Several other studies found increased risk in HAM operators.
And of course the smoke inhalation from sending smoke signals.
The original wireless.
If the raw material for the smoke signal was cannabis bushes - would that qualify as a High frequency transmission?
Most likely correlation not causation. Amateur radio operators use a wide range of equipment and frequencies, so without know all factors such research means nothing. Many hobbyists solder the own electronics or have jobs in engineering, so there could be any number of other factors at play.