You don't get very far down the air purifier aisle before your head starts swimming with questions about how all of these units compare with each other. So far, various organizations have come up with new ratings to try to make it easier to compare one to the other. Each of the ratings seems to fall short in determining the actual air cleaning ability of the unit.
MERV (Minimum Efficiency Reporting Value) ratings apply to the efficiency of an air filter, while the CADR (Clean Air Delivery Rate) rating applies to a self-contained air purifier. The real information you should be concerned with is how much air is this machine capable of cleaning and how clean is it going to be when the machine or filter has done its job.
It's not the ratings that are lacking; it's the fact that there is no frame of reference. If a filter is MERV 10, is that better or worse than a MERV 9. Also, if it is better, it is difficult to tell how much better. You have the same situation with the CADR rating. Is the 4 rating better than the 5, and then, how much better?
What it all comes down to is; how much air is resulting and how clean is the air? In order to understand how clean the air is you have to know the particles you are trying to remove. For instance, pollen, dust and animal dander can be controlled with a system that is effective down to 1 micron (one micron is one millionth of a meter). If you want to clean the air from second hand smoke and bacteria, you have to find a system that is effective down to 0.3 microns. One fact that is not always reported is that the most harmful particles are those smaller than 1 micron.
In order to determine how much clean air is being delivered (this is what the CADR rating is attempting to accomplish), you must do a little math. The simple formula is:
CFM (cubic feet of air moved per minute) x 60 (minutes per hour), then divide by 6 (for number of air changes per hour), then divide by the height of the room (ex. 8ft), then calculate the square root. This will give you the size of room that the unit is capable of cleaning.
The formula looks complicated, but if you walk through it once or twice, it will start to make sense. Lets say you are looking at a unit that has a 500cfm fan. You also know that the height of your ceiling is 8ft. So you multiply 500 x 60 = 30,000. Now divide by 6 air changes per hour = 5,000. Then divide by your room height of 8ft = 625. Now find the square root = 25. The machine is capable of cleaning a room size of 25 x 25 x 8ft. This should give you a rough idea of what room you could put this unit in.
It is important that you include the number of air changes per hour. Each air-cleaning device should be rated for 4-8 air changes per hour to ensure that all of the air in the room is getting "scrubbed". When you can determine how much air is being cleaned and how clean that air is, you will know what the air purifier manufacturer don't want you to know.
I hope this information has been helpful in helping you understand a very simple technology that sounds much like magic.
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