Grandma was right: the sun helps kill germs inside

Even before Florence Nightingale recommended that hospitals design daylight, people found that the sun contributed to your health. But there has not been much research to explain why, especially inside buildings.

Researchers at the University of Oregon have come up with a study on dusty rooms the size of a dollhouse in order to compare what's happening in the rooms exposed to the light of the day through ordinary windows, in rooms exposed only to ultraviolet radiation and in the dark. They used a mixture of dust collected in real homes in the Portland area and let the miniature rooms sit outside while maintaining the interiors at normal room temperature.

After 90 days (because that's about the time it takes most people to vacuum, they said), they sampled the dust and analyzed the types of bacteria present.

What they found surprised them and confirmed what your grandmother already knew: the rooms exposed to the light of day have less germs. In fact, the study showed that lighted rooms contained about half of viable bacteria (those that are able to grow), compared to dark rooms. Parts exposed only to ultraviolet light contained only slightly fewer viable bacteria than those exposed to daylight. Their research was published Wednesday in the journal Microbiome.

As we spend a lot of time indoors, new research could provide insight into the relationship between air quality and light. "I think it's a new study because they have examined the effect of visible light, as well as on true bacterial communities and household dust," said Linsey Marr, professor of engineering. Civil and Environmental at Virginia Tech, specializing in interior equipment. air quality and did not participate in this study.

The main author of the study, Ashkaan Fahimipour, postdoctoral fellow at the Center for Biology and Built Environment of the University of Oregon, says he is surprised that visible light and ultraviolet light act in the same way to contain the bacteria.

The researchers looked at both types of light because UV is known to be a good disinfectant and is used to clean drinking water. Yet, typical windows filter most of the ultraviolet rays.

Another surprising thing was the amount of viable microbes in the dust. Previous studies did not suggest that it would be the same, says co-author Kevin Van Den Wymelenberg, co-director of the Center for Biology and Built Environment at the University of Oregon. That 's because indoor dust is like a desert – it' s too dry for most bacteria or other things to grow. This study found that 12% of bacteria in dark rooms were viable, compared to 6.8% in rooms with daylight and 6.1% in rooms exposed to UV light only.

Although it may seem like little, "6% of the millions of cells still represent a lot of microbes," says Van Den Wymelenberg. "Until now, daylight [illuminating a building with natural light] has been on visual comfort or wide health. But now, we can say that daylight influences the quality of the air. "

The study study rooms also contained fewer types of bacteria associated with human skin, which people lost as they moved in, and looked more like outdoor bacterial communities. Some of the species of bacteria associated with humans that have not survived in bright halls are known to cause respiratory diseases.

In their future work, the researchers indicated that they wanted to design studies to determine the amount of light needed to kill microbes so that architects could start designing buildings in this spirit.

In addition, researchers learned by trying to eradicate all the germs in the clean rooms of hospitals and laboratories that it was very difficult to get rid of the bulk microbes. "Sanitation is not the best approach," says Fahimipour. And some microbes are actually good for us, like those in yogurt. One day, he says, "it may be better to enrich an indoor environment with non-harmful or even harmful microbes. [with those that are] beneficial. "

Kathleen O Neil is an independent science writer who lives in the Washington, DC area.

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