Humans spend 90% or more of their time indoors. 90%, that’s more than some species of whale spend underwater. Spending most time indoors means breathing mainly indoor air, and it thus goes that its quality has important impact on our health. Most of us care about what we eat and drink, yet we rarely question the quality of air we breathe.

While standards exist for quality of indoor air, they are mostly limited to concentrations of pollutants and temperature. There are limited and rare regulations for concentration of CO2 (levels of which are even more rarely measured) and virtually no regulations on concentrations of airborne pathogens. That’s not enough.

Indoor CO2 and aerosols

CO2 levels affect our immediate cognitive ability. In areas with low CO2, cognitive test scores have been reported 61% higher than in areas with relatively high CO2. Mind you, this study designated ‘high CO2’ as a level just half of what has another study found to be a common level in classrooms! Poor air quality not only makes us dumb, it also makes us sick. Indoor airborne transmission is the major route of spread for the most common viruses such as influenza, and now SARS-CoV-2 as well. And not only directly from someone coughing at us, but also from breathing indoor air where they remain suspended in the air for extended amount of time as aerosols. One study estimated that half of all infections happen in just 5% of all indoor areas, high occupancy public buildings such as airports, schools, gyms, exhibition halls, theaters, music venues, etc.

Ventilation is helpful, but not sufficient

We cannot, and should not, opt out of indoors. This is most of where we work, socialize, meet our needs, and enjoy conveniences. We, therefore, must ventilate and sterilize the indoor air.

‘Ventilate?’, you say, ‘aren’t we doing it already?’ We are, but very very poorly. The high occupancy areas described above tend to have ventilation with air exchange per hour (ACH, a common way how to quantify the rate of ventilation in an indoor environment) of 1 or 2. At this rate, it will take hours before airborne contaminant concentration is reduced by 99.9%. That’s incredibly slow. For comparison, at ACH 6 it is still pretty slow and will take over an hour for the same reduction. As another point of reference, pharmaceutical manufacturing spaces, where environmental quality is very important for the manufacturing process, often require several hundreds (!) of ACH.

Clearly, we have a long way to go before significantly improving the quality of our indoor environments. Technologically, we are not even quite there yet. While ventilation with very high ACH is possible, it needs to be designed for at construction (something that is not possible in existing buildings), requires high energy input and frequent maintenance, and may actually not be pleasant for building occupants (e.g., due to noise and air streams).

Far-UVC inactivates airborne pathogens

There is an early-stage alternative that holds a lot of promise: far UVC. Far UVC, is light at the high energy spectrum. With wavelength of ~220 nm it does not penetrate human skin or retina, but being UV it does kill microorganisms including viruses, bacteria and fungi. Early scientific studies showed that it can efficiently sterilize indoor air and surfaces (Buonanno, 2020 and Eadie, 2022). It can deliver 50-100-fold improvement over current ventilation systems in terms of relative air exchange (ACHe) and could also be mounted in already existing buildings. Bringing manufacturing costs down, as well as carrying out epidemiological studies needs to be done before large scale deployment.

Plants have many qualities, but air-purification is not one of them (yet)

UVC seems great, but you must be wondering how about good old plants – aren’t they also cleaning the air? Can’t we just have more plants everywhere? I would love that as well, so I looked into it. It turns out that the only thing plants can do reasonably well is regulating indoor humidity. When it comes to CO2 removal, plants sequester only negligible amount of the indoor CO2 (Llewellyn and Dixon, 2011). This is because they need just a tiny amount of it for photosynthesis. Moreover, some potted plant systems are even net positive CO2 emitters due to the composition of substrate in the pot (!). Finally, even if plants would photosynthesize like crazy, they would still only deal with CO2.

I am not ready to give up on plants. Being a synthetic biologist, I am aware that there are efforts to engineer plants for higher CO2 sequestration rates. Similarly, their microbial symbions can be engineered to do the same. There are also startups that engineer plants to remove pollutants from the indoor air. I remain a plant-optimist. When coupled together with building design that brings in more light and improves passive indoor ventilation, I believe plants will have a small but important role to play in improving the quality of indoor air in the future.

Become an indoor clean air activist

So what can you do while waiting for UVC to hit the mass market and synthetic biologists to engineer more plants? You can certainly use the information you saw here to remind yourself to ventilate your indoor spaces frequently. I would also get a CO2 meter and measure the ambient concentration in spaces where you spend the most time (at home in your living room and bedroom, at the office). When you walk into a crowded space, be proactive and ask for it to be better ventilated if you can. If you are maybe feeling slightly sick, like your immunity may not be in the best shape that day, consider skipping that party or concert that you know will be crowded and poorly ventilated – the chances that you catch an airborne virus are quite high, and that may not be worth it. If you are an employer, ask your employees about the indoor environment and whether they recall getting sick while at the office, especially if you implement some changes (occupancy, ventilation, …) or move offices. Show your love for local businesses that make the effort to create a nice indoor environment. If you consistently notice the place has fresh air, is well ventilated, is not overbooked, and maybe even uses UVC (such as this bar in Boston) – thank the owners and spread the word.

It will take a while but together, we will make indoor spaces healthier environments for everyone.

This post was inspired by a great article by Juan Cambeiro and Brian Potter over at Institute for Progress.