Room Air Filter for Allergies

Allergist suggests a hepa air filter for kid’s room, but I am dubious. Are in-room air filters worthless if the window is open? If the door of the room is open to the rest of the house (it’s a little kid’s room – we never close it)? We already have cental a/c with its own filter, does a separate room air filter add anything when the a/c is on?

Thanks.

By UWS2 |