If a power line is 300 yards separated from a house and there are many
trees 50 yards from the power line that block any view of the power
line from the house, the question is: can the trees block the magnetic
fields
created by the power line, so that the house is protected?

In the comment sorwin-ga says:

" I think this is a serious subject which requires much more research..."

    In reality, sufficient amount of research was done to demonstrate
that there is no health hazard from the power lines (short of stepping
on a wire which fell down)

The arguments, both empirical and theoretical are reviewed here:
 
http://answers.google.com/answers/threadview?id=770816

It may be an interesting topic of research in study of rumormongering 
and follies of popular press.

Hedgie

---

Clarification of Answer by hedgie-ga on 11 Dec 2006 19:19 PST