Open The Windows
Although it might sound a bit obvious, keeping the airflow fresh by opening the windows is the easiest and of course cheapest way to improve the air quality in our homes, even in winter. And while it might seem counter-intuitive to open the windows when the temperatures outside are dropping but it is crucial, even for a short time, to allow the oxygen in and that bad air out.
It will also help reduce the humidity in your rooms, something that dust mites thrive on. This becomes ever more necessary if you happen to be using chemical-heavy cleaning or decorating products.