Aerosols – invisibly fine particles that float in the air around us for minutes or hours – appear to be a major player in the transmission of the coronavirus. That’s a big change from the early days of the pandemic, when larger droplets from coughing or sneezing and the contamination of surfaces were thought to be the main ways COVID-19 spread.

With this new understanding comes the question: how do you slow the spread of disease if hordes of invisible floating particles are a serious source of infection? The good news is that the old standbys of washing hands, social distancing, and, when you can’t socially distance outside the home, wearing a mask are still critical mainstays. But the rise of aerosols as a transmission vector requires new additions to those pandemic-fighting tools. They involve deliberate steps to clear the air.

What’s floating in the air – and how it does or doesn’t float in different situations – has been a source of COVID-19 uncertainty. Pandemics are the domain of epidemiologists and infectious-disease experts. Aerosol science is rooted in physics, chemistry, and engineering. It took time for those versed in aerosol science to impress upon the medical community that the long-understood nature of aerosols, combined with the scant evidence of COVID-19 transmission through droplets, demanded a rethinking of how the coronavirus spreads. The watershed moment came with the publication of a July 6 challenge to the World Health Organization signed by 239 scientists from around the planet, “It is Time to Address Airborne Transmission of COVID-19.”

Hard science around airborne coronavirus transmission

Among the signatories was Jose-Luis Jimenez, a University of Colorado at Boulder atmospheric chemistry professor. He has since emerged as a national force in the efforts of aerosol-science experts to impress upon the medical community and the rest of us how aerosols behave and why the science and circumstantial evidence strongly suggests aerosols’ culpability in the spread of COVID-19. One of his key points is that much of what the medical community calls droplets – and assumes fall quickly – are in fact aerosols that can float for many minutes or hours.

What’s more, he adds, when you take those “droplets” out of the picture, the droplets that remain in play are so big that the only way they’re likely to cause infection, besides contaminating surfaces, is through a hard landing in someone’s mouth or eye at a distance of a couple of feet – not terribly likely, he argues. All told, Jimenez suspects that perhaps 75% of infections are spread through aerosols, the touching of contaminated surfaces (technically called fomite transmission) accounting for much of the rest. (Jimenez’s team has created an online aerosol transmission estimator to provide a rough sense of the transmission risk in various settings, including a classroom, a supermarket, a bus, a stadium, a rally, and others.)

The question of whether SARS-CoV-2 viruses previously found in aerosols could be infectious went from strongly circumstantial to scientifically concrete with a University of Florida team’s preprint article posted on Aug. 4. The paper awaits peer review, but the work appears to be rock-solid.

With that background, let’s talk about what to do about these aerosols.

First, avoid places where aerosols accumulate or can waft between people at close range. The Japanese came up with the idea of avoiding the “Three Cs” – closed spaces with poor ventilation, crowded spaces, and close-contact settings such as close-range conversations.

At a recent webinar on the University of Colorado’s work to clear coronavirus from the air for students, faculty and staff, Shelly Miller, a CU engineering professor and environmental air-quality researcher, said common sense can be a good guide here.

“If it’s a hot, stuffy environment, chances are that there’s probably not enough outside air in that space,” said Miller, who has also become a prominent voice in raising awareness about aerosol-borne coronavirus risk. A more sophisticated approach is to use carbon-dioxide monitoring, she adds: if CO2 levels exceed 1,000 parts per million, either fewer people or fresher air are the answer. (Jimenez wrote this about how to go about doing the measurements.)

The “close-range conversations” part of that third “C” is surprisingly important: talking emits five to 30 times more aerosols than breathing – and singing and shouting emit even more, Jimenez says.

Second, wear a mask in indoor or crowded outdoor environments. Studies increasingly point to masks’ effectiveness in stopping large percentages of not only larger droplets, but also aerosols Research shows that most masks protect both the wearer and others.

Jimenez suggested during the webinar a rule of thumb: imagine that those in the space you’re entering were smoking. Then consider if you could smell that smoke more than in passing. If so, wear a mask.

Third, do it outside. Few cases of outdoor COVID-19 transmission have been recorded – despite many mass gatherings such as political protests and crowded beaches. Chinese researchers who in a preprint article considered more than 7,300 cases driven by 318 outbreaks found only one outdoor outbreak – from a conversation between two men in a village in Henan. If the weather permits, head out.

Fourth, keep indoor numbers down. Chris Ewing, CU’s assistant vice chancellor for Facilities Management, said in the webinar that CU’s classrooms will be at between 20% and 30% capacity this fall – the result of spacing out students to ensure six-foot gaps. That’s to minimize possible short-range (six feet or less) aerosol transmission. Fewer bodies mean fewer aerosols and less work for HVAC (heating, ventilation, and air conditioning) systems and other viral countermeasures. Ewing’s colleague Shannon Horn, a CU Facilities Management engineer also on the webinar, said lower capacity should allow for about 70% more outside air per person in campus buildings even without the other measures the university is taking.

Fifth, amp up the HVAC. Building ventilation systems vary. Some bring in outside air; some recirculate inside air with outside air; and some rely on open windows for any outside air at all. Typically, building managers turn down or off active ventilation systems in the dead of night as an energy-saving measure. But during the pandemic, CU is running these systems 24/7. The university is also upgrading the filters used in its HVAC systems to MERV 13, which a 2013 study as well as the industry group ASHRAE recommend as a means of capturing more virus-carrying particles.

Sixth, use portable air cleaners. CU has its share of older buildings in which getting fresh air means opening a window. In such spaces, the university is adding portable air cleaners. Choosing a portable air cleaner is no easy task – there are hundreds of options with vastly different pricing and specs. Miller and Jimenez both recommend air cleaners with HEPA filters and nothing else.

Air cleaners must be matched with the size of the space involved, Miller adds. The Association of Home Appliance Manufacturers’ (AHAM) certification program provides numbers, the key one being CADR, for clean air delivery rate. AHAM calculates it separately for dust, pollen and smoke. While those numbers, expressed in cubic feet per minute, tend to be similar for a given air cleaner, the CADR for smoke is the best proxy for lingering, virus-carrying aerosols, Miller says.

Miller recently collaborated with Harvard’s T.H. Chan School of Public Health researchers on a portable air cleaner calculator for schools. The calculator can also serve as a guideline for portable indoor-air cleaning in other public spaces. Between opening windows, running HVAC systems, and employing portable air cleaners, shoot for five air exchanges per hour in a given space, Miller says.

For a 25-foot by 25-foot classroom with eight-foot ceilings and typical school ventilation, for example, Miller’s calculator suggests a CADR of 300 or more. Smaller spaces can get by with smaller air cleaners, and you can also use more than one air cleaner per room, Miller says.

Portable air cleaners – alone or in multiples – capable of refreshing a 625-square-foot room’s air several times an hour can cost hundreds of dollars. A homebrew option Miller and Jimenez suggested combines a 20-inch by 20-inch MERV 13 or HEPA filter with a common box fan.

Affix the air cleaner to the intake side of the fan, turn the fan on low (easier on the motor given the extra resistance), and voila – one has an air cleaner capable of scrubbing perhaps 90% of the fine aerosols in the 0.3 micron range, the toughest to capture (aerosols ranging from 0.2 microns to 50 microns are suspected of carrying SARS-CoV-2 viruses, which are themselves 0.12 microns wide – about one six-hundredth the breadth of a human hair). Ninety percent is less than the 99.97% of such particles HEPA filters remove, but as with HEPA-filtered devices, air quality will improve as room air passes throughmultiple times. Plus, it’ll cost less than $50. (For more on this, check out this video and this story.)