I agree with Ruthi. You can use either preposition here. It is certainly possible to say 'harmful for your health.' If you say 'Smoking is harmful for you health' you are treating the word 'harmful' like any other negative adjective : 'it's bad for your health', 'it's dangerous for your health', and so on. When we say 'Smoking is harmful to your health', we are treating the word 'harmful' as the adjectival form of the noun 'harm'. The noun 'harm' collocates with the preposition 'to', for example 'Smoking does harm to your health'. This is why you can - if you wish - say 'harmful to xx'. There is no difference in meaning between the two sentences.

Look here: http://www.oxfordlearnersdictionaries.com/definition/english/harmful?q=harmful 'harmful' is used only with preposition 'to' with 'good' it's different: he's good to me = he is kind to me it's good for me = it benefits me

I'm a native U.S. English speaker). I think either is fine, with no obvious difference in meaning. When they first began putting warnings on cigarette packages in the United States, the wording was "Caution: Cigarette Smoking May be Hazardous to Your Health." It made big headlines at the time, and the wording of the message became a sort of familiar saying. Later versions of the warning also used "to," "dangerous to your health," "serious risks to your health." The warning has a kind of status as a catchphrase or saying, and people make joking references to it, like "Warning, high fees in retirement plans are hazardous to your wealth." It's possible that these warnings have made the phrasing "dangerous to your health" a familiar pattern that "rolls off the tongue," particularly in the context of smoking.

Thanks,Ruthi

http://www.thefreedictionary.com/harmful