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High tension wire
There are now close to 100 larger and smaller epidemiologic studies that address the long-term exposure issue. In brief, there is a suggestion which has appeared in many (but not all) studies of children living near power lines that such exposure can approximately double the risk of leukemia and/or brain tumors. Evidence is less clear for adults, but suggests the possibility of similar risks. Risks of breast cancer, depression, and other negative health effects are based on much more limited evidence and are even more speculative. There is enough information to have some concern, but not enough to set exposure standards. There is also a lack of clear evidence from laboratory evidence from laboratory experiments to indicate what protective measures (if any) would be appropriate or to demonstrate to everyone's satisfaction a mechanism which could explain such an effect of powerline fields.
The question of a "safe distance" is, at least in my opinion, a complete unknown. The epidemiologic studies indicated above used a "cut off point" of 2 or 3 mG (average magnetic field level) to define people as "exposed," but you should realize that this is somewhat arbitrary level. If you set the cut off point too low, the exposed group is not really much different from the group you are comparing them to, and if you set it too high you will never find enough people in the exposed group to allow statistical analysis. These experiments cannot really give an indication of safe levels: you would need a very clear effect, show a sharp change in effect over a small difference in exposure. Given the current state knowledge, I don't think anyone can indicate safe or unsafe levels, only magnetic field levels which are unusually high compared to what most of us are exposed to. Many people adopt a "prudent avoidance" position where they reduce personal exposure levels to 1 or 2 mG if it doesn't cost a lot to do so. In doing this you are not establishing "safe" conditions (because we don't really know what is safe), but are establishing conditions which do not expose you to an unusually high magnetic field.
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