If air, water, or land is polluted, it is dirty in a dangerous way — not a bit dusty, but unsafe to breathe, drink, or grow food in. We say a polluted river, polluted air, a polluted beach. The dirt usually comes from people: cars, factories, farms using too much chemical spray. It is the -ed form of pollute, used as a describing word, so it tells you the state of the place, not who did it. News, science class, and everyday talk all use it this way. Though actually, people stretch it a little: “the city feels polluted” can just mean the air is heavy and grey, not that a scientist has measured it. It still points at the environment, not at a messy desk.