The benefits of ventilation reach far beyond the coronavirus. What if we stop taking colds and flus for granted, too?
When London vanquished cholera in the 19th century, it took not a vaccine, or a drug, but a sewage system. The city’s drinking water was intermingling with human waste, spreading bacteria in one deadly outbreak after another. A new comprehensive network of sewers separated the two. London never experienced a major cholera outbreak after 1866. All that was needed was 318 million bricks, 23 million cubic feet of concrete, and a major reengineering of the urban landscape.
The 19th and early 20th century saw a number of ambitious public-health efforts like this. The United States eliminated yellow fever and malaria, for example, with a combination of pesticides, wide-scale landscape management, and window screens that kept mosquitoes at bay. One by one, the diseases that people accepted as inevitable facts in life—dysentery, typhoid, typhus, to name a few more—became unacceptable in the developing world. But after all this success, after all we’ve done to prevent the spread of disease through water and insects, we seem to have overlooked something. We overlooked air.
This turned out to have devastating consequences for the beginning of the coronavirus pandemic. The original dogma, you might remember, was that the novel coronavirus spread like the flu, through droplets that quickly fell out of the air. We didn’t need ventilation or masks; we needed to wash our hands and disinfect everything we touched. But a year and half of evidence has made clear that the tiny virus-laden particles indeed linger in the air of poorly ventilated areas. It explains why outdoors is safer than in, why a single infected person can super-spread to dozens of others without directly speaking to or touching them. If we are to live with this coronavirus forever—as seems very likely—some scientists are now pushing to reimagine building ventilation and clean up indoor air. We don’t drink contaminated water. Why do we tolerate breathing contaminated air?
It’s not just about COVID-19. The scientists who recognized the threat of airborne coronavirus early did so because they spent years studying evidence that—contrary to conventional wisdom—common respiratory illnesses such as the flu and colds can also spread through the air. We’ve long accepted colds and flus as inevitable facts of life, but are they? Why not redesign the airflow in our buildings to prevent them, too? What’s more, says Raymond Tellier, a microbiologist at McGill University, SARS-CoV-2 is unlikely to be the last airborne pandemic. The same measures that protect us from common viruses might also protect us from the next unknown pathogen.
To understand why pathogens can spread through the air, it helps to understand just how much of it we breathe. “About eight to 10 liters a minute,” says Catherine Noakes, who studies indoor air quality at the University of Leeds, in England. Think four or five big soda bottles per minute, multiply that by the number of people in a room, and you can see how we are constantly breathing in one another’s lung secretions.
The particles emitted when people cough, talk, or breathe come in a range of sizes. We’ve all been unwittingly sprayed by large droplets of saliva from the mouth of an overenthusiastic talker. But smaller particles called aerosols can also form when the vocal cords vibrate to air rushing out from the lungs. And the smallest aerosols come from deep inside the lungs. The process of breathing, says Lidia Morawska, an aerosol scientist at Queensland University of Technology, in Australia, is essentially a process of forcing air through the lungs’ moist passages. She compares it to spraying a nebulizer or perfume bottle, in which liquid—lung secretions, in this case—becomes suspended in exhaled air.
Even before SARS-CoV-2, studies of respiratory viruses like the flu and RSV have noted the potential for spread through fine aerosols. The tiny liquid particles seem to carry the most virus, possibly because they come from deepest in the respiratory tract. They remain suspended longest in the air because of their size. And they can travel deeper into other people’s lungs when breathed in; studies have found that a smaller amount of influenza virus is needed to infect people when inhaled as aerosols rather than sprayed up the nose as droplets. Real-world evidence stretching back decades also has suggested that influenza could spread through the air. In 1977, a single ill passenger transmitted the flu to 72 percent of the people on an Alaska Airlinesflight. The plane had been grounded for three hours for repairs and the air-recirculation system had been turned off, so everyone was forced to breathe the same air.
In official public-health guidance, however, the possibility of flu-laden aerosols still barely gets a mention. The CDC and World Health Organization guidelines focus on large droplets that supposedly do not travel beyond six feet or one meter, respectively. (Never mind that scientists who actually study aerosols knew this six-foot rule violated the laws of physics.) The coronavirus should get us to take the airborne spread of flu and colds more seriously too, says Jonathan Samet, a pulmonary physician and epidemiologist at the Colorado School of Public Health. At the very least, it should spur research to establish the relative importance of different routes of transmission. “We had done such limited research before on airborne transmission of common infections,” Samet told me. This just wasn’t seen as a major problem until now.
At the University of Maryland, Donald Milton—one of the few longtime airborne-transmission researchers—is about to embark on a multiyear, controlled trial aimed at understanding influenza. Flu patients and healthy participants will share a room in this study. And they will take different precautions, such as hand-washing plus face shields or having good ventilation, which would presumably stop either droplet or aerosol transmission. The trial is meant to prove which intervention works the best, and thus which transmission route is dominant. When Milton had managed to get funding for a different aerosol study in the 2000s, he said a public-health official told him, “We’re funding you to put the nail in the coffin of the idea that aerosols are important.” Now, Milton says, “We’ll find out which direction the nail is being driven here.”
A virus that lingers in the air is an uncomfortable and inconvenient revelation. Scientists who had pushed the WHO to recognize airborne transmission of COVID-19 last year told me they were baffled by the resistance they encountered, but they could see why their ideas were unwelcome. In those early days when masks were scarce, admitting that a virus was airborne meant admitting that our antivirus measures were not very effective. “We want to feel we’re in control. If something is transmitted through your contaminated hands touching your face, you control that,” Noakes said. “But if something’s transmitted through breathing the same air, that is very, very hard for an individual to manage.”