Where There's Smoke...

We received a notice today that the street department will be conducting sanitary sewer smoke testing next week. Say what? Apparently the best way to identify leaks in the sewer lines is to back fill it with smoke.
Under normal circumstances, if your plumbing is working properly, the smoke used to conduct this test should not enter your house.
They go on to ease fears by declaring the smoke to be "harmless to pets, food, and material goods" but fail to mention what to do if your toilet starts smoking other than opening a window.
Has anyone else ever heard of such a thing? It makes sense but just imagine the poor guy who comes home to find smoke billowing from the washing machine!