 Miners used to take canaries with them in coal mines. The canaries were sensitive to methane gas. And this is kind of morbid, but if the canary died, it means it was present of gas and it was a heads up to the miners to get the heck out of there before it killed them. Hence the term canary in a coal mine. Early warning system is what that means. Long before it becomes lethal for human beings, this is a heads up.