The answer to this question depends on your personal health situation and the environment in your home. Buy an air purifier if there is a nearby environmental factor, or if you have allergies or some other respiratory issue due to particles in your home. Purchasing an air purifier without an explicit need for one is not a worthwhile investment.
An air purifier must be actively maintained in order to be effective. Failure to replace filters on a timely basis can create a health hazard. Additionally, UV-based air purifiers can be actively dangerous, because they produce ozone, a gas that is harmful when taken into the lungs.
Place an air purifier next to a door, window, or hallway. These areas are natural places where air flows in and out of the room you are looking to purify. Since the purifier is located there, it becomes easier for it to do its job, essentially acting as a “gatekeeper” for air entering the room.
