Is Sunlight Actually the Best Disinfectant?

Answers to your questions about the news.
Aug. 9 2013 12:25 PM

Is Sunlight Actually the Best Disinfectant?

It depends on how much time you have.

A hiker drinks from a water bottle in front of Heart Lake in the Red Mountains of Yellowstone National Park, Wyoming.
If you’re trying to disinfect anything other than water, sunlight isn’t the wisest option

Photo by Lucy Nicholson/Reuters

Edward Snowden was back in the headlines after Russia granted the NSA leaker temporary asylum last week, leading President Obama to cancel a planned summit with Russian President Vladimir Putin. Those defending Snowden’s exposure of secret NSA surveillance programs—Guardian reporter Glenn Greenwald among them—have alluded to Supreme Court Justice Louis Brandeis’ 1913 maxim that “sunlight is said to be the best of disinfectants.” Does sunlight actually work as a disinfectant?

Yes, but it’s not the best there is. The ultraviolet radiation in sunlight does work as a natural disinfectant and is used regularly to disinfect drinking water in countries such as India, Kenya, and Peru, where more complex forms of water purification are less accessible. According to CDC estimates, more than 2 million people worldwide use solar disinfection to clean their drinking water. The process is remarkably simple: Just fill a transparent bottle, shake the water, and place it under steady sunlight for at least six hours. While the process will eliminate microbiological pathogens, it will have no effect on chemical contaminants, like lead or toxic pollutants.

If you’re trying to disinfect anything other than water, however, sunlight isn’t the wisest option, as it’s is most effective against water-borne pathogens. For example, if you try to disinfect your cellphone (studies have shown it’s probably dirtier than your toilet seat), sticking it in the sun for an afternoon won’t do the trick, because there isn’t enough ultraviolet light in the sun’s rays (at least within the Earth’s atmosphere; the ozone layer blocks the majority of powerful UV radiation). Rather than relying on the sun’s relatively feeble rays, some hospitals have begun using UV-emitting machines to disinfect rooms.


Hypochlorite cleaners—bleach, essentially—have been shown to significantly reduce bacteria in household environments. And while bleach is effective at cleaning surfaces, it’s still not necessarily the best option when it comes to sanitizing.

So what is the most powerful disinfectant? The one you pair with a backup. No single method of disinfection is guaranteed to eradicate bacteria entirely—living things are that resilient and adaptable. If you really want a surface squeaky clean, using a combination of methods, for example, both chlorine and heat, will ensure maximum germ elimination.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Kevin McGuigan of the Royal College of Surgeons in Ireland and Bill Rutala of the Center for Infectious Diseases at the UNC School of Medicine.

Ciara McCarthy is a Slate intern.



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