India, firstly you should realize that it is virtually impossible to "prove" a negative proposition. However there are now numerous studies that have shown some of what you are asking for. Try the ASA site and others for access to this info. Just keep in mind that you cannot ever proove a negative and if you are in a discussion with soemone who feels that you must first prove the "absolutely no harm" theory you are breaking some of the most important words to live by.

Never argue with a fool:
1) Those watching can't tell who is who.
2) They will only drag you down to their level and then beat you with experience.

regards,
Max Blast