dust and such particles in the atmosphere do not give the effect of twinkling to starlight. due to refraction of light at different layers of atmosphere(as the earth get hotter during day time the layer of air near it is very hot. it gradually decreases as we move up), the stars or planets appear at some above postion from the actual.
the light coming frrom stars are much less than the amount of light coming from the planets as they are nearer.
as the atmospheric conditions keep on changing continuously, the temperature as well as density of different layers keep changing.so the apparent position of the planets or star is not stationary. this gives that flickering effect in stars.(as the amount of light coming from the stars is very less so that shows twinkling effect.