At a green tinge on the drip test, the water can irritate the fish's lungs. This causes them distress, which lowers their ability to fight off infection. This combination makes them more prone to illness just like us. Infection can set in and they'll be unable to fight it off, which weakens them further until they succumb and die.
This is what is meant by technically any level is dangerous. Please do right by the lives you've accepted responsibility for, give them a safe place to live by making sure their water is clean and dechlorinated, even if that means daily water changes until the drip test shows bright yellow after five or so minutes left to develop.
They'll do their healthy fishy thing for you in return, which is fantastic!