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Go to Brilliant.org/SciShow to check out their course Probability Fundamentals. [♪ INTRO]. This episode was filmed on June 9, 2020.

If we have more recent episodes on the COVID-19 pandemic, we will include them in our playlist and in the description below. It's on everybody's mind, to some extent, right now. If a surface is contaminated with the SARS-CoV-2 virus, how long does it pose a risk of infection?

The virus is thought to mainly spread through respiratory droplets. These are produced in a cloud when a person coughs or sneezes, or even talks. Some potentially-virus-laden droplets might end up getting breathed in by other people in the vicinity.

But many of them end up landing on objects like door handles or water faucets. When that happens, infectious disease experts refer to that door handle as a fomite. And if a person then touches the fomite while the virus is still infectious, they can then spread it to new surfaces, or actually infect themselves.

Fomites aren't just for viruses -- any type of pathogen can create fomites -- but we're talking about viruses for… obvious reasons. The good news is that SARS-CoV-2 virus particles don't last forever -- or even all that long. Eventually, the protein coat that allows the virus to actually bind to and infect cells will degrade.

This neutralizes the virus and leaves it as a small pile of ineffective protein and genetic material. So the question is, how long does this take to happen? As is the case with pretty much everything to do with this virus, we still don't know for sure.

There are still a lot of details to nail down. Some studies on other coronaviruses have suggested they're pretty hardy, by virus standards. The viruses that cause SARS and MERS can survive in some environments for more than a week, and maybe even four weeks if it's cold.

For SARS-CoV-2 in particular, one of the ways scientists have tested how long the virus might stay active outside the body is essentially by re-creating a sneeze. They spray a watery mixture containing the virus onto certain surfaces. Then, later, they swab the surface and try to infect cells in a petri dish or test tube with what they collected.

They can watch to see if infection does occur, and in how many of the cells, and use those numbers to calculate how much of the virus survived. This is totally different from the nasal swab test we use to see if someone's infected. That test essentially just looks for genetic material.

This test is specifically looking to see if active, still-able-to-infect virus is present. One of the things we've figured out using this method is that what type of surface the droplet falls onto makes a big difference. One study from the New England Journal of Medicine, for example, found viable virus, though at low levels, on steel and plastic after 72 hours.

On cardboard, the virus couldn't be detected after 24 hours. And copper, which has antimicrobial properties, made it disappear in four hours. Another paper in the Lancet found viable virus after a whole six days on steel and plastic.

Meanwhile, it seemed to disappear on glass and banknotes after about four days, two days for wood, and three hours for paper. It's important to note that while the scientists in both studies found evidence of viable virus, it's hard to say how infectious these viruses are. That's because you need a higher dose of some pathogens than others to actually get infected with them -- and we don't know what that is for SARS-CoV-2 yet.

For one thing, the New England Journal of Medicine study found that while some of the virus lasted a long time, the amount decreased exponentially over time. This means most of the individual virus particles didn't last very long. Outside the body, changes in things like temperature, humidity, and UV radiation can quickly degrade the virus.

Even though these studies found the virus after days or even weeks, the amount was generally really small -- a tiny fraction of what was there at first, and probably not enough to infect anyone. The Lancet group's experimental method also used a solvent specifically designed to transport viruses, and they note that this is not exactly comparable to a real-life contact scenario. But why is there such a difference in the amount of time between surfaces?

Well there, the field of physics can help us out. A paper out this week in the journal Physics of Fluids modeled how long large, respiratory-sized droplets of water could last on various surfaces and conditions. They assumed that SARS-CoV-2 would die when the droplet evaporated, though we don't know that for sure yet.

They found that temperature and humidity affected the lifetime of these droplets in ways you would expect, with higher temperatures and lower humidities drying them out faster. They also modeled how much the droplets can spread out on surfaces, which in physics is called the contact angle. On some surfaces, like glass, the droplet spreads out really wide and thin, which makes it evaporate faster.

On others surfaces, like steel or smartphone screens, which can be covered in a water-repellent coating, the droplet stays more spherical, which limits the surface area and makes it evaporate more slowly. That might be part of the reason why the earlier researchers were able to find viable virus on those surfaces after such a long time. What's the good news?

Well, all these tests assume nobody's cleaning. While viral particles might be able to stick around for a long time in the perfect environment, they are also still susceptible to disinfection. Which means - yes, it's a good idea to wipe down those door handles.

It also hasn't stopped being important to wash your hands after you've been out and about, touching potentially contaminated surfaces. Which we are all still doing, right? Models like the one we talked about today rely on understanding probabilities.

And one way you can learn more about probability is with Brilliant's course Probability Fundamentals. Brilliant has tons of courses in math, science, engineering, and computer science. They're all hands-on and meant to help teach scientific thinking.

And they're designed by experienced educators from Duke to Caltech and beyond. Right now, the first 200 people to sign up at Brilliant.org/SciShow will get 20% off an annual Premium subscription. So thanks for dropping by! [♪ OUTRO].