Can I use a BMX helmet for commuting?

You can, but it's not ideal. BMX helmets are heavier, hotter, and lack the visibility features (reflective elements, light mounts) that make commuter helmets safer for road use in traffic. They also lack aerodynamic optimization, which matters more at the sustained speeds of road commuting. If you ride BMX to work occasionally, your BMX helmet is better than no helmet. But for regular commuting, a dedicated commuter or road helmet will be more comfortable, visible, and appropriate for the riding environment.