6 Things You Need To Know When Buying An Air Purifier [STUDY]

Many people have asked us our views on which purifiers are most effective. So we decided to use the Airveda monitor to do a small study on 3 popular Air Purifiers:

Purifiers in study


  • What levels of filtration are needed? : Given high PM2.5 concentrations in India, it is essential to buy a purifier with a HEPA filter which protects you against very small PM2.5 particles. You also need a pre-filter which catches the bigger particles preventing the HEPA from getting clogged quickly. Additional levels of filtration are nice to have.
  • Which specification of the purifier is most important? : The Association of Home Appliance Manufacturers (AHAM) developed the CADR, otherwise known as the “Clean Air Delivery Rate”. Based on our study, CADR is the best metric to determine effectiveness of a purifier. If you are trying to pick between a few purifiers of same cost, go with the one with the higher CADR.
  • How does CADR translate into room size?: AHAM recommends that filters should be used in rooms whose volumes, expressed in cubic feet are equal or smaller than the filter’s CADR rating multiplied by 12. Assuming height of the room to be 8ft:
    • For a 818 sq ft room, CADR should be at least 545 (818*8/12).
    • For a 680 sq ft room, CADR should be at least 453 (680*8/12).
    • For a 450 sq ft room, CADR should be atleast 300 (450*8/12).
    • For a 200 sq ft room, CADR should be at least 134 (200*8/12).
  • Do AHAM prescribed CADR work well for India?: The prescribed values by AHAM are absolute minimum. In our study we saw that, in room-size 450sq ft, with a purifier with CADR 300, it took an hour to get the room from PM2.5 of 300ug/m3 to under 30ug/m3. AHAM prescribes these numbers for less polluted spaces than what we have here in India. We recommend getting a purifier with CADR at least 50-100 above the minimum prescribed value per the AHAM formula above.
  • What CADR will work for my room space?: The effectiveness of a purifier depends on the size of the room and the leakage in the room. For rooms with greater size, or high leakage, you will need purifier with higher CADR or multiple purifiers. The best way to determine effectiveness is to measure levels in the room with an air quality monitor after the purifier has been running for at least 30 minutes. Also note that the specified CADR is always based on purifier running at highest speed. If you run your purifiers on 'auto' or at lower speeds then you will not see the same level of effectiveness.
  • How accurate are the inbuilt air quality indicators on the purifiers?: We found that both Sharp and Honeywell indicators was completely unreliable - the colours did not match the levels of pollution at all. Phillips has a sensor which also shows PM 2.5 number but we noticed that the numbers and even the trends were way off mark. Ask your dealer to send a sales person with an independent monitor to ensure that the purifier is effective in your house or better yet buy an air quality monitor.


  • Room-size: 450 sqft. This was the lowest claimed by the three purifiers.
  • Leakage: Room had no windows and a well sealed heavy door, so leakage was low.
  • Purifier condition: All purifiers were new with original company filters.
    • Data: Each purifier tested individually in the same room size 450sqft, by creating pollution levels using Dhoop and Agarbatti up to PM 2.5 of 999 and then turned on the purifier, letting pollution levels drop.


a) Below is the amount of time taken by the three purifiers to clean the room:

Purifiers in study

PM2.5 trends with the three purifiers.

Details in tabular form:

Purifiers in study

b) Time taken to clean the room was directly proportional to the purifiers’ CADR.

The Association of Home Appliance Manufacturers (AHAM) developed the CADR, otherwise known as the “Clean Air Delivery Rate”. This helps you identify how quick and efficient your potential air cleaner is.

Purifiers in study

The 2.7 simply means that the AHAM tested in a cleaner environment. But more importantly, the time taken to clean the room is directly proportional to the CADR in an almost exact way. This implies that CADR prescribed by these purifiers is quite accurate and is by far the best metric to determine how effective a purifier will be.

We hope this study will help you in your next purifier purchase. Do provide feedback on this post, as well as other studies you would like to see in the comments below. If you found this study useful then do share on your timeline.

Disclaimer: This post is not an endorsement from AIRVEDA for any of the brands. Our insights are based solely on this study and we do not make any claims about the performance of these or any other purifiers outside this study.

About the author

Airveda team is a group of people helping everyone breathe well and live well.