The number of air exchanges is important when choosing an air purifier. You need to consider how many times the air is changed each hour. The higher this number is the better.

We use air changes to tell how many times the air in a room is cleaned. A purifier that provides 4 changes per hour, can clean all the air in a room 4 times in an hour.

To calculate this you need to know how big your room is and how powerful the air purifier's fan is. Most fans have a Cubic Feet per Minute listed. If it doesn't, you can use the CADR rating instead.

You can use the CADR ratings to calculate how many exchanges a certain purifier will give you. You cannot assume the CFM rating of your air purifier is the CADR rating. Be careful, some companies list the CFM as the CADR rating. An air purifier will only have a CADR rating if it has been tested by AHAM.

Once you know how big your room is and how poweful your fan is, you can figure out how many times that air purifier will change the air. For allergies and asthma, we recommend 5 to 6 exchanges. 4 exchanges would be the lower limit of what we recommend. Less than that and you are not really cleaning the air. More than 6 is better of course.

To figure exchanges follow these steps. We will use a sample room that is 10 feet by 12 feet and an air purifier rated at 140 Cubic Feet per Minute (**CFM**).

Volume of Room | |

Volume = Length X Width X Height | 10 foot X 12 foot X 8 foot = |

Fan Airflow Volume | |

_____ CFM or | 140 CFM |

Cubic Feet per Hour | |

_____ CFM X 60 | 140 CFM X 60 |

Air Exchanges | |

_____ CFH / Volume | 8400 CFH / 960 CF = |

You can also do the calculation backwards. You know how many air changes per hour you want and you know how big your room is. Using these two you can find out what the air flow of the fan has to be.

We can use our same 10 foot by 12 foot room. We know we want to exchange the air 6 times.

Volume of Room | |

Volume = Length X Width X Height | 10 foot X 12 foot X 8 foot = |

Desired Number of Air Changes per Hour | |

Exchanges | 6 air exchanges |

Needed Airflow of Fan | |

(Volume X Exchanges) / 60 | (960 * 6) / 60 = |

Using the second set of calculations will tell you how strong the fan needs to be. In our example, we need a fan that has an airflow rate of **96 CFM**. We can take this number with us when we go shopping. Look at each air purifier and see if it provides 96 CFM of airflow. If it does, you will know that this will work in your room.

Do this check for each air purifier. You *will* find that the recommended room size is not always the same as what you calculated. Often the recommended room size will only provide you with 2 air changes per hour.

One popular air purifier is advertised as cleaning a 12 foot by 18 foot room **2** an hour. (That's 2 air exchanges an hour.) Using the second set of calculations we can find the CFM of this air purifier.

Volume of the room is 12 * 18 * 8 = 1728 cubic feet

CFM = (1728 * 2) / 60 or

**57.6 CFM**

This air purifier's fan is rated at 58 CFM. What size room should we use this in to provide 6 exchanges an hour? How do we find this out? That's right. We use the other calculations.

First we take 58 CFM * 60. That's 3480 CFH.

Divide this by the number of exchanges. 3480 / 6 = 580 CF

Divide cubic feet by 8 to get square feet. 580 / 8 =

**72.5 SF**

This air purifier will clean a space 72 square feet 6 times per hour. That's a room about 8 foot by 9 foot. Even a traditional college dorm room is bigger than that.

Do you really want an air purifier like *this*?

Air exchanges are very important. The more air changes per hour an air purifier provides the better it will clean the air. We recommend at least 6 air exchanges per hour. More than this is OK. We will accept as low as 4 exchanges sometimes. But lower than 4 and YOU really are wasting your money.

All these calculations are done assuming a square room with no furniture or other obstructions in it. Once there is furniture or walls in the room the air flow is affected. The number of air exchanges will decrease. To be on the safe side, pick the air purifier with the higher air exchanges or use it in a smaller room.

Follow the advice here and you will pick the right air purifier. You *will* find one that will really clean your room.

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