i once was very concerned about dust. everyday must take out lens from dry cabi and inspect for dust. but then after i went out to shoot almost everyday during dec for jobs i realised dust is part of the fun. lol.
Dust have diameters less then 500 micrometre. So how it gets in? y not? lenses are not air tight..... and even it is air tight, there will still be dust! present in the thing but jus didn't colate to let u know its presence