Leaving an air purifier on night or day will leave the air in your room cleaner, but will require frequent filter changes. The air purifier filter will become less effective as the filter becomes more dirty. If you plan to run an air purifier all the time, it is important to check the filter often. 
If you plan to run your air purifier for an extended period of time, it is also important to make sure the purifier does not over heat. Although this has never happened in our experience when testing purifiers, there is always a risk of overheating when running any machine for a long period of time.
How Long Can You Run An Air Purifier?
An air purifier should run as long as you would like, but a dirty filter will eventually limit it’s ability to clean the air.
In order to test if it is ok to run an air purifier all the time, we selected 3 different air purifiers and ran them for a week straight. The results were as expected. All 3 of the air purifiers ran extremely well for the entire time, but after a week of using the purifier all night and all day, the HEPA filters were already getting dirty.
Although the filters were getting dirty, the time it takes for the filters to become dirty also depends on how clean that air is in your home. If the air in your home is exceptionally clean, then these air purifiers can run for extended periods of time before requiring a filter change. On the contrary, if the air in your home is extremely dirty, then it will not take long
How Long Should you Run an Air Purifier For the Best Results?
The best time to use an air purifier is while you are present in the same room. In order to get the best results out of your purifier and to limit the number of filter changes, the air purifier should be turned off when leaving the room.
One feature that people often forget about air purifiers is their ability to sense the room air quality. Many of today’s air purifiers will monitor the quality of air in your house and turn on and off as needed. Using this feature is the best way to not only conserve the filter but also produce effective results.
Also worried about pet allergies? Learn how to reduce the effects of pet allergies. And check out air purifiers to help.
Frank is the chief editor and director at Specialty Air, where he oversees testing, research, and editing for all air quality-related articles. Frank is an expert in the air quality space with extensive experience testing, researching, and reviewing air purifiers, air conditioners, air filters, air fresheners, fans, and more. He also has over 4 years of experience with issues surrounding mold remediation, radon mitigation, moisture management, air duct systems, HVAC, and insulation.