Can you run an air purifier with a window open? In this article, we’re going to check whether it makes sense to vent your room while you run your air purifier.
Actually, the answer is quite obvious: of course, you can run your air purifier.
However, as soon as you open your window, particles like pollen and bacteria enter your room.
Is that a reason to stop your air purifier? Should you wait until the window is closed to run it again?
Is running an air purifier with an open window a waste of electricity?
Should you open your window at all? (Yes!)
I find it funny how such a simple question turns out to be so complex. To find a definite answer, you have to immerse yourself in physics, experiments about room air exchange rate, air purifier power consumption, and so much more.
To all these views, I want to add a different approach, a very systematic one. It’s going to be interesting!
The summarized answer:
Yes, you can run an air purifier with a window open. In most parts of the Western World, outdoor air is clean enough and the airstream can even help support indoor air purification. Of course, if you live in a polluted area, an open window acts as an infinite source of particles, which reduces your air purifier’s particle reduction rate and its filter lifetime.
Let’s have a look at how we can inspect the problem systematically:
Can you run an air purifier with a window open?
Let’s postulate a couple of statements that we believe to be true.
- Venting is necessary.
- Venting creates an airflow inside your home.
- Outdoor air may be or not be polluted.
Now, let’s investigate each of those statements in detail.
1. Venting is inevitable
As long as all your windows and doors are closed, your home is a closed space. Theoretically.
In practice, air leaks in through tiny gaps below doors, and windows, and through imperfect insulation.
Still, indoor and outdoor air have a diminishingly low exchange rate if you don’t vent your room.
Over time, moisture (from your breath, the shower, from cooking) accumulates indoors.
This moisture has to be vented!
You cannot healthily live at home without venting. Venting is mandatory.
“My experience is unless there is heavy smog smoke or pollen it is always best to flush the home air with fresh air at least once a day.”DarkVandal on reddit
So, the question is NOT whether you can run your air purifier instead of venting your room. Air purifiers do not remove moisture, and they do not refresh oxygen levels.
The following statement is our guiding star:
You have to vent your room regardless of whether you run an air purifier or not.
So, here’s an interesting intermediate result:
- You HAVE to vent your room, regardless of whether you use an air purifier.
- Your air purifier does NOT replace venting your room.
Now, the question boils down to: Should I keep running my air purifier while the window is open or not?
2. Venting creates an indoor airflow
The smaller the particles in your room, the better a HEPA filter can collect them. That’s quite an interesting result.
“The fact that a HEPA filter’s removal efficiency increases as particle size decreases below 0.3 microns is counterintuitive.”Donaldson Company
In the figure below, you can see the filtration efficiency for two different filter types, HEPA and FERENA filters. Interesting for us is just the blue curve for HEPA filters.
As you can see, the filtration efficiency has a minimum at a particle size of around 0.1 microns (= 100nm).
So, the smaller the particles, the better a HEPA filter can collect them.
Now, imagine your home has no air movement. Except for the absolutely smallest particles (which are suspended in the air due to their size and Brownian motion), most particles would settle down over time.
In a home with little airflow, these particles start clumping together, eventually sticking to larger particles and forming dust. Dust acts in itself like a filter, since it attracts particles.
Now, dust particles are larger and, therefore, harder to collect by the HEPA filter.
Here’s where venting enters the game:
Regular venting creates an indoor airflow that lifts small particles up. An air purifier can pick up small floating particles easier than clumped-up larger particles.
Following this logic, venting causes an indoor airflow that could even help your air purifier pick up those particles that stick to the floor, walls, or furniture due to electric charge.
Of course, through venting, additional particles enter your home.
And of course, an air purifier creates its own airflow as well. So this effect might be comparably small.
3. Outdoor air may or may not be polluted
When you open your window, outdoor air enters your room and indoor air leaves your room.
If you live in a rather rural area, or a couple miles from the next large city, outdoor air may not be as polluted as you think.
Let’s have a look at the air quality map below.
The word air quality report ranks air quality worldwide.
The large majority of regions in the US and Western Europe are either blue (meets the WHO air quality guideline for PM2.5 particle concentration) or green (WHO air quality guidelines are surpassed by a factor of lower than 2).
Australia has very high air quality and is mostly tagged with blue quality markers.
In the yellow regions, air quality surpasses the WHO air quality guideline by a factor of 2 to 3.
If you compare that to dark red or even violet-colored India and Southeast Asia, you can tell our air is a lot better than theirs.
So, using an air purifier in the US, Western Europe or Australia might be good for picking up dust and particles from the air.
But it’s usually not a necessity.
So, what happens when you open your window?
If you’re in the US, Western Europe, or Australia, chances are, the outdoor air is less polluted than your indoor air. Venting, therefore, is just another way to purify the air.
In this case, it doesn’t matter whether you run your air purifier while the window is open.
