Did you know that sunshine on your shoulders not only makes you happy (as the John Denver song says) but also kills germs? It’s true and, frankly, it’s too cool for school. Exposing a bottle of water to sunlight for 6 hours makes it safe to drink.
But wait, it gets better: the sanitizing power of sunlight works even after it passes through window glass! When you pull back the curtains and raise the shades, you are letting in a powerful purifier. This is especially important when there are children in the house since they are vectors for the common cold and flu.
Scientists have known since 1877 that sunlight is a potent disinfectant and bactericide. “Sugar water left in the shade became cloudy, indicative of bacterial growth, but if exposed to sunlight, it remained clear.”
In 1890, German microbiologist Robert Koch demonstrated that sunlight killed tuberculosis bacteria. Subsequent studies found that E. coli bacteria were destroyed in 12 feet of water and in waste stabilization ponds.
In the past, sunlight was used to disinfect hospital rooms. An article which appeared in May 1901 titled “Methods of Disinfection Recommended by the Department of Health of the City of New York” opened with this bold statement:
“Sunlight, pure air, and cleanliness are always very important agents in maintaining health and in protecting the body against many forms of illness.”
The 1901 instructions for hospital sanitation said that “It is important to remember that an abundance of fresh air, sunlight, and absolute cleanliness not only helps protect the attendants from infection but also aids in the recovery of the sick. Sunlight is one of the most effective disinfectants known, killing all germs directly exposed to it within a few hours.”
We now know that these claims were exaggerated somewhat – daylight does not kill 100% of the bacteria indoors. But modern science has linked sunshine with a reduction in disease-causing microbes.
A research team looked at how bacteria in household dust changed after exposure to daylight. Their test environment was eleven identical replicate miniature model “rooms” that were dollhouse-sized.
The little windows were glazed to test visible, ultraviolet, or no light so the resulting bacterial communities could be measured and compared. The team’s press release explained:
“After 90 days, the authors collected dust from each environment and analyzed the composition, abundance, and viability of the bacteria present.”
The scientists put bacteria-laden dust collected from real homes in Portland into the model rooms before putting them outside. The inside temperature was kept at a normal room temperature.
The study results showed that, after 90 days, the daylit rooms had half the living bacteria compared to the dark rooms:
“Researchers found that 12% of bacteria in the dark rooms was alive after 90 days, compared to 6.8% in the sunlit rooms and 6.1% in the rooms exposed to UV light.”
Living bacteria can be rendered inactive by window-filtered sunlight. They also discovered that sunlight was just as effective in reducing bacteria as ultraviolet (UV) light, a known disinfectant.
The dust from the dark test rooms had types of bacteria closely associated with respiratory diseases. There were almost none of these infectious germs in the rooms exposed to daylight.
The bacteria that were able to survive in the sunlit test rooms were, by and large, identical to the ones commonly found in the air outdoors.
This research suggests that sunshine is not only good for visual comfort and overall health but an anti-bacterial agent. Kevin Van Den Wymelenberg, one of the study’s co-authors and co-director of the Biology and the Built Environment Center 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 research team plans to find out just how much light it takes to kill microscopic germs so future architects can design buildings accordingly.
Ashkaan Fahimipour, the study’s lead author, pointed out that killing all germs may not be the course of wisdom. He recommended avoiding using commercial products that promise to overkill their targets:
“Sanitizing isn’t the best approach. 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.”
Dr. Fahimipour said his team’s findings uphold what many of us – and our parents and grandparents – suspected all along:
“Our study supports a century-old folk wisdom, that daylight has the potential to kill microbes on dust particles.”
Indeed, how many of you know someone who hung stained white laundry and sheets on a line outside to bleach in the sun?
Any piece of furniture or household item you can manage to drag outside will disinfect and lighten after prolonged exposure to natural light, including:
- Dingy whites
- Stuffed animals
- Stained Tupperware
- Moldy Items
- Outdoor Furniture
Now that we know for sure that daylight promotes good health and lowers the risk of bacterial diseases, as the song from the musical Hair goes:
“Let the sun shine, Let the sunshine in!”