The U.S. government is banning bulk shipments of highly flammable lithium-based batteries on passenger airlines.

Congress in October required the Department of Transportation to mandate the rules as part of broader legislation setting aviation policy. It doesn’t apply to passengers bringing electronics as carry-ons.

The new restriction, announced Wednesday, will have little immediate effect because most U.S. carriers have been adhering to the guidelines, which were recommended by the United Nations International Civil Aviation Organization in 2016.

Years of research on how lithium-based batteries burn have raised concerns about their safety. A 2016 study by the Federal Aviation Administration found that bulk shipments of lithium-ion rechargeable batteries could smolder and explode. More recent research found that a single battery in a checked bag could overpower an airliner’s fire suppression system.

Three cargo aircraft accidents, including two fatal crashes, have been linked to lithium-based batteries that caught fire, according to investigators in the U.S. and other nations. Cargo carriers, which can still carry the batteries, have taken steps in recent years to improve packaging and encourage manufacturers to hew to higher standards.

The Pipeline and Hazardous Materials Safety Administration, which works with the FAA on regulating transportation of batteries, issued the rules.