As mask usage decreases in the United States and worldwide, the risk of aerosol transmission of SARS-CoV-2 increases inversely. A maskless gym, grocery store, or gas station is much more dangerous than those settings in a masked context. We ought to be acutely aware of the levels of risk maskless settings pose as world governments roll back Covid policies while Covid continues to thrive.

In our recent series on airline travel, we discussed exhaled CO2 as a Covid-19 risk proxy aboard aircraft. The concept is that the higher your surrounding CO2 levels, the more significant the proportion of air exhaled from nearby people, and thus the greater the risk of SARS-CoV-2 particles in your immediate vicinity.

This concept can be extended past airplanes into our daily lives. In the initial months of the pandemic, we were in fewer situations with high CO2 levels, reducing the risk of infection. More recently, many are resuming daily activities with office jobs, eating at restaurants, and going on vacations. Monitoring CO2 levels, either personally or by the business or company hosting a social gathering, may grant us a better sense of Covid risk in the coming months.

In the first year of the pandemic, many suggested that SARS-CoV-2 could not be transmitted via aerosols, i.e., exhaling virus particles. Their view restricted virus transmission to larger droplets, such as with sneezing or coughing, as well as fomites, which are object surfaces to which the virus may cling, for example, door handles or countertops. This dissenting view yielded skepticism about mask usage and is one reason behind a lack of true mask mandates in the United States as many other countries implemented them.

This theory was eventually dispelled, in part, due to the examination of CO2 in social settings. Studies such as that of Peng and Jimenez analyzed CO2 in closed indoor environments and quantified CO2 levels in terms of Covid-19 risk. As expected, closed environments with higher CO2 levels are much more likely to harbor transmission if an infected person is within the system.

In unventilated areas, gases move rapidly about space to fill the volume of the contained environment. Much like how liquids take the shape of whatever container they inhabit, gases do the same at a much higher rate. This is why being in the same room as an infected person may get you sick even if you do not touch them. The gas carries virus particles across the room towards you.

That is why CO2 levels are an adequate proxy for Covid risk. The more CO2 that is in the air, the more likely that CO2 is transporting virus particles around the room if an infected person is there as well. With this in mind, we can introduce three tools to help reduce aerosol-based infection.

The first tool is the CO2 monitor. The CO2 levels of an enclosed space correlate to the risk of Covid infection, assuming there is someone infected in the room. Personal monitors can be purchased for as low as $50 and could be a strong investment to prevent Covid infection and potential long Covid. Additionally, businesses and companies could provide CO2 level monitoring in their stores to assure customers of their safety.

We recently measured the CO2 levels of various locations on a typical day in New York City. CO2 levels varied by location and density of people, ranging from 458 ppm in outdoor spaces to 2366 ppm on a very crowded subway. We cannot avoid many aspects of life, such as subway transport in New York, but being aware of CO2 levels to some degree may help mitigate risk to some extent.

The second tool is the high-efficiency particulate air (HEPA) filter. Particulates as small as 0.3 microns can be removed from the circulating air by HEPA filters. While SARS-CoV-2 particles are as small as 0.1 microns, HEPA filters can take in contaminated CO2 and filter the air for dust, debris, and even virus-carrying droplets, reducing the chance that virus particles will be carried from person to person. Modernized aircraft widely use HEPA filters, which often have lower CO2 levels than taxis and subways. We highly recommend using a mask on subways and in taxis when riding alongside strangers. CO2 levels in cars may be inflated by engine combustion, but closed spaces present the opportunity for virus spread.

The third tool is UV light sources. There is evidence showing that viruses are inactivated by continuous exposure to UV irradiation, particularly on fomites, but also in the air. UV light is invisible to human eye receptors. The widescale implementation of UV lights in regularly crowded areas could decrease virus transmission by inactivating live viruses in the air and on surfaces.

We recommend all public spaces, such as schools, restaurants, theaters, libraries, public transport, and stadiums, be designed with a high volume of air exchange, as well as a large network of HEPA filters and UV lights to sterilize spaces as much as possible. Just as there are fire codes for new buildings, there ought to be air safety codes as well. These requirements should be mandated at a local, state, and federal level, with the same impetus we place on other building regulations.

As we echoed in our analysis of airlines, the onus is now on the individual to protect themselves from Covid-19 as world governments reverse course on Covid safety policies. The pandemic, however, is far from over. We highly emphasize continued mask use, vaccination, and the tools listed above as measures one should take to protect themselves against the disease.