15. Ventilation and Filtration - why both still matter
Indoor air quality requires thoughtful design and careful data analysis.
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As the world seems to be heading towards a sense of Covid ambivalence - an acceptance that this will remain with us for years to come - now is the time to think about the importance of indoor air quality. This is even more important as some communities move towards fewer mask requirements. Your lungs will thank you later.
There are all sorts of nasty things in the air that we breathe. From pollutants like tobacco smoke, oil droplets from cooking, and exhaled droplets from our family members, coworkers and classmates. Yuck.
What’s a breather to do? It’s simple: Filter and ventilate.
What’s the difference?
Filtration is when air is passed through some sort of filtering device, like a HEPA filter, which captures the tiny particles suspended in air. Clean, filtered air exits from the other side of filter and recirculates back in to your room. You can filter your air by using a portable device, or by using filters installed within heating and ventilation equipment, like a central heating/air conditioning system, that blows heated or cooled air into a room.
Ventilation is when you mix in outside air, which is presumably cleaner (but not always) into the room you occupy. It can be as simple as the opening of windows or doors. In less common cases, you can ventilate using a forced air heating and cooling system. It is often a misnomer that for most ‘Heating, Ventilation, and Air Conditioning (HVAC)” systems that are in operation, they manage heating and air conditioning pretty well but usually provide no ventilation. You find HVAC ventilation more common in commercial buildings, and less often in residential ones.
Which is Better for my Health?
As you might imagine, the answer can be complicated. The best protection is the one that is available to you and is one that you will use consistently. While much of the recent discussion on indoor air quality revolves around Covid-19, health risks from air quality have been around for much longer.
Let’s tease this apart.
The case for filters
Filtration captures and removes floating particles in your air. We’ve known for a long time that these little particles make us quite sick, but it has become clear that some of these little droplets also carry infection to people.
Filters work well for the particles we traditionally research - exhaust from vehicles and factories, woodsmoke, wind-blown dust, for example. But they also work just as well for droplets that are emitted from people. The more they operate, the more particles they scavenge and capture.
There are at least three downsides to filters - they can be quite expensive to install and operate, and they have to be the right size for the room you are trying to clean and, when undersized, take a lengthy amount of time to remove particles. Also, if the room is enclosed with no ventilation, filters will do nothing for carbon dioxide, which builds up in rooms where people occupy. Your air has fewer particles, but is sometimes perceived as stale.
Carbon dioxide in rooms is not typically a hazard we worry much about - but it is easy and relatively inexpensive to measure whether air in a room is stagnant and needs to be cleaned. Higher carbon dioxide usually means that there is buildup of exhaled breath in a room. Many commercial HVAC systems monitor carbon dioxide to tell the system when to turn on or off. Filters work to remove particles, but do not remove carbon dioxide.
The case for ventilation
Ventilation, on the other hand, does a great job at removing carbon dioxide, as well as sending nasty particles from your space to somewhere else. By the simple act of opening a window or door, you can dilute infectious particles really quickly because lots of air can rush in to replace the stagnant air. When this happens, the particle concentration changes (though not always in the way you hope), and carbon dioxide often goes back to outdoor levels, which is usually around 425 parts-per-million. Ventilation can often provide clean air to a room much faster than most filtration systems.
But, there are some downsides. Ventilation doesn’t always work as expected. Sometimes when a window is opened, fresh air from outside rushes in, sending your polluted air back in to the building. Or the reverse happens - air rushes out of the window, and the air in your room is replaced by stagnant air from another part of your building. This can vary based on wind direction and speed, your location in a building, or other activities occurring within that building. One way to reduce this is to close off the room where you are occupying (such as a classroom) by closing the door that leads to the remaining parts of the building. But this can reduce how effectively that open window ventilates.
In other cases, the air from outdoors can be more polluted than the air indoors. Maybe your place of work or school is located next to a busy highway, or wildfire emissions are impacting your community. You’ve just replacing polluted indoor air with polluted outdoor air - not much of a trade in my book.
And while it’s easy to recommend ventilation, we often neglect that not everyone has access to operable windows. So for many, this is simply not an option. Too many scientists seem to forget this.
What strategies to you use to improve your air quality? Be sure to comment.
The case for all of the above
There is no perfect solution, and one has to use some judgment. Here, I offer some advice that works for classrooms, offices, residences, or businesses and wherever people congregate.
If you have a HVAC system, turn it on, even if it’s just in ‘fan’ mode that circulates air. And make sure you use a high quality filter that meets 'MERV-13’ standards or better. It’s also okay to open your windows to let in fresh air, even when your fan is running.
If no HVAC system is available, then what does your outdoor air quality look like? If it’s pretty good, open your windows as much as you can to ventilate your space. When possible, close off the rest of the building. Not sure what your local air quality looks like? Check EPA predictions here (US locations only). When it’s in the orange color or higher, use filtration instead of ventilation and avoid opening those windows.
Don’t have a central HVAC system or access to clean outdoor air? You’re a candidate for portable filtration. If a few hundred dollars for a portable HEPA filter is outside of your budget, go with a do-it-yourself option and build a CR Box. If you can wrap a birthday present, you can make a CR box. In either case, make sure it’s the right size for your space. You can do this by looking at the specifications of your product for their “clean air delivery rate”, CADR, which tells you how much clean air they can put out per minute. A good rule of thumb1 is to find a filter where the CADR value is about the same as the square footage of your room (length x width). The higher the CADR, the better. If the manufacturer doesn’t make this information easy to find, try searching the web for the model. If you still can’t find it, use another product.
And ignore manufacturer promises about room size - this is just marketing.
I understand these are complicated options, and there is no one-sized-fits-all solution. And because it is often and invisible problem, air quality can be easy to ignore. As an exposure scientist, I know we ignore these risks at our own peril.
We have tools available to protect public health. We just have to use them.
After reading this piece, take a deep breath (of filtered or ventilated air), and consider subscribing. It’s free!
It is best to have CADR capable of ‘changing’ room air 4-6 times per hour - more is better. If you have a room that is, for example, 12 feet by 15 feet (180 square feet), a filter that delivers a CARD of 180 cubic feet per minute of clean air will filter the air 6 times per hour. This assumes a 10’ tall ceiling. And when in question, get the higher capacity filter - you cannot ‘over-clean’ your air.