Between all those door handles, credit card keypads and even cell phones, we touch so many surfaces daily. It’s just a fact of life. But when it’s flu season — or there’s an outbreak of any other virus — this simple act of touching stuff can spread germs.

In many cases, it’s cause for concern because some viruses can live on surfaces for hours — or even weeks. What’s not always clear is how long a surface, like a credit card terminal at the gas pump, might stay contaminated if a sick person sneezes on it.

Part of the uncertainty is because viruses are diverse and have a variety of surface survival rates. There isn’t even a hard-and-fast rule for how long a virus can survive outside of a host. The type of surface and environmental temperature and humidity all come into play, too. So which surfaces are safe to touch, and how often do we need to disinfect them?

Before we even discuss how long viruses can live on a surface, we have to understand how viruses work.

No Virus Is an Island

Viruses don’t have the right enzymes to create the chemical reactions necessary for reproduction. Instead, viruses need a host cell, which can be bacteria, fungi, a plant or an animal, including a human. With help from the host, viruses are then able to multiply. That’s good for the virus but generally bad for the host.

Without the host cell, a virus cannot survive long term; however, it does have a short window of time during which it can function in hopes of attaching to (aka infecting) a new host.

Outside its host, a virus can be divided into two categories — either it can be intact and remain infectious or it is simply identifiable, which means it has enough genetic material to be identified but is no longer capable of attaching to host cells, Julia Griffin and Nsikan Akpan wrote in article for PBS News Hour. At the point that a virus on a surface is only identifiable, it won’t be able to cause harm.

How Long Can Viruses Live on Surfaces?

The length of time that viruses can live on surfaces and remain infectious varies greatly by pathogen, Dr. Alicia Kraay, postdoctoral fellow in epidemiology at Emory University, explains in an email. There are baseline differences between viruses. For example, rhinovirus — the viruses that cause the common cold — will survive for less than an hour on surfaces. However, others such as the norovirus, which is a virus that can cause vomiting and diarrhea — can survive for weeks. Not surprisingly, with its ability to live this long outside of a host, norovirus can spread both through infected people and through contaminated foods and surfaces.

The research into how long COVID-19 can survive on surfaces is new and ongoing. A March 13 study by researchers at the National Institutes of Health (NIH), the U.S. Centers for Disease Control and Prevention (CDC) and multiple universities compared the novel coronavirus (SARS-CoV-2) with SARS-CoV-1, the most closely related human coronavirus and the virus responsible for the 2003 epidemic. The non-peer-reviewed study found that the two viruses have similar viability in the environment, however, the study determined the novel coronavirus could survive up to three days on stainless steel and plastic surfaces. Survival on other surfaces was lower — just one day on cardboard and four hours on copper. The results indicated that novel coronavirus can live in the air for hours and on surfaces up to days.

Another research study published March 17, 2020, in the New England Journal of Medicine by the National Institute of Allergy and Infectious Diseases and Princeton University also found that the stability of novel coronavirus (SARS-CoV-2) was similar to that of SARS-CoV-1 under the experimental circumstances tested. However, novel coronavirus was more stable than SARS-CoV-1. In their experiments, SARS-CoV-2 remained viable in aerosol form for up to three hours. Viable coronavirus was and detected on plastic and stainless steel up to 72 hours after application. No viable coronavirus was measured after four hours on copper surfaces, and 24 hours on cardboard.

What Factors Affect Virus Survival Rates?

If it seems like it should be a simple test to pinpoint an outside-host survival period, it’s more complicated than just spraying some virus on a surface and waiting to see what happens. In fact, in the article for PBS News Hour, Griffin and Akpan wrote that there isn’t a lot of “rigorous data” on how long cold and flu viruses remain infectious.

“Generally, survival of pathogens on fomites [objects or materials likely to carry infection] is determined by inoculating a surface with a known quantity of virus and then sampling at various time intervals to determine the amount recovered,” Kraay says. “Scientists use this information to estimate a decay curve for the pathogen on the particular surface, which can be extrapolated to longer time intervals.”

The NIH and CDC team who studied surface variation for coronavirus is already looking into virus viability in different matrices, as well as in varying environmental conditions.

Although viruses have differing baseline rates of survival on surfaces, additional factors affect their ability to endure outside of a host. Temperature, humidity and surface properties can all affect survival, according to Kraay.

“In general, viruses survive longest at lower temperatures, higher humidity and [on] non-porous surfaces (like stainless steel),” she says. “However, some viruses do well at low humidity.”

In addition to surface material and environment, the amount of virus on the surface can also help determine how long it will survive, explains James M. Steckelberg, M.D. in an article for the Mayo Clinic. While it is possible to spread viruses like cold and flu through sharing objects, personal contact is the most common mechanism of spreading viruses.

There have been a lot of theories about whether coronavirus will lessen during warmer months because dry, cold air tends to provide favorable conditions for flu transmission. But Dr. Marc Lipsitch, professor of epidemiology and director, Center for Communicable Diseases Dynamics, Harvard T.H. Chan School of Public Health says when it comes to coronaviruses, the “relevance of this factor is unknown.”

Can You Get a Virus From a Surface?

If you touch a surface that is contaminated with a virus — including COVID-19 — does that mean you will get the virus? Not necessarily. But if you don’t immediately wash your hands, and then touch your mouth, nose or eyes, you could transmit the virus. However, the CDC says surface contamination isn’t considered the most likely way to get coronavirus. Without a host, viruses begin to degrade pretty quickly, so what is on the surface becomes less and less potent.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), explained during the March 13, 2020, CNN/Facebook Global Coronavirus Town Hall that when considering the viability of a virus on various substances, it is probably measured in a couple of hours. While he recommends wiping down surfaces — like doorknobs and cellphone screens — when you can, he cautioned against worrying about money and mail.

In the end, despite the differences in viability on surfaces among pathogens, fomites and contexts, the No. 1 recommendation for preventing the spread of viruses is standard. Wash your hands.

This article was first published on March 16, 2020, and last updated on March 18, 2020.

By CARRIE WHITNEY, PH.D. HowStuffWorks March 16, 2020