DHS Study: Solar Light and Humidity Can Reduce the Half Life of COVID-19 from 18 Hours to Two Minutes

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PHOTO GALLERY – In this photo taken on Friday, Aug. 10, 2018, a sea turtle soon after hatching, strains against the surf to reach the waters of the Mediterranean Sea as the sun sets on Cyprus’ protected Lara beach. Cyprus’ Green and Loggerhead turtles have made a strong comeback thanks to pioneering conservation efforts stretching back decades, after being hunted to near extinction in the first half of the last century. (AP Photo/Petros Karadjias)


President Trump was ridiculed in February for suggesting that “the heat, generally speaking, kills this kind of virus.” CNN’s Anderson Cooper, Alisyn Camerota and many others openly mocked him over his naiveté.

Well, it turns out he was right. A new study from the Department of Homeland Security (DHS) has shown that sunlight and humidity kills COVID-19.

At Thursday’s White House briefing, William N. Bryan, the acting undersecretary for Science and Technology at DHS, told reporters:

We’ve identified that heat and humidity is a weakness in that chain. We’ve identified that sunlight, solar light, UV rays, is a weakness in that chain. That doesn’t take away the other activities, the White House guidelines, the guidance from the CDC and others on the actions and the steps that people need to take to protect themselves. This is just another tool in our toolbox, another weapon in the fight that we can add to it. And we know that summer-like conditions are going to create an environment where the transmission can be decreased, and that’s an opportunity for us to get ahead.

Bryan said his team has seen the “powerful effect” that solar light has on the virus “both on surfaces and in the air.” They’ve seen a similar effect with both heat and humidity.

Bryan presented a chart which shows the dramatic effect that these three variables have on the virus which can be viewed here.


He explained that, under normal circumstances, COVID-19 has a half life of 18 hours (on surfaces). “Normal circumstances” is defined as temperatures between 70 and 75′ and humidity of 20%.

If humidity is increased from 20% to 80%, the half life decreases to six hours (on surfaces).

If humidity remains at 80% and the temperature is increased to 95′, the half-life drops to one hour (on surfaces).

Next, Bryan adds the effect of solar light. A scenario is a summer day, the temperature is between 70 and 75′, humidity is at 80%. The addition of solar light will reduce the half-life of the virus (on surfaces) to two minutes which is pretty stunning.

The situation is different for aerosols (saliva droplets in the air). Under normal circumstances, indoors (no solar light), temperature is between 70 and 75′ and humidity is at 20%, the half life is approximately one hour. If you step outside on a summer day, with temperature and humidity remaining the same, the half life plunges to one and a half minutes.

So, the virus, Bryan concludes, survives best indoors and in dry conditions and it dies the quickest under the presence of direct sunlight. He noted that “extra care may be warranted or dry environments that do not have exposure to solar light. Increasing the temperature and humidity of potentially-contaminated indoor spaces appears to reduce the stability of the virus.”


The DHS has also tested the effects of of various disinfectants on the virus and Bryan said, “I can tell you that bleach will kill the virus in five minutes, isopropyl alcohol will kill the virus in 30 seconds, and that’s with no manipulation, no rubbing.”

He stressed the following point: “It would be irresponsible for us to say that we feel that the summer is just going to totally kill the virus and that it’s a free-for-all and that people can ignore those guidelines [social distancing and other health guidelines], that is not the case. We have an opportunity though, to get ahead with what we know now, and factor that into the decision-making for what opens and what doesn’t.”

I highly recommend watching Bryan’s presentation below (seven minutes).


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