Home air improving gadgets?

Do you use anything to improve air quality in the home, particularly through the winter when it's too cold to have windows open,  humidifiers,(or are they dehumidifiers?)  those things you hang on radiators with water in, salt lamps or anything else? Air conditioning systems are not common in homes in the UK, but if anyone does have it does it really help to keep the air good?

Hello Bluebird there really is no substitute for fresh air, even if it is just a brief blast when it's cold.