Although we speak about ‘pimples’ in a generic sense, there are actually several different types of pimple. Some pimples are simply the result of bacteria invading the pores of the skin. But most pimples are formed as a result of a mixture of dead skin cells, oil, dirt and bacteria collecting in the pores. When this happens there are two possible outcomes.
Blackheads are pimples which form when the pore remains open, and oxygen is able to enter. The mixture of oil, dead cells and dirt then oxidises, the oil turns rancid, and the mixture turns black. Whereas whiteheads form when the pores are clogged and oxygen cannot enter. Here, pus gathers inside the pore, but as there is no oxygen it cannot oxidise, and does not turn black.
In both these cases, the contents of the pores are toxic, and lead to inflammation of the skin. This is the reason that pimples become painful and swollen. By popping the pimple and removing the contents, the skin is allowed to heal. However, care needs to be taken to ensure that this is done hygienically, with a proper pimple popper rather than bacteria laden hands.
There are other types of pimple, which cannot be popped. Red filled lumps, for example, will not respond to pimple popping, and you will damage the skin if you try. Allergy bumps also cannot be popped. But for blackheads and whiteheads, it is a safe strategy.