Some of the dead squamous cells from the outer stratum corneum fall down into the areas around the hair follicles and plug the sebaceous glands causing acne in selected individuals. Dead cells and Sebum clog one blocked exit, without any getting out. This can turn into what the scientists call a lesion . The non-inflamed lesions don't have any redness to them. They can stick out from your pores and are very noticeable.
Some people think they're filled with dirt.
But really, the content of the clogged pore can appear black as a result of dark pigment or oxidized oil , not because of dirt. What we all know as pimples are really inflamed lesions. This is caused by the bacteria that live in a healthy follicle, and can break down when the pore's been plugged. Little irritants found inside the pore get everything worked up and inflamed. Pimples pop up for a bunch of reasons.