Starting Tuesday, January 23, 2007, Canadian, Mexican and Bermudan air travelers, as well as U.S. citizens flying home from those countries or the Caribbean, must display their passports to enter the United States. The only valid substitutes for a passport will be a NEXUS Air card, used by some American and Canadian frequent fliers; identification as a U.S. Coast Guard merchant mariner; and the green card carried by legal permanent residents. Active members of the U.S. military are exempt. For now, the rules affect only air travelers. Land and sea travelers will not have to show passports until at least January 2008. Air travelers who cannot produce a passport will be interviewed by customs agents, who will decide whether to let them into the country.