Physician assistants are an increasingly critical part of the health care workforce, which in many states isn’t keeping pace with a growing and aging population. By 2028, the nation as a whole will be short some 100,000 critical healthcare workers — doctors, nurses and home health aides — according to a new report from Mercer, a management consulting firm. The looming shortage is one reason why 13 states have joined the PA Licensure Compact, a multistate agreement that allows PAs to practice in any participating state, without having to get an additional license.