Air purifiers perform at their most efficient when all the doors and windows remain shut. By doing so, the air purifier has the ability to clean the air in the space with no disturbance from corridors or other spaces. As air moves everywhere that have an aperture, the air cleaner is going to look to suck in air from outdoors unless the doors get completely locked.
Of course, this might sound a bit counterintuitive, considering that people commonly open doors and windows to allow fresh air in. Yet, due to the fact, tiny debris like dust and pollen can be challenging to see, pretend it's snowing outdoors. Your air cleaner will be working hard to remove snow from your room, though with the windows open, it will keep putting more snow into the room, resulting in a coating of snow that will never disappear.
Summertime, when the heat can reach the triple digits, you may want to put the air cleaner next to a wide-open window to catch a few particles once those reach the room. However, make sure you look for the safest spot to prevent any harm to your air cleaner buddy - your air cleaner hates spending time in ultra-hot environments just like you do.