I thought that the high voltage lines were only really a concern in built up areas, and not directly from the electric fields. From the pollution in the air, getting too close the fields, ionising (or something like that) and reforming into carcinogenic stuff. So higher density of cancer causing air particles around city based high power lines.
But I was reading about this 20 years ago, and haven't heard anything in recent times, so maybe just a bit of theory that didn't play out in the real world?