First off, @ alanstr0ke you can't set taxes higher then 20%. Second, I have used this trick to get some quick cash in the beginning of a city's life. Dont' touch the taxes, then zone your industry. Let DI fill the zoned area. Then set the DI tax to 20%. Th Dirty industry will not leave your city at all, unless you bulldoze it. I think if education is raised high enough then it will slowly leave but I have never seen dirty industry leave on it's own. Only a bulldozer can do it quickly.