My Answer (I toke notes from websites and put them all together in my own words):
Dental assistants receive their formal education through academic programs at community colleges, vocational schools, technical institutes, universities or dental schools. Graduates of these programs usually receive certificates. Although the majority of academic dental assisting programs take nine to eleven months to complete, some schools offer training, part-time education programs or training via distance education. Lastly, Dental assistants can become certified by passing an examination that evaluates their knowledge.
Dental assistants are eligible to take the CDA examination if they have completed a dental assisting program accredited by the Commission on Dental Accreditation. People who have been trained on the job or have graduated from non-accredited programs are eligible to take the national certification examination after they have completed two years of full-time work experience as dental assistants. Some states also recognize passage of components of the CDA examination, such as the Radiation Health and Safety examination, or the Infection Control examination, for licensing and regulatory purposes.