Sunlight can kill infection-causing bacteria indoors, study finds

Rooms exposed to daylight have fewer germs, and compared to dark rooms, sunlit rooms have about half the amount of bacteria that can grow, a study published in Microbiome found.

The researchers set up a study of dusty, miniature model rooms to see what happened in rooms exposed to daylight through regular glass, rooms exposed to only ultraviolet light and rooms kept dark.

They used a mix of dust collected from real homes and let the miniature rooms sit outdoors while keeping the insides at a normal room temperature. The researchers then sampled the dust and looked at the types of bacteria present after 90 days.

In addition to finding that rooms exposed to daylight have fewer germs, the researchers found rooms exposed only to UV light had slightly less viable bacteria than ones exposed to daylight.

The researchers evaluated both visible light and UV light because UV is known to be a good disinfectant, but typical window glass filters out most UV light.

The study authors were surprised by the amount of microbes that were able to grow in dust since earlier studies did not suggest it would be as much, co-author Kevin Van Den Wymelenberg, PhD, told NPR.

The study found 12 percent of bacteria in dark rooms were viable compared to 6.8 percent in rooms with daylight and 6.1 percent in rooms exposed to UV light only.

Although it sounds like a small number, "Six percent of millions of cells is still a lot of microbes," Dr. Van Den Wymelenberg said. "Until now, daylighting [illuminating a building with natural light] has been about visual comfort or broad health. But now we can say daylighting influences air quality."

The researchers aim to develop studies that determine how much light is needed to kill microbes so architects can design buildings with these facts in mind.

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