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Wiping surfaces is fine for the coronavirus, but the main risk is through the air. How to tell if your ventilation is OK.

Fresh air is key. But in aging buildings, such as Philadelphia schools, that may not be an option.

Home delivery manager Jenna Swartz wipes down bulk food container lids at Weavers Way Co-op in Mount Airy. Store employees regularly disinfect frequently touched surfaces to reduce potential coronavirus transmission.
Home delivery manager Jenna Swartz wipes down bulk food container lids at Weavers Way Co-op in Mount Airy. Store employees regularly disinfect frequently touched surfaces to reduce potential coronavirus transmission.Read moreTIM TAI / Staff Photographer

Once an hour, sometimes more, employees at Weavers Way Co-op grocery stores diligently wipe disinfectant on all “high-touch” surfaces: the checkout counters, the banisters, the plastic nozzles that dispense gourmet granola. At Whole Foods Market, workers sanitize even more often, wiping the credit-card readers between every customer.

“Deep cleaning” is the coronavirus catchphrase of the moment as more retailers, schools, and offices increase their indoor operations. And in at least one case, the goal seems to be deep and long-lasting: Witness American Airlines’ plan to use a cleaning spray that is said to keep surfaces virus-free for seven days.

There is nothing wrong with good hygiene, and for goodness’ sake keep washing your hands, especially as COVID-19 is joined in the coming months by the flu. But as more indoor spaces prepare to reopen this fall, infectious-disease experts say we don’t need to be quite as fanatical about cleaning surfaces — at least not to the extent that it distracts from the main route of coronavirus transmission: through the air.

That means continuing to stick with methods that work. Wear masks. Practice social distancing — an effective safeguard against larger virus-laden droplets that fall to the ground within a few feet. And when going indoors — given that smaller viral particles can remain aloft — consider the ventilation.

At the Weavers Way location in Mount Airy, for example, it may help that the red-brick-and-stucco structure, built in 1925, still has some of its drafty original windows, said facilities manager Steve Hebden.

“We can smell on the second floor when a neighbor has some good barbecue going,” he said.

The science of air flow is complex — more below on how to tell if an indoor space is adequately ventilated. But first, a reminder that it can be hard to tell how an individual person was infected with the virus.

Despite evidence that the virus can live on surfaces for days, there are few, if any, confirmed reports of people getting COVID-19 from touching a germ-laden object, said Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. One patient in Germany may have picked up the virus from touching a salt shaker, for example, and then presumably rubbing his eyes or nose. But other than that report, in Lancet Infectious Diseases, there is little else in the medical literature.

» READ MORE: Coronavirus spreads mainly between people, not via surfaces, CDC says. But nothing is certain.

That doesn’t mean surface transmission of COVID-19 can’t happen. It almost certainly does, given our knowledge of other viruses, said Gregory A. Poland, a vaccine researcher at the Mayo Clinic.

But short of a controlled laboratory experiment in which scientists expose people to a virus on purpose, such things can be difficult to tease out. Epidemiologists are left to trace how people were exposed after the fact — a challenge with a virus that’s often spread by people who do not realize they are infected, said Adalja, a spokesperson for the Infectious Diseases Society of America.

“Imagine two people are on the couch talking, and they’re also eating cookies off the same coffee table,” he said. Say one of them is infected, and the other gets sick a few days later. What is to blame, the talking or the cookie tray?

Regardless, the primary route through which the coronavirus spreads, say Adalja, Poland, and other infectious-disease experts, is the air.

That leaves two primary options for when indoors, said Michael S. Waring, a Drexel University professor of civil, architectural, and environmental engineering: Bring in fresh air, or filter the air already in the room. Or both.

Here are the basics for keeping spaces well-ventilated:

HVAC units

Office buildings tend to have boxy rooftop units to heat and cool indoor spaces while also adding air from outside, as indicated by what HVAC stands for: heating, ventilation and air-conditioning. Dampers can be opened to admit varying degrees of fresh air into the system. But in extremely hot or cold temperatures, it costs more to condition the outside air to a livable temperature, and there is a limit to how much fresh air some HVAC systems can handle, Waring said.

A high-quality system will refresh the air in a space multiple times an hour, though the ideal rate depends on the size of the room and how many people are in it.

» READ MORE: Influenza is about to make this coronavirus year even worse. What to know about flu shots.

Filters help

Commercial HVAC units generally are equipped with filters, rated with a system called MERV: minimum efficiency reporting value. Look for one with a rating of MERV-13, Waring said. That means it is designed to remove 90% of particles that measure one micron — a millionth of a meter — or more. Higher-rated filters are available, but they require more powerful fans to push the air through.

And just as important as the filter’s efficiency is how well it fits, Waring said. Gaps and leaks defeat the purpose.

In buildings without HVAC units, another option for removing viruses from the air is a standalone air-cleaning device equipped with a HEPA filter, Waring said.

Be wary of bells and whistles

Some air-handling systems tout the use of ultraviolet light, which is known to kill viruses. But be aware of its limitations, said Joshua Santarpia, an associate professor of pathology and microbiology at the University of Nebraska Medical Center. Do not expect it to help much in air that is moving through ductwork. UV light takes tens of seconds to inactivate a virus, and the air in an HVAC system is moving faster than that.

“You don’t really get enough time,” he said.

UV light might be useful, on the other hand, if the goal is to prevent microbial growth on a specific surface of an HVAC system, such as a cooling coil, he said.

Other fancy add-ons may do more harm than good, said Waring, director of Drexel’s architectural engineering program. For example, some standalone air cleaners emit ozone, which can irritate breathing.

“There’s a big market out there right now,” he said. “I think a bunch of people are trying to make some money.”

» FAQ: Your coronavirus questions, answered

Windows and box fans

In older buildings without HVAC units, including many schools in Pennsylvania and New Jersey, opening windows is a commonsense alternative.

Cross-ventilation works best: opening windows on opposite sides of the room. For improved flow, place an outward-facing box fan in one window and an inward-facing fan in the other.

But opening windows is not a workable solution on a hot day in mid-September, or an ice-cold one in January. Many school districts, including Eastern Regional in Camden County, have cited a lack of adequate ventilation as a reason they are starting the year with remote instruction.

Harvard public health experts provide more school-specific air-quality tips at

At the Weavers Way grocery in Mount Airy, the air-handling system does not admit outside air, though the doors are often open and there is that benefit of the drafty windows. The co-op’s Chestnut Hill and Ambler locations, on the other hand, have advanced HVAC units.

In all three stores, COVID-19 cases among employees have been low, said general manager Jon Roesser. Since the beginning of the pandemic, just four of the co-op’s 250 staffers have become infected — at least two of whom think they caught the virus outside the store, he said. All have recovered.

He attributed the success to a variety of precautions. One is reduced occupancy — just 12 people at a time are allowed in Mount Airy, 20 in Chestnut Hill, and up to 50 in Ambler. Another is a requirement to wear masks. Disposable gloves are provided for customers who wish.

And the cleaning? Roesser figures it can’t hurt.

“It probably doesn’t matter whether you disinfect every hour or every couple hours,” he said. “It makes people feel good.”