 ACH stands for Air Changes per Hour, and it is arguably the most important and least understood factor in air purification.
An ACH rating tells us how many times an air purifier filters all of the air in a given room during one hour. An ACH rating of four, for example, means
that the air purifier cleans the room's full volume of air four times per hour.
The effectiveness of an air purifier is directly related to its ACH rating. The more air changes per hour, the more effective the filter is.
But before you can calculate the ACH, you must know the Clean Air Delivery Rate (CADR) for the air purifier in question.
Clean Air Delivery Rate & AHAM
 Some air purifiers have CADR ratings that have been certified by the Association of Home Appliance Manufacturers (AHAM). Such air purifiers display CADR
seals that look like this:
Note that an air purifier's CADR may be different for different particles. For instance, according to AHAM, the CADR for
the
Blueair 403 air purifier is 250 for dust, 240 for tobacco smoke, and 255 for
pollen. The recommended room size is 365 square feet.
It's important to know that AHAM works under the assumption that all rooms have ceiling heights of eight feet. Also, all of AHAM's room size
recommendations are based on the CADR for tobacco smoke.
The Math Behind ACH
An air purifier's ACH is calculated by dividing a room's volume by the air
purifiers' CADR, then dividing that result into 60.
In the Blueair 402 example above, if we assume that the 365 square-foot room has a ceiling height of eight feet, then we can calculate the air
purifier's ACH for tobacco smoke:
365 square feet x 8 feet = 2,920 cubic feet;
2,920 cubic feet / 240 cubic feet per minute = 12 minutes;
60 minutes / 12 minutes = 5 ACH.
So, when it comes to tobacco smoke, the Blueair 402 air purifier provides five air changes per hour in a 365 square foot room (assuming that the
ceilings are 8 feet high). Under normal circumstances, this ACH rating would be sufficient for someone who has allergies or asthma.
ACH Recommendation for Allergy & Asthma Sufferers
We recommend a minimum of four air changes per hour for allergy sufferers, and all room size recommendations
at AchooAllergy.com are based on an ACH rating of four.
A minimum ACH rating of four is also recommended by ASHRAE (The
American Society of Heating, Refrigerating and Air-Conditioning Engineers) for patient rooms in hospitals. ASHRAE recommends a minimum ACH of six for intensive care
units and a minimum ACH of 25 for operating rooms.
How to Recognize Marketing Ploys
Many retailers list the recommended room size based on an ACH of two, and their customers often end up with air purifiers which cannot effectively remove allergens
from the air in the room.
Other retailers list the recommended room size based on an ACH of six, and their customers often end up paying more money for a large air purifier when a smaller
model would have worked just fine.
All room size recommendations at
AchooAllergy.com are based on an ACH rating of four. We want to make it easy for you to
choose the right air purifier for your home.
 A Word about Ozone
Unfortunately, many stores and catalogs offer air cleaners that emit ozone, which is a dangerous lung irritant that can cause asthma attacks. If the word "ionic" is
used to describe an air cleaner, then it most likely produces ozone. To learn more about ozone, see Ozone: A Powerful
Lung Irritant.
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