According to the Occupational Safety and Health Administration, all aerial platform operators must be trained according to OSHA work safety standards. However, no certification is required. The standards must merely be met.
According to OSHA, aerial lifts may be modified for purposes other than their original function. However, such modifications must be certified in writing by the aerial lift manufacturer or by a national testing laboratory to have met provisions outlined by the American National Standards Institute (ANSI).
While certification is not required, obtaining certification that is approved by national or international standards organizations (e.g. the Canadian Standards Association) is often recommended for safety purposes and to prove to potential employers that you are able to operate aerial lifts properly.