What is the difference between teeth whitening at dental clinics versus aesthetic spas as the latter tends to be much cheaper?

Dental professionals and dental clinics are the only entities licensed to perform teeth whitening by the Ministry of Health in Singapore.

It is illegal for non dental professionals to dispense whitening gels and perform whitening treatments.

I have written an extensive answer on why having a non dentally trained person perform your teeth whitening is not a good idea.