Air sounds simple but it isn't really.
To find good air today is a sort of a problem
Rules are simple:
stay away from industrial or dusty areas and keep the air humidity around you between 50% and 60%
Air temperature in the area should drop down under 0 degrees C at least once a year (it destroys/washes out some bacteria and viruses)
Keep plastics and foams out and away of your house (softmakers (plasticizers) in plastics and other chemicals are pure poison) !
Ventilate your house/shelter fully at least once a day
Wet-clean your house at least once a week
Apply mechanical filtering through tissue or ceramics where possible to keep micro-particle quantities, suspended in the air, down.
With the above rules folowed, you should have an acceptable air quality in the house.
Some dust pre-filters are strongly suggested if proximity to big cities or other dusty areas, like deserts or construction sites, is unavoidable.
Below there are some simple and self-explaining steps to how to do an improvised face mask (dust-mask or wearable air-filter) from an old t-shirt
I kept the design as simple as possible so that even a 6 year old child would be able repeat the steps.
From the upper part of the long sleeve from the t-shirt a simple air filtering neck piece is easily done.