What Is Air Changes Per Hour? (ACH)

Arguably the most misunderstood metric in an air cleaner. If you notice the label ACH or air change per hour in an air purifier box or specification, you must be wondering what the heck is it? In this post, we will explain what ACH is, why it is important, and how to measure it? A must-read for those new to the air purifier world. Let's begin.
Short answer
ACH = * Clean air in an hour.


What Is ACH Rating?

ACH is the abbreviation for air changes per hour shows how many times an air purifier can filter the air in a room hourly. For example, an ACH rating of 2 indicates an air purifier releases 2 times clean air in an hour or every 30 minutes. Likewise, ACH rating of 4 indicates an air purifier releases 4 times clean air in an hour or every 15 minutes. The more air changes per hour, the more times the air purifier will filter the air; the faster indoor air can be cleaned.

What Is The Best ACH For Hyperallergic And Asthma Sufferers?

An allergic or asthmatic person requires an air purifier with at least 4 air changes per hour. That is the industrial standard and recommendation by The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) for patient rooms in hospitals. ASHRAE further recommends a minimum of 6 ACH for intensive care rooms and a minimum of 25 ACH for operating rooms.

The more clean air exchanges, the fewer pollutants in the air that will trigger your allergies. Most ACH ratings for air purifiers are between 1 to 6. For general air cleaning purposes, 2 ACH ratings are sufficient for those without breathing issues or respiratory disorders.

What Is Air Changes Per Hour Important?

Air changes per hour are important because it creates a standardization on measuring an air purifier’s air cleaning coverage. You can accurately compare different air purifiers’ performance based on the exact ACH rates. In other words, apple to apple comparison. ACH will also help you determine the correct size air purifier based on your needs.

Here’s where the confusion started. Most manufacturers published their air purifiers’ suggested room size based on 2 ACH; it is not a consensus decision. Some manufacturers would publish their air purifiers’ suggested room size based on 1 ACH ratings to boost the air coverage without any disclaimer. The lack of transparency led many people to purchase the wrong unit because they were unaware or confused about the ACH baseline. For example, if you are an asthmatic person that requires 4 ACH in a 200 sq. ft. room, you will need an air purifier with a suggested room size of 400 sq. ft. purifying coverage. Likewise, you might end up paying more money for an unnecessarily large air purifier because the recommended room size published by the manufacturer is measured at 5 ACH.

Until there is a regulation on ACH usage, buyers will have to pay extra attention to the fine details when purchasing an air purifier.

How To Calculate With ACH?

It’s not hard to get the room size based on ACH with a little bit of math. Before we begin, you will need to know the air purifier’s cubic feet per minute (CFM). Look it up on the specification sheet, product manual, or the product’s website.

ACH is calculated assuming the air purifier runs at max speed with the highest volume of moving air. Multiply the cfm by 60 (minutes) to get the total volume of moving air an air purifier can clean in one hour. Divide the number by 8 (average ceiling height) to get the total square footage. Lastly, divide the number of ACH you need to reflect the room size an air purifier can cover.

Let’s use Coway Airmega 300 with 260 cfm as an example:

260 cfm x 60 minutes = 15,600
15,600 / 8 feet (ceiling height) = 1,950
1,950 / 4 ACH = 487.5 sq. ft. room coverage.

Air changes per hour are connected to the purifying coverage. The higher the ACH, the smaller the room size an air purifier can cover. Using the above example, Coway Airmega 300 can cover a wider 975 square feet of room size with 2 ACH. To get the maximum ACH of the air purifier, just switch the final equation with the room coverage.

ACH Limitation With Ozone

As crucial as ACH is, it should not be the only factor in looking at an air purifier. ACH will not discount any “fake” air purifier that produces ozone. Ozone is a toxic, colorless gas that can cause many respiratory issues, lung scarring, and aggravate asthma attacks. There are still ozone generators in the market that mask themselves as ionizers or air cleaners to confuse the buyer. Besides looking at the ACH, you should look at the types of filtration used, and keywords such as “no ozone”, “ozone-free”, or “CARB approved”. An air purifier listed on the California Air Resource Board (CARB) board passes the stringent requirement of low ozone emissions limit therefore deemed safe for home use.

Final Thoughts

By now, we hope this post will give you a better understanding of why air changes per hour (ACH) is an important metric to measure and compare the air purifier effectiveness. Always look at the ACH ratings when shopping for an air purifier. If you have more questions regarding air purifiers and their many technical jargons, feel free to explore our other helpful posts.




Max Fernandez

A loving father and a dedicated reviewer for airfuji.com with more than 1000 air purifiers under his belt. Max Fernandez is also one of the million patients currently suffering from asthma. Feel free to nudge him if you have any questions.
How Can I Help You Today?

I need air purifier that is to deal with for to be placed in the for with