Airlines must put redesigned oxygen systems back in aircraft lavatories, one year after they were ordered removed on concerns they could be used by terrorists as weapons, the U.S. Federal Aviation Administration said.

Airlines have 37 months to install new oxygen supplies, according to a directive published today on the U.S. Federal Register's website. The rule doesn't specify the new standards.

Oxygen supplies, required in the event an aircraft loses cabin pressure at high altitude, last year were deemed a hazard in lavatories because passengers could tamper with them, according to a statement issued last year by the FAA. The pure oxygen could be used to start a fire, according to the statement.

The FAA, working with the Transportation Security Administration and the FBI, ordered the oxygen systems disabled in lavatories on March 8, 2011. It was working with industry to find a tamper-proof oxygen system, according to last year's statement.

Most oxygen systems are cans containing chemicals that, when activated, cause a reaction producing pure oxygen.

The costs to the airline industry of installing the new systems is estimated to be US$44.2 million, the FAA said. It cost airlines US$935,000 to disable the old devices. The rule applies to about 5,500 aircraft in the U.S., according to the directive.