The best place to get the correct answer to how close power lines can be to your deck or even your house might be found at your local building department.
It sounds however like this is a low level line coming into your house from the power line poles off your property. If so, there probably isn't much you can do about altering that inexpensively.
But I do not know what the health issues are about line kind of power line. If there is an electrician reading this perhaps they might add a comment or two to help out.
I won an inflatable 6 person inter hot tub. I have a large deck. 12x 20 on both sides of the house. With a header to the house and the deck was built 6
I'm putting a swim spa into an inground concrete bunker and will build a deck around it. I have been advised to keep the deck joists a minimum of 6 inches