This is an old (from 2010) article, but still fantastic. Static electricity still happens, yo. https://www.dcrainmaker.com/2010/04/troubleshooting-your-heart-rate.html
Please note that Bluetooth is now the standard communication between device and strap so electrical interference is practically nil...with one exception.
Note the photo below:
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This is a chest strap. It reads the electric signals from your heart. The part circled in red is the conductive part that needs to be moistened before your workout begins, then pressed directly into your skin. You probably know this already. What I need to make clear is that NOTHING that carries electric impulses should be near this part of the chest strap while you are working out.
Takeaway: If you listen to music while you run, please keep the cords from your headphones as far from this area as possible. If the cord is next to either side of the monitor, you could get a bad reading. You will know this is happening when you see spikes up to the 200s followed by massive dips to baseline as the wrist device starts rejecting the readings. Consider an arm band for your iphone.