SAN DIEGO  Fire departments around the nation are cutting jobs, closing firehouses and increasingly resorting to “rolling brownouts” in which they shut different fire companies on different days as the economic downturn forces many cities and towns to make deep cuts that are slowing their responses to fires and other emergencies.

Philadelphia began rolling brownouts this month, joining cities from Baltimore to Sacramento that now shut some units every day. San Jose, Calif., laid off 49 firefighters last month. And Lawrence, Mass., north of Boston, has laid off firefighters and shut down half of its six firehouses, forcing the city to rely on help from neighboring departments each time a fire goes to a second alarm.

Fire chiefs and union officials alike say it is the first time they have seen such deep cuts in so many parts of the country. “I’ve never seen it so widespread,” said Harold A. Schaitberger, the general president of the International Association of Fire Fighters.

The risks of cutting fire service were driven home here last month when Bentley Do, a 2-year-old boy who was visiting relatives, somehow got his hands on a gum ball, put it in his mouth, started laughing and then began choking.