Viruses thrive in the great indoors, Charles Gerba, a virologist, and microbiologist at the University of Arizona, says. Case in point: A pre-print study (not peer-reviewed) on 318 Covid-19 outbreaks in China found that 80 percent of outbreaks happened at home.
"If somebody walks in a home with a virus on their hands, it'll be on 90 percent of the surfaces in about four hours because of all the activity that goes on in a home," Gerba tells Inverse. "It's interesting how fast a virus spreads in an indoor environment today."
Considering that Americans spend about 90 percent of their time indoors and indoor spaces, ranging from restaurants to the office buildings, are starting to reopen in some states, that ease of spread is an important factor to consider as you meditate on how to stay healthy as the pandemic continues.
Crucially, the first and most important factor that determines whether somewhere is risky is if someone who is infected is there, says Kevin Van Den Wymelenberg, a professor of architecture and the author of a recent study on how SARS-CoV-2 may travel in buildings. If that person is coughing, sneezing, or talking loudly and releasing droplets, risk will increase.
"After this, there must be a caveat that there is still much to learn," he tells Inverse.
With that caveat, there are a few factors that scientists say influence how risky a place is:
- Its population density.
- How much time you spend there.
- The amount of highly trafficked surfaces that could contain virus-containing droplets. (Droplets can linger up to 72 hours on plastic or stainless steel, for less than 24 hours on cardboard, and for less than 4 hours on copper)
- To some extent, airflow.
With that in mind, here is what we know about how the virus spreads in four places every town has.
Restaurants – In a study published in the journal Emerging Infectious diseases, scientists in China found that sitting nearby someone with confirmed Covid-19 infection, as you might in a restaurant, can result in infection.
In that case, a family of four (in which one person was unknowingly infected) ate lunch with three other family members (they're called Family A in the study). Nearby, two other families were also eating. Over a week later, 3 members of one family sitting to the left of Family A had coronavirus, and two members of the family sitting to the right of Family A had coronavirus. The tables were close together, only about one meter apart.
The scientists traced those cases back to the restaurant where all parties ate lunch and theorized that droplets were responsible for transmission. Importantly, they believe that the air conditioner, which may have blown smaller sized droplets around was a "key factor." Of the 83 people in the restaurant, 9 fell ill.
To help mitigate risk, sitting outside is a good idea, Gerba says.
Other than infected individuals coughing into the air, the additional the risk in restaurants probably stems from the way that surfaces are sanitized, he explains.
Gerba notes that surfaces like children's high chairs and plastic menus are particularly at risk. If you touch either, try to remember to wash your hands or use hand sanitizer. That's especially relevant considering people touch their faces between 16 and 23 times per hour, on average. (Though that habit is breakable.)
The office – Offices are tricky because there are many communal surfaces and people tend to spend long amounts of time there. An April study that analyzed a South Korean call center illustrates how fast SARS-CoV-2 can spread in that kind of environment.
That study details an outbreak in an 11-story building with residential apartments and a call center between the 7th and 11th floors. The South Korean government tested 1,143 people from that building. Of those, 97 people tested positive for Covid-19, and 93 of those people worked on the 11th-floor call center. The infected call center workers' seat assignments are highlighted in blue below.
The majority of those 93 people worked on the same side of that 11th floor, in close proximity. The researchers conclude that a "high density" work environment is a high-risk environment for Covid-19 spread.
The area you may want to look out for the most, if you do return to the office, might be the coffee break room. In a break room or in a communal kitchen area, there can be both a high concentration of people in a small space and some highly trafficked surfaces, like coffee making machines or countertops.
That's the place, Gerba says, where "there's both germs and gossip being exchanged at the same time."
The grocery store – Gerba's past work has highlighted shopping carts as the most high-risk areas of the grocery store. His 2012 analysis of 85 grocery carts from San Francisco, Sioux City, Los Angeles, Portland (Oregon), and Atlanta found that 62 contained coliforms, bacteria often found in feces.
Gerba says that carts may also be hot spots for SARS-CoV-2 transmission as well.
A model theorizing how a cough might spread droplets in a grocery store.
Carts aside, a model released by Finnish researchers in early April suggests that, after a single cough, small particles about 20 micrometers across can form a cloud that lingers in the air — and moves across grocery store aisles.
Those clouds could linger for "several minutes" reads the report, which was not published in a peer-reviewed journal.
More recent research suggests that while small droplets can linger in the air for about 8 to 14 minutes, the chances of those droplets containing the virus are less than one percent for a droplet about 10 micrometers large.
Public restrooms – Most people fear the doorknobs on the exits of public restrooms, says Gerba. But the real "hotspots," he says, are the faucet taps where you go to wash your hands post-business, as well as the floor, where fecal bacteria can accumulate.
For that reason, he says to beware of touching taps with bare hands and suggests that it's a good idea not to put your purse or other bags on the floor of a restroom.
In terms of coronavirus, taps may simply be high trafficked surfaces where the virus could linger. But bathrooms could pose a threat because scientists are unsure whether the virus lingers in feces.
Feces has tested positive for Covid-19. Studies suggest that infectious diseases can possibly be spread through toilet plumes, aerosolized particles that emerge after a flush. However, as the CDC notes, there have been no confirmed coronavirus cases that originate from that particular route.
Even as places are beginning to reopen, we still can't know for sure just how well coronavirus spreads.
That's why Van Den Wymelenberg argues that we actually need to be testing buildings for Covid-19, as well as people (where testing is also essential.)
"This type of system would provide more awareness to individuals making decisions about occupying buildings as we reopen our economy and therefore reopen our buildings," he says.
But for now, Gerba suggests that it's best to be careful where we go and what we touch until we know more.
"It just pays to be extra cautious at this time," he says.