The government is giving airlines more than three years to restore emergency oxygen to aircraft lavatories, although safety advocates say that's too long to be without the potentially life-saving supplies.

The Federal Aviation Administration is ordering chemical oxygen generators be installed, in a directive to be published in the Federal Register today -- a year after ordering airlines to take them out because they could be used to start a fire.

The FAA said at the time that the decision to remove them was made in conjunction with the Transportation Security Administration and the FBI as a precautionary measure unrelated to a specific terrorist threat.

The agency said then it would take two to four years for aircraft manufacturers to develop a secure oxygen system in lavatories.

No designs for such a system have yet been approved, though manufacturers and aircraft modification experts "are working on acceptable designs," the FAA says in today's directive.

Safety advocates and flight attendants say passengers, attendants and pilots using a lavatory could be seriously harmed or killed if there is an aircraft decompression and emergency oxygen is needed immediately at a high altitude.

An analysis of FAA service-difficulty data by USA TODAY and former airline mechanic John King last year showed that airlines reported deployment of emergency oxygen masks on 105 flights from 2001 through October 2011, including 37 flights from 2008 through 2010.

The FAA says rapid decompression events on commercial planes "are extremely rare." An FAA advisory committee researching high-altitude flights found 2,800 "instances" in a 40-year period -- an average of 70 a year -- in which supplemental oxygen was needed on commercial and general aviation flights.

Passenger-rights advocate Kate Hanni says "it's about time" the FAA ordered emergency oxygen systems back into lavatories. She says studies show that at an altitude of 40,000 feet, passengers can lose consciousness without adequate oxygen in 15 to 20 seconds.

"We are relieved that the airlines will be correcting the failure to protect the public while using a lavatory at 33,000 feet but are left wondering why the FAA did not order oxygen replaced sooner," Hanni says.

In its directive, the FAA says it will not require airlines to provide a temporary emergency oxygen supply in lavatories until a secure oxygen system is installed.