Most of the purifiers come with an indicator light which alerts you if it’s time to change the filters. Unfortunately, not all the purifiers comes with such convenient feature. If you have such device in your house, you need to check the manual to find out the recommended time of effectiveness for the filters. If that time has passed, you will have to change the filters. The time of filter change varies between three months to a year. So, never ever leave your doors or windows open. Besides the above issue it will also put immense pressure on the purifier.