A comedo, commonly called a blackhead, occurs when a pore becomes clogged with sebum, dirt and old skin cells. When the passageway is clogged, the sebum cannot come out and it forms a stopper. When the sebum reacts with the oxygen in the air, a so-called oxidation occurs and the melanin in the sebum becomes black (open comedo). A black blackhead is thus not dirt, but oxidised sebum in the opening of the pore. If you squeeze around the opening (which one should preferably avoid!) The sebum that comes out may look like a small worm, hence the name. A sebum stopper deeper into the skin, a white blackhead, is covered by the top layer of skin. These are easily confused with acne, small lumps in the skin that consist of keratin.
Blackheads are often very stubborn and it takes patience and consistency to treat them. Some products can suppress sebum production and act as astringents. Exfoliation is an important part of the treatment. BHA (salicylic acid) is a fat-soluble acid that effectively dissolves the sebum and old skin cells in the pores. BHA also has an anti-inflammatory effect, which soothes irritated skin. Ingredients that build up the skin from inside and stimulate collagen production have indirect effects on the pores. When collagen and elastin are strengthened, the skin becomes firmer and more resilient, resulting in an astringent effect.