I find this extremely important, and sometimes overlooked.
Ideally you should get one with ozone function, as Dr. Wilson recommends. But it is possible you will find even a simple one with some HEPA filter will improve symptoms, sleep quality, so on. This may be because of allergies, which sometimes go undiagnosed.
If you are in Europe, and it’s hard to get the one recommended by Dr. Wilson, or too expensive, at least get a simple one, for starters. A simple one for example is Xiaomi MI Air Purifier.. whatever version.
Hope you’ll find this information useful.. and I’ll think of some more to add.