It depends on whether they belong to an international union association. If you happen to be an ironworker, you'd belong to the International Association of Bridge, Structural, Ornamental and Reinforcing Iron Workers, and they have scope right across the country. Even down in the States, when certain states might run short of iron workers, for instance, they'll call up the unions in Canada and ask them to send their people down.
On April 16th, 2008. See this statement in context.