29 C.F.R. § 1912.29
Any person appointed by the Assistant Secretary to an advisory committee has a right to be present at any duly called meeting. If any person representing the interests of employers, employees, or the States is unable to be present at a duly called meeting, he may notify the Assistant Secretary or his designee, and request that another member of the Committee representing the same interests be permitted to vote in his place on any matters coming before the advisory committee in the particular meeting. The request may be oral or in writing, and shall be accompanied by a statement of reasons for the anticipated absence. The Assistant Secretary or his designee shall grant the request whenever he is convinced that the reasons for absence are valid and that number of requested proxies for any particular meeting will not be so numerous as to impede materially the deliberations of the advisory committee.
29 C.F.R. §1912.29