How Long Does It Take to Purify the Air In a Room?


When I bought my first air purifier for my bedroom, I was excited to unpack it and to turn it on. After that, all I had to do was to wait for it to work.

But how long does it take for an air purifier to purify the air in a room? You can expect an air purifier to purify most of the air in a room within the first 45 minutes to 3 hours. Just how quickly it can clean the air depends on many factors like the selected power setting, filters, and ACH (rate of air changes per hour) of the air purifier.

While some air purifier manufacturers may claim that the air in an entire room can be exchanged in as little as 30 minutes, that’s somewhat unrealistic. Their tests are performed in a controlled setting with brand-new, unused filters, and don’t take into account what a real home looks like (for example, furniture nearby blocking airflow).

For that reason, at least 45 minutes is more realistic to expect the air to clean out.

The answer isn’t as simple or straight-forward and requires a more detailed explanation.


How Fast, How Far, and What the Air Purifier Can Clean

There are four factors that need to be considered together before you can understand just how effective an air purifier is at cleaning the air inside a room.

  • The power setting of the air purifier.
  • How fast the air purifier can cycle the air.
  • How much volume of air the air purifier is rated for.
  • What size of particles the filters can trap.

The Power Setting

Most air purifier features are marketed using the highest power setting. In other words, once you turn the power down a notch to the lower setting, the capabilities of an air purifier drop precipitously. Keep this in mind, especially during the first day of using an air purifier. The assumption is that you’ll need to turn the air purifier on at the power setting to see results.


How Fast the Air Purifier Cleans

When trying to determine just how fast an air purifier cleans the air, the most important number to look for is the ACH, or the rate of air changes per hour. The ACH tells you how many times the same air can cycle through the air purifier and be cleaned per hour. A good air purifier should be able to cycle the air in a room about 4 to 5 times per hour. In other words, the same air must pass through the air purifier every 12 to 15 minutes.

Air purifiers that cycle the air once every 20 minutes (3 times per hour) are average, but they’re obviously not as good as those that have a faster ACH rate. Stay away from air purifiers that don’t change the air out fast enough. The air inside most homes tends to be quite polluted. An air purifier that takes an entire hour to change the air out won’t be powerful enough.

And remember, the ACH is given by the manufacturer’s tests and will inevitably be different in a real home. For example, furniture blocking airflow, nearby cooking, and normal wear-and-tear of the air purifier’s filter will lessen the ACH.


How Far the Air Purifier Cleans

A powerful air exchange rate is excellent. But even if your air purifier is able to clean out the air every 10 minutes, does it really clean out all the air inside the room? How much volume of air can it really push through, and how far is its reach?

For this, you’ll want to look for an air purifier that has a CADR rating (short for clean air delivery rate). The CADR is a standardized test that shows how many cubic feet per minute an air purifier is able to move to clean out 80% of the pollutants in the air. Unfortunately, some other labels can be misleading. I like the CADR rating because it’s an independent test with a clear label that leaves no confusion.

(I’ve talked about CADR in this post – use it to find out what rating your room needs for the cubic feet per minute by referring to the table.)

What the Air Purifier Cleans

So far, so good. You’ve found an air purifier that can clean a large 800-square foot space with high ceilings, and it exchanges all the air every 15 minutes.

But there’s one more thing to think about, and that’s the filters. Just because the air purifier is able to move all the air in your room doesn’t mean that it will be able to clean all of it. You’ll have to understand what kind of pollutants the filters are able to trap and remove from the air.

Most air purifiers will be able to remove medium and large particles like pollen and dust, but the fine particles is where the differences arise. Fine particles are indoor air pollutants that are 2.5 micrometers or below by definition, but you’d be surprised to know that most of them are in the 0.1 to 1.0-micrometer range. These can include things like bacteria, viruses, small mold spores, gases, VOCs, among other things.

HEPA filters are a popular way to combat those fine particles, but they’re not all created equal.

For example, air purifiers with a True HEPA filter are able to filter out 99.97% of the pollutants as small as 0.3 micrometers in size. Compare that to filters that are marketed as “HEPA-like” or “HEPA-style.” These filters are able to filter 99.0% of the air pollutants as small as 2.0 micrometers in size.

The gap between 0.3 and 2.0 micrometers can be quite large once you realize just how many pollutants are smaller than 2.0 micrometers. Add to that the idea that an air purifier goes through tens, if not hundreds, of air changes every day, and the difference in air quality can become quite large over time.


How to Use This Information Practically

Before trying to answer how long an air purifier will take to clean the air, you’ll first have to answer a few questions. First, what do you want the air purifier to clean? If you have specific pollutants you want to eliminate, you’ll need to understand what kind of filters are necessary to achieve this. Next, you’ll have to calculate the volume of the room and find an air purifier that has the CADR rating to match the size of the room.

Once you have picked the right filters and CADR rating, the airflow exchange is the next thing to consider. The air purifier has to have the ability to quickly move all the air in the room through its filters.

Only after you’ve resolved these 3 factors can you realistically expect an air purifier to clean the air in a room in about 1 to 3 hours.¬†You could even notice the difference only after 20 minutes or so, depending on how powerful the air purifier is and what its air change rate is.

And remember, air purification is an ongoing process. Just because the air purifier was able to clean the air inside a room doesn’t mean that the process is complete. Keep your air purifier on 24 hours a day, even if it means it will run at the lower setting.

Letting the Room Air Out Naturally

Of course, you could just simply open all the windows to create a draft, letting the air be purified naturally. Assuming a decent outdoor air quality with no pollution, the rules are the same – 45 minutes to 3 hours. Remember, however, that while natural ventilation may help remove some stagnant pollutants from the surfaces in your home, you may be introducing some other pollutants like pollen and smog.