Here are some articles for you to check out. I think it is a matter of your comfort level. Another thing to consider is that enough people prefer not to live near power lines and that could make a property in close proximity to power lines less desirable in the marketplace when compared to a similar home not in proximity to power lines.
The first link describes the difference between high voltage transmission lines and lower voltage distribution lines.
Regardless, I'd still have someone come out to take some measurements if only for peace of mind. Maybe your realtor can include that as part of the home inspection clause (???). The tool used to measure EMF is called a gaussmeter and the inspector must be able to make readings as low as 0.5 mg (milligauss) to provide an accurate picture.
I'm not an expert, just a parent who after the birth of my daughter became very concerned about a line running behind my home (30ft). I purchased a gaussmeter (online approx. $150) and I discovered that things like sitting with a computer on my lap, having the baby phone on my nightstand, driving my car exposed me to much higher levels of EMF than the power line. That said my husband & I are lucky that although the line behind our home is part of the main distrubtion system for our community we are near the end of the line so the measurements were very low. The numbers I measured in other parts of my community were far higher ...by the way, you'll need to decide for yourself what 'acceptable' is since this is also subject to extreme debate. I settled on
Hope this helps.
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