Godzilla vs. Hedorah (1971)

Coming to Earth on a meteorite that falls to Earth, a microscopic alien life form later named Hedorah feeds on Earth's pollution and eventually grows into a gigantic, ever-evolving, poisonous smog-spewing and acidic sludge-secreting monster. Godzilla, Earth's defender, senses the threat and meets ''the Smog Monster'' head-on in a literal battle for Earth's survival.