New Orleans ? Some of the city’s most popular neighborhoods officially reopened to residents Friday, a move that could bring back about a third of New Orleans’ half-million inhabitants.

The newly opened areas, including the French Quarter and Garden District, all escaped major flooding when the city was devastated by Hurricane Katrina a month ago before receiving a second blow from Hurricane Rita last week.

Mayor Ray Nagin has pushed aggressively to reopen the city despite concerns raised by state and federal officials. Serious health hazards remain because of bacteria-laden floodwaters, a lack of drinkable water and a sewage system that still does not work, said Stephen Johnson, chief of the Environmental Protection Agency.