Check for exhaust leaks.

obvious answer! but what about pollen filters etc?

Pollen filter (Activated Charcoal Type) can not filter all exhaust smell, so replacing it won't fix the actual problem.

What engine is in your car?

Usual problems / smells are oil smells... due to oil leaking from the Valve Cover Gasket & the Chain Tensioner Gasket, that then drips onto
the exhaust manifolds & catalytic converter.... the smell then makes its way into the car via the fresh air intake / pollen filter.

Exhaust flex pipes are common leak areas.

Thomas