Portable air cleaners – by which I mean domestic appliances that use a fan to pass air over a HEPA filter and are sometimes referred to as PACs – often claim to remove 99.9% of bad stuff (viruses, smoke, dust) from the air. If that were true, why are we all not using them? Why isn’t there one in every building?
Well, firstly those quoted percentages are classic marketing – factual truths, but not the whole story. A HEPA filter will remove over 99% of the dust or virus from the air that passes through it. The real question is how much air is passing through it? It could take up to an hour for all the air in a typical living room to pass through a typical home air cleaner, during which time I could still be exhaling virus or smoking, or bushfire smoke could be seeping in. It’s more like a race – can the air cleaner remove particles faster than they are being replaced?
Having said that, my experience of using PACs in research projects has led me to be not just impressed but convinced we should be using these devices much more than we are. Testing appliances in homes in neighbourhoods with significant outdoor air quality problems arising from widespread wood and coal burning, I’ve found that PACs routinely reduce the levels of particulate matter in the home by 50 – 80 %. That’s the equivalent of picking up that home and moving it from a polluted to a clean air neighbourhood.
Participants in our PAC trials have overwhelming reported positive responses – reduced symptoms, reduced anxiety, less dusting.
There appear to be only a few downsides to the PAC, yet they seem to be significant enough to prevent widespread adoption. Ostensibly these are noise, energy and filter cost. But there is also the more subtle issue of the missing feedback.
Firstly, cost. Very few PAC manufacturers or retailers forefront the running costs of their devices. Those devices that are cheaper to buy tend to be “always on” which implies a higher electricity cost. Some smarter devices are available which rely on the user to decide when it should run high or low, or use sensors to automate fan speed. This results in higher purchase cost but lower operating cost. But if it is exclusively indoor contaminants you are dealing with (the outdoor air is reasonably clean) what PAC sellers rarely tell you is that there is another air cleaning “mode” which is usually far more effective but often costs nothing. It’s called opening a window. Aka ventilation.
Not that it’s an “either/or” – ventilation and filtration are not exclusive. Air filtration using a PAC can be thought of as having an additional open window – one that doesn’t let the weather in.
Where filtration really comes into its own, though, is in situations where opening a window is not an option. This could be because it is too cold, or too hot, or too windy or rainy. Or noisy outside. Or there are no windows. It is especially true when it is the outside air that is the source of pollution (or there are sources both inside and outside). If you are in a location where this is pretty much always the case you may be happy to have your PAC on permanently. But where the problem posed by outdoor air comes and goes, you may be in the market for a smarter sensor-driven device that knows when to crank up the fan and get filtering, and when to slow down and save energy.
And noise. Many PACs working at full tilt are very noisy. Especially in noise-sensitive locations like bedrooms, studies, or when you’re trying to have a conversation or watch TV. Noise has been one of the major drawbacks to using PACs in school classrooms, with studies showing that the white noise they generate being particularly difficult when the teacher or students are operating in a second language.
At the moment there are two ways of managing PAC noise. One is, again, to have a variable speed device that you can turn down if needed (albeit reducing its air cleaning effectiveness). A smarter device may have the option to reduce speed for a pre-set period – an hour, or 8 hours, say. The other way is just to have a larger PAC. It might be counter-intuitive but larger devices are generally quieter devices. They can also be run at low speed for the similar effect as a smaller, noisier PAC running at high speed, whilst also have lots of spare power in reserve for emergencies (bushfires, epidemics, etc). But they do take up more room, and tend to be more expensive to buy and buy replacement filters for.
One of the subtlest problems with PACs today is the relative lack of feedback. What I mean by that is that every PAC gives you at least incentives to turn it off – noise and cost. What are the counter-incentives to keep it on? We keep using heating systems because if we don’t we feel cold. But we don’t always notice any change if we stop using a PAC. Some of the better PACs have onboard air sensors and display the current air quality. But, unless you are really, really paying attention (you are not) that doesn’t tell you anything about what the device is achieving.
If the unit says something like “particles low, air quality good” you may think “great, my PAC is working”. But how do you know the PAC achieved that? Maybe air quality was good anyway. Maybe you’re paying good money for nothing. If your PAC says “particles high, air quality poor” then what use is it?
In both of these scenarios our instinctive conclusions could quite well be wrong. We have fallen prey to black-and-white thinking. Or at least the PAC has allowed us to do that. What the PAC has not told us is this: what would air quality be like if you DIDN’T use me? Or what if you operated me in a different mode? In other words, “how much difference have I made?” This is the question you are asking yourself, consciously or not, when you decide whether to keep your PAC plugged in, or not; whether to replace its filter, or not.
So, for me, the needs are clear. PACs are a very effective air control device. But to be more acceptable they need to be more flexible in how they allow the user to define their own trade-off between power, noise and effectiveness. That also requires them to monitor and report their own effectiveness. An ideal system will also work with the user to change that trade-off if conditions change, e.g. as the relative hazard between indoor and outdoor contaminants change. This could mean responding to major changes in the environment, such as moving from “normal” to “bushfire” to “epidemic” conditions, or more subtle changes, like transitioning from a mostly-indoor hazard like cooking to a mostly outdoor hazard like woodsmoke as a winter’s evening progresses.
Finally, whilst saving energy, sensor-triggered PACs are a reactive technology. They have to wait until a space becomes polluted before they start working to clean it up. In principle, PACs could operate in a preventative or even predictive mode, being triggered to ramp up to full speed before the contamination builds up. This could be based on routine patterns of behaviours observed and “learned” by the system, or could be related to early warning forecasts, like bushfire smoke forecasting. This could effectively guarantee clean air at all times and probably reduce power consumption overall.
Whether dumb or smart, stand-alone or part of a wider sense-and-control system, portable air cleaners have a lot to offer, and are worthy of a little R&D to make them indispensable.
