What do you mean by "dirty?" It there actual dirt? Or do you just mean that your skin looks darker there? I don't know what your ethnic background is, but I do know that for some people (mostly men) of Asian backgrounds, they have hyper-pigmentation in certain areas (e.g. armpits, elbow crease, back of knees, groin area, and buttocks). This is genetic and nothing can really be done. There are skin bleaching creams that exist, but I do not know about the safety of these creams. I hope that this was helpful. Don't worry about it, I am sure that you care about it a lot more than anyone else would. No one has perfectly even pigmentation of their skin.