When did breathing clean air become so complicated? HEPA this, ACH that. Shopping for an air purifier reads like Dr. Seuss’ attempts at German. There are lots of fancy features on lots of great air purifiers, but before you get fancy, you need to know the two most common pieces of jargon in the industry.

hepa vs not

HEPA stands for “high efficiency particulate air.” “This type of air filter can theoretically remove at least 99.97 percent of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns,” says the EPA.

Look for HEPA when you’re buying an air purifier. Don’t be fooled by marketing materials that say HEPA-like. Get the real deal, because it’ll capture more of the nasty stuff. That’s the whole point of buying an air purifier, right?

check the ach to find square footage

When an air purifier cleans all the air in one room, it’s called an air change. You’ll see this written on air purifier marketing materials as air changes per hour (ACH). The CDC says to aim for 5 ACH, and the EPA endorses it.

The problem is, air purifier companies make it hard to find this information out. “ACH” isn’t household terminology, so when a company (more often than not) says that it’s suitable for rooms up to 1,860 square feet, it looks impressive. Wow! Just look at that! It must be sooo powerful!

Then you do some digging and find out that that statistic only applies for a single air change per hour. Take that square footage number, if the fine print does say that it’s for 1 ACH or one air change per hour, and divide it by 5 to get the size of the room that the air purifier can handle while providing 5 ACH.

While it’s not difficult to do the math to figure out 5 ACH from the 1 ACH figure (just divide the given square footage by five), lots of air purifier companies, even the most renowned and credible ones, only give the 1 ACH figure because it looks better when they’re trying to sell their products.

These are the two most important aspects to look into when you’re buying an air purifier. All the automatic air quality detection and app controls are secondary to buying one with the right performance and suitable for the size of the room in which you’re putting it.