heard a grandma claim that letting sunlight into the house kills indoor germs, you probably just smiled and nodded along. But recent research shows that you might want to listen to her: She may have been onto something long before scientists finally caught on.
An October 2018 study published in Microbiome analyzed a group of dusty dollhouse-size rooms to compare areas that had been exposed to daylight through glass, spots that had been exposed to ultraviolet light, and spaces that had been left dark. Results showed that the rooms exposed to daylight had fewer germs compared to the dark rooms. As it turned out, these exposed rooms actually had about half of the viable bacteria as the amount present in non-exposed rooms. Interestingly enough, the ones exposed to just ultraviolet light even had slightly less viable bacteria than the areas exposed to the daylight.
Even though the rooms analyzed in the study were tiny, it’s worth noting that the researchers used real dust collected from actual people’s homes in the study. This gave the researchers a unique opportunity to properly analyze the type of bacteria that was present at the beginning — and how much of it truly stuck around in the same rooms after a period of 90 days.
As NPR reported, the study’s lead author, Ashkaan Fahimipour, PhD, was surprised that the daylight and the ultraviolet light performed in such a similar way to reduce the number of bacteria in the rooms that weren't dark. Many other experts voiced their shock at the novel study as well. But we bet grandmothers everywhere are simply just grinning right now and saying, "I told you so" — as they rightfully should.
Now, the researchers want to work next on figuring out the optimal amount of light to kill bad bacteria in people’s homes. In the meantime, feel free to let in as much light as your heart desires!