system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] Why is dust one of the most common allergies? Isn't dust one of the most common things in the world?
gpt
People are usually allergic to *specific kinds of* dust. For example, the kind that is made of human skin flakes. Or they are allergic to the tiny creatures called *dust mites* that live in some dust.

A current theory is that people are more likely to develop an allergy when the environment they grow up in is too clean.