Dust in lens! :(.
Dust on or near the rear element is the important thing. Not a problem towards the front.
I wonder of these lenses are dust suckers - might be worth it to put a uv filter over the front and use the lens hood.
Zoom lenses are capable of sucking in air and dust while zooming. The lens extends and sucks in air and dust.
Primes can suck in dust during focussing when the lens extends during focussing.
An filter in front of the lens will not help, air is sucked inby every (small) gap in the lens.
Most dust in the lens will not effect the IQ. When it does, let a profession remove the dust, lenses are very hard to assemble right! A badly assembled lens WILL have big infuence on the IQ!