Source Player Core pg. 458
Effects with the metal trait conjure or manipulate metal. Those that manipulate metal have no effect in an area without metal. Creatures with this trait consist primarily of metal or have a connection to magical metal.

These planes consist of chaotic and shifting structures and oceans of metal. Metal planes tend to exist in a state of change and decay, leaving plentiful pockets of air for visitors to breathe or move within. Creatures unlucky enough to be entombed in the plane’s substance risk suffocation if they can’t phase through metal. Wood creatures find the lack of stability and soil on a metal plane disconcerting and often fail to thrive in such environments.