Paul, I've read the Factsheet you kindly provided, and as I understand it, the following two facts emerge:-
1) Long-term exposure to values above 0.3-0.4 microteslas cause a twofold increase in childhood leukaemias.
2) The level underneath powerlines is generally 20 microteslas.
Thus, living under or very near a high-tension line would surely be rather unwise???