Why does ventilation matter?

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After extensive studies of how COVID spreads, scientists and policy makers are focusing on indoor air quality. Here’s everything you need to know:

What do we know?

One of the most powerful tools to limit the spread of COVID-19 has been right in front of our noses: purifying the air we breathe. After extensive studies of disease outbreaks, super-spreading events and aerosol dynamics, epidemiologists who specialize in respiratory diseases have warned that keeping 6 feet of distance from others is not enough to avoid indoor infection. It has taken the World Health Organization more than a year to recognize that COVID is not only transmitted through large respiratory droplets that quickly fall to the ground, but also spreads in minute particles that can linger in the air like fine mists for hours. With mask mandates lifted, this underscores how important it is to maintain air flow in crowded indoor spaces, by equipping buildings with ventilation systems that pump virus-laden air out, and filtration devices that trap viral particles. Think of the virus that spreads in aerosols as cigarette smoke, said Joseph Allen, director of the Harvard Health Buildings Program. “If I smoke in the corner of a classroom and you have low ventilation/filtering, that room will fill with smoke,” Allen said. But outdoors, he said, “you could be two feet from me, depending on which way the wind is blowing, you might not even know I’m smoking.”

How is the coronavirus transmitted?

Every time we exhale, air rushes out of our lungs and through our nose and mouth in a warm cloud of respiratory fluid. The droplets that appear when you scream or cough can be as wide as a tuft of human hair, but the coronavirus is primarily transmitted through millions of aerosols — droplets just a few millimeter wide — escaping with each breath. In a crowded, poorly ventilated room, up to 4 percent of each inhalation is someone else’s breath; University of Oregon researchers found that there was no significant difference between the number of aerosol particles shared between people standing 4 feet apart versus 11 feet apart. However, moisture helps. Studies show that in dry places, such as many offices and restaurants, respiratory droplets travel longer and longer distances.

Where does infection occur?

Mostly indoors. There have been very few reports of outdoor transmissions, even when tens of thousands of people gather for concerts or sporting events, since there is unlimited ventilation outside. One of the first COVID superspreader events — at the March 2020 Choir Rehearsal in Washington State — highlighted the limitations of indoor social distancing. Despite taking steps to publicize the singers in the church, 52 of the 61 participants tested positive for COVID within a few weeks, and two choir members died. Scientists suggest that the more people are indoors, the higher the risk that one of them will be infectious and emit invisible clouds of viruses. This explains why crowded restaurants, gyms, and poorly distributed meeting rooms cause so many outbreaks.

How can ventilation help?

It brings in fresh air and pumps up the air that people exhale, thus diluting the concentration of potential coronavirus particles. (It also reduces the risk of catching flu and cold viruses, which are also spread through the air.) A study of more than 10,000 classrooms in Italy found that good ventilation systems reduced COVID transmission by 82.5 percent. Airplanes are another good example of ventilation in action. Sitting in the cabin of an airplane is one of the safest ways to travel, with half of the passengers breathing air coming from outside the plane, and the other half being recycled through High Efficiency Particulate Air (HEPA) filters that provide a minimum capture efficiency of 99.97 percent — better than most buildings. However, you can become infected on an airplane if you are sitting next to an infected person.

What steps should be taken?

Opening the windows only improves ventilation, especially when you are on opposite sides of the room. Portable HEPA filters are effective, and UV light has also proven effective when used to treat air that passes through a building’s heating, ventilation, and air conditioning (HVAC) system. Experts also spread the folly of using plexiglass spacers to protect people from each other’s breath. Studies show that while these barriers may prevent droplet aerosols, such as sneeze guards in salad bars, they also impede air circulation and may work on upload risk of injury.

Is good ventilation common?

No, which is why the White House recently released an action plan to improve indoor air quality to fight the spread of COVID. For decades, engineers have prioritized making buildings more energy efficient, preferring recirculation over ventilation. Outdoor air pumped indoors needs to be cooled or heated, and in a typical commercial building, HVAC systems account for up to 40 percent of total energy expended; This percentage rises when thicker MERV 13 filters are installed to capture aerosols. It’s expensive to repair HVAC systems, so there will be resistance, said Lynsey Marr, a professor of engineering at Virginia Tech. She said, “To put some teeth into this, standards and regulations would be needed. And that would take years.”

Air quality problems in schools

The average age of a public school building in the United States is more than 45 years, and many have outdated or poorly performing HVAC systems, according to the Government Accountability Office. During the pandemic, some desperate schools left windows open during the frigid winter months to improve ventilation. Facing tremendous pressure to improve air quality and reduce the risk of a COVID outbreak, public schools have committed an estimated $4.4 billion to HVAC projects. A study of Georgia schools found that improved ventilation and HEPA filtration reduced the rate of COVID by 48 percent. As a temporary solution, school districts across the country have also spent tens of millions of dollars on portable air purifiers, which often overestimate their effectiveness and are known to release ozone particles, which can trigger asthma in developing lungs. The best solution is to upgrade your heating, ventilation, and air conditioning systems to provide frequent air circulation, said Tracy Washington Inger, an indoor air specialist with the Environmental Protection Agency. She said that to enable the transition to living with COVID as an “endemic disease,” schools need “effective, long-term management strategies that address indoor air quality.”

This article was first published in the latest issue of the week magazine. If you want to read more like her, you can try six risk-free issues of the magazine here.

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