Portable chargers and power banks might be useful additions to your carry-on luggage, but they’re also a potential fire risk. Now Southwest Airlines is tightening rules around their use on board aircraft.

Citing safety concerns, Southwest Airlines will no longer allow passengers to charge devices with a power bank stowed inside a carry-on bag—going further than current FAA guidance.

Southwest says it will limit battery charging to devices that are easily visible, in case chargers catch fire.