Air circulation can fight coronavirus transmission — or help it spread. Here's how to spot a faulty system.

• Stagnant, poorly circulated air could foster coronavirus transmission in some indoor spaces.
• But it can be hard to tell whether a building's heating, ventilation, and air conditioning (HVAC) system is working properly to minimize risk.
• Two HVAC experts offer tips on how to spot low-quality systems.

Imagine coronavirus particles in a space like glitter in a pool: The glitter is the virus, while the water is the air.

That's how Theresa Pistochini, an engineer specializing in heating, ventilation, and air conditioning (HVAC) systems, likes to visualize it. To get glitter out of a pool, a system should have three main components: it must move water around (circulation), remove glitter from the circulating water (filtration), and bring in new water to dilute the glitter (ventilation).

The same goes for air in buildings. After people expel coronavirus particles via breathing, talking, or coughing, those particles can remain temporarily suspended in the air. But unlike glitter, coronavirus particles are invisible, and humans can only tell if air is getting circulated (since that means a space doesn't feel stuffy), not whether it's properly filtered or ventilated.

"You walk into a building that's too cold or too hot, and you know immediately," Pistochini, who works at the Energy Efficiency Institute at the University of California, Davis, told Business Insider. "But people in a building have absolutely no perception if the system is filtering or ventilating."

To ensure people can safely spend time in offices, stores, restaurants, and schools, experts say, building managers must update airflow systems. That's because most cases of documented coronavirus transmission have occurred in tight, poorly ventilated indoor spaces; one recent study (though not yet peer-reviewed) even suggested that a person might be nearly 20 times more likely to spread the coronavirus inside as they are outside.

"The biggest risk is these closed, indoor environments," Don Milton, a virologist at the University of Maryland, previously told Business Insider.

Unfortunately, research suggests many buildings aren't meeting even basic standards for air circulation and ventilation. A June report by the Government Accountability Office estimated that at least 36,000 schools nationwide had dysfunctional HVAC systems. A 2018 review also found that the majority of US buildings "have multiple faults residing in the systems causing either energy, thermal-comfort, or indoor air-quality penalties."

Pistochini offered several tips on how to spot faulty HVAC systems.

Older grocery stores are probably safer than newer office buildings

First, identify the building's purpose, since that can inform whether ventilation and filtration are high priorities. In hospitals, for example, the design must ensure sick patients don't contaminate each other; airflow in grocery stores helps keep produce fresh. Offices and school buildings, on the other hand, may not invest in high-quality ventilation or filtration.

Next, consider the building's age.

"Everyone has a preconceived notion they should be more worried if they're in an older building," Pistochini said. "Being in an older building is actually safer than a newer one. If you're in a new building with broken ventilation, you're in real trouble."

That's because most buildings over 50 years old tend to leak air, and consequently require greater amounts of energy for heating and cooling. Designers started sealing them up more thoroughly in the 1970s to reduce energy use. It worked, but airtight spaces mean decreased ventilation when a system isn't up to par.

LEED- and WELL-certified buildings are notable exceptions to the old-versus-new building rule, however, since structures with these certifications must have high quality HVAC systems, Pistochini said.

Test a building's CO2 levels

Although there's no simple, easy, or cheap test to measure coronavirus particles in the air, high levels of carbon dioxide can be a "canary in the coal mine," according to Roger Silveira, director of facilities maintenance and operations at San Jose's East Side Union High District. Silveira's work focuses on improving school air quality.

CO2 builds up when a lot of people exhale in a closed space, so a high concentration of it indicates that a building isn't well ventilated. In a buildings with good ventilation, CO2 levels generally stay under 1,100 parts per million, Silveira said.

If you have about $100 to spend, you can buy a carbon-dioxide monitor online at CO2meter.com, or on Amazon or other sites. Silveira, who worked with Pistochini to develop a healthy air plan for schools during the pandemic, plans to install a meter in every classroom in his district.

If all else fails, ask the manager — and open windows

If you have to spend a lot of time in a particular building, like an office or a school, it's probably worth asking the building manager or maintenance director about its HVAC system.

Some good questions to start with: Has this building been updated to comply with COVID-19 standards set by top HVAC experts? If not: Has this building's ventilation system been tested recently, and what's the airflow rate? Different buildings require different airflows, but most should have at least 15 cubic feet per person per minute.

You could also find out which types of filters the building uses — Pistochini recommends HEPA or MERV-13 or -14 filters, since these can block most coronavirus particles if used properly and replaced regularly.

If you can't ask a manager and also can't opt out of being inside a particular building, experts suggest simply trying to stay near open windows. That's no substitute for a state-of-the-art HVAC system, but Pistochini says open windows are still better than no ventilation at all.