If you are, however, in a polluted area, I recommend turning off your air purifier while the window is open. Here’s why:
Why you shouldn’t run your air purifier with a window open when the outdoors is polluted
If the outdoor air is polluted where you live, and you run your air purifier with a window open, essentially the following happens:
Dirty air from the outdoors enters your room. Your air purifier collects the particles and blows the clean air outside.
The cycle repeats until your air purifier’s filters are full.
When your window is open, it acts as an infinite source of pollution. Running your air purifier while it is connected through an air channel to infinite particles to filter is not a good idea.
It wears down your air purifier, and it accumulates pollution in your living space.
Now, you can argue your air purifier cleans the dirty outdoor air and it’s no problem to vent. In principle that’s true. Of course, while your air purifier is running, indoor pollution levels decrease slightly.
But as long as more pollution constantly enters your room through the open window, your air purifier essentially fights against an infinitely large opponent. This wears down the filter and does not benefit you much.
In a polluted area, shut off your air purifier while venting. Limit the venting time. And let your air purifier clean the air after you close your windows.
This approach ensures that your air purifier does not blow any clean air outdoors. It does not collect more pollution than necessary. And you save a bit of electricity.
Air purifier experiment results with open windows
The guys from smartairfilters.com did the following pollution level measurements in their flat in Beijing.
As you can see, the red bar indicates that when your window is open, your air purifier only reduces particles by 59.6% (as compared to when it’s not running).
When the window is closed, the reduction rate increases to 89.9%.
I strongly assume the reason is the infinite inflow of new pollution (as explained in the previous section). Your air purifier just can’t keep up.
Running an air purifier with a window open reduces particle filtering efficiency. However, that’s only the case if you live in a polluted place.
If outdoor air was cleaner than indoor air (which is the case in many places), instead of a net inflow of particles, there would be a net outflow of particles, which could change this result drastically.
In which case you should keep your air purifier running while the window is open
There is one case I can think of in which you should keep your air purifier running in combination with an open window:
You can run your air purifier with a window open if the source of pollution is indoors.
For example, the fastest way to get rid of smells from smoking, gases from strong cleaners, or from cooking, is to vent your room and run your air purifier at the same time.
In this case, air purification and air exchange add up.
Your air purifier reduces the particle count, while venting removes smelly gases and moisture.
How much does running an air purifier with an open window cost?
Opening the window while your air purifier is running decreases the particle reduction efficiency (as we’ve seen in the experiment earlier) if you live in a polluted place.
However, it does not affect the air purifier’s running cost. An air purifier’s power consumption depends mainly on fan speed.
An open window does not affect the fan speed setting, and thus, it doesn’t affect the power consumption.
I emphasize this, because for many devices, like heaters or air conditioners, an open window would affect power consumption. For example, if cold air enters your room, a heater (with built-in temperature control), would increase power consumption to hold the temperature steady.
That’s not the case for air purifiers. An air purifier has no kind of built-in control.
An average air purifier on “Medium” airspeed consumes 33W.
Larger air purifiers, or air purifiers on the highest air speed setting, consume up to 60W of power.
At an average electricity rate of 16 cents per kWh, running an air purifier for one hour costs
33W × 1 hour × 16 cents per kWh = 0.53 cents per hour
So, even if you vent your house for 1 hour a day, that would still waste less than $0.01.
But there’s a real cost we can associate with venting in polluted areas:
Filter degradation cost
The average HEPA 13 air purifier filter lasts 160 days if you use it for 8 hours a day. After that, the filter’s efficiency degrades to only 50%.
That amounts to a filter lifetime of
160 days × 8 hours = 1280 hours of usage
A good set of replacement filters (click to view it on amazon) costs $35.
That’s $17.50 per filter.
Based on that you can associate a cost to each hour you use the filter:
$17.50 / 1280 hours = 1.36 cents per filter-hour
Okay, honestly I expected it to be more. I am underwhelmed by the result.
The total cost of running your air purifier per hour would then be
1.36 cents + 0.53 cents = 1.86 cents
In a polluted area, you’d waste 1.86 cents in electricity and filter costs per hour if you keep your air purifier running with a window open.
Interpretation of the results
Financially, running an air purifier is never going to make a large impact. Regardless of whether your windows are open or not.
However, practically speaking, your filters last longer and you have to exchange them less frequently if you turn off your air purifier when windows are open.
There’s no single right or wrong answer, unfortunately.
Here’s my concluding guideline:
Shut off your air purifier when you open your windows if:
- you live in a polluted area
- you vent for an extended time (>10 minutes)
Keep your air purifier running if:
- outdoor air is clean – in this case, venting and air purification go hand in hand
- the source of pollution is indoors (from smoking, cooking, or cleaning)
I hope this article was helpful! Any further questions? Shoot me an email!