The smoke and grit in the air in New York City that came from the collapse of the World Trade Center towers pose only a small health risk, U.S. EPA officials and some doctors said yesterday. They equated the danger to a smoggy day in the city, when people with already-weakened immune systems should be especially cautious. Other independent health experts were gloomier about the health implications of the Tuesday tragedy, expressing concerns about the mixture of soot and toxic chemicals that was created as the planes exploded and the buildings fell down.