The COVID-19 pandemic has challenged the status quo in a number of ways. From helping the world realize that you can run scientific conferences that are accessible for people who cannot (or will not) travel, to rising the very thorny question of “should kids be learning in the classroom, even though their risk of infection will be higher?”
Particularly around the issue of indoor gatherings, the use of air cleaners has been getting more attention … even some quite striking headlines from Australia!
As an air quality scientist, who have worked on atmospheric chemistry for more years that I care to count I have been asked my thoughts on air cleaners and to be honest my response is quite simple. FILTER ALL THE AIR!
Now, some might remember my position on outdoor air scrubbers and ask why I recommend air cleaners for indoor environments. Well, and this may come as a shock to some, indoor environments are smaller than the whole atmosphere đ Put simply, portable air cleaners work really well for small enclosed environments like offices, classrooms or homes because the volume of air they need to process to make a difference in those environments is relatively small.
Take a “not too large” room of 60$m^2$ and ceilings at 3m … that’s 180$m^3$ of air and a mid-size air cleaner can deliver 400$m^3$ of clean air per hour.
High Efficiency Particulate Air (HEPA) filters are really good at removing particles from the air and therefore removing the pathogens that are attached to those particles. Apart from these filter-based solutions, there are other air purifiers that use ionization or uv-light to sanitize the air but there are quite significant issues with these technologies
The problem is relatively simple, we need to clean the air from airborne pathogens, and we can, in principle, use similar solutions than whatâs used to sanitize surfaces, water or clothing. However, the use of certain chemicals is not suitable for air that is intended to be breathed in.
Ionization: Do you want some ozone with that?
A number of air purifiers use ionization techniques to help with the cleaning of the air. The basic idea of these devices is the use of ion generators (e.g. corona discharges) that charge the particles that pass through it, which then are attracted to the surfaces in the air cleaner, greatly increasing their filtering efficiency.
That sounds harmless enough, right? Well, the main problem with these devices is that the same process that generates ions, also generates ozone which is a pollutant that has harmful acute impacts on peopleâs health.
Indoor air chemistry is a largely under-researched topic but the information that does exist points to an unacceptable risk with the general use of ionizers in indoor environments. The US-EPA warns that only a very small number of devices, specifically designed to use ozone as a disinfectant agent, are approved by the US-FDA for use under certain conditions but in general, adding ozone to an occupied indoor environment is adding unnecessary risks. A study specifically looking at the benefits, or otherwise, of the use of ionizing air cleaners in school environments found that air ionizers in school classrooms reduced particulate matter concentrations which led to some improvements in respiratory health among 11-14 year old children, but that the ionizers had an adverse effect on heart rate variability (a measure of cardiovascular health), meaning that any benefit to the lungs came at a cost to the heart.
UV-C: Do you want a smog chamber? Because thatâs how you get smog chambers
Ultraviolet light (uv) has long been used to disinfect water because the high energy uv light denaturalizes some proteins which render pathogens inert. The same principle has been used in hospital settings where quick sanitization is required, and the objects are not suitable to be put through a high-pressure steam treatment in an autoclave. This process has been identified as safe, as long as people are not directly exposed to this uv radiation (because if itâs denaturalizing proteins in a bacteria, it will do the same to the cells in our body ⌠enter cancer). However, because of the general disconnect between health research and indoor air chemistry, the secondary issues of using high energy light sources like uv lamps have been largely ignored. It has been reported that uv-light can induce photolysis in certain types of volatile compounds (VOC) and can also help generate conditions for new particle formation.
So … what then?
There are many ways of framing this but in essence, the question we should always keep in mind is why add a risk to a situation if there are perfectly viable solutions that donât add unknown risks?
In this case, the perfectly viable solutions are straight-up filters. There is no need to add ionization, or uv-light sources because the filters already do a hell of a goot job in removing particles (and the pathogens attached to them) from the air so instead of trying to over-complicate the question, just get a bunch of good quality HEPA filters for your indoor environments and leave the uv-treatments for the hospitals (where the risk calculation is different) and the ion generation processes for aerosol scientists.
