As listed in the certification section of this site, there are a number of national certifying boards, spread across Australia, Canada, the European Union, Japan, New Zealand, & the United States.
There are national administrations like OSHA that enforce health and safety legislation as established by national agencies like NIOSH in America and CCOHS in Canada.
All of these organizations act under their Country's respective labour departments
(US & Canada)
What all of this signifies, is that crucial findings of even one ergonomist (or usability professional in countries where laws legally require websites to be accessible) can trickle up to their country's labour department and become established as occupational law.
These organizations (both certifying & informative) also connect provincial and state organizations with one another. For example, in Canada, ergonomists in the Quebec-based association that advocates workplace safety in social affairs (ASSTSAS english ) could be connected to ergonomists in the Ontario-based association advocating safety in community & healthcare (OSACH) via the national Association of Canadian Ergonomists (ACE).