Grandma Was Right: Sunshine Helps Kill Germs Indoors

I wait for windy weather to clean house.  I open all the doors and windows and let mother nature blow all the dirt out.  Luckily, I live in Southern California where it is breezy and sunny most of the year.  Luckily, I live in a house that has large glass sliding doors in every room (with the exception of bathrooms which I have to clean myself without mother nature’s help).

Turns out my house may be more microbe free than yours . . . .

Letting Sunshine In by Peggy

University of Oregon scientists used real dust from inside homes around Portland to test the effects of sunlight, UV light and darkness on bacteria found in the dust. Researchers “. . .  set up a study of dusty, dollhouse-size rooms to compare what happens in rooms exposed to daylight through regular glass, rooms exposed to only ultraviolet light and those kept dark.”

“After 90 days (because that’s about how long it takes most people to get around to vacuuming, 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: Rooms exposed to daylight have fewer germs. In fact, the study showed that the lit rooms had about half the viable bacteria (those that are able to grow), compared with dark rooms. Rooms that were exposed only to UV light had just slightly less viable bacteria than ones exposed to daylight.*

This 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.

While it may not sound like much, “6 percent of millions of cells is still a lot of microbes,” “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 daylit rooms in the study also had less of the types of bacteria associated with human skin, which people shed as they move around indoors, and more closely resembled outdoor bacterial communities. Some of the human-associated bacteria species that didn’t survive in the lighted rooms are known to cause respiratory disease.


*Research was published in the journal Microbiome.”


4 comments on “Grandma Was Right: Sunshine Helps Kill Germs Indoors

  1. This is a really fascinating study. I’m going to resist cracking a lot of jokes (they come to mind easily,) but focus on the implications of sunlight in our homes and on our health. As I’m writing at 6:30 AM, the sun just opened up bright yellow and poured into the room where I’m typing. Hello, Sunshine!

    I’m not surprised to learn about the positive impact of sunlight and fresh air. My mom, who had Alzheimer’s , lived at an assisted living residence that maintained close ties with UCI Mind, the medical team conducting research on the disease. One of their conclusions was that people exposed to hours of bright light functioned better and longer than those who stayed indoors in darker rooms. So the residence made a point of getting more light into rooms and trying to get residents outdoors more often.

    It was also really easy to see how some residents suffered from sundowner syndrome, becoming morose and difficult as evening set in. I know this isn’t exactly what you’ve written about here, but it’s obvious that sunshine has positive effects on how people feel and thus how they behave. I think it’s not only beneficial to let the sunshine in but let the people out.

    Love your adorable cartoon, Peggy. Great article, you two.

    OK – one dumb joke: My house dust is cleaner than your dust – its sunlight is much healthier since it hasn’t been vacuumed in about a year.


  2. Fascinating article, I had no idea that there had been studies made on this. I do know the effect of sunshine and freshly aired rooms now. Grew up with a mother who loved opening windows in all rooms and I am afraid – cleaning every week – . I guess many bacteria were drowned in wind and water.
    Like her, I air a lot and have my home in a sunny position.
    We are lucky.



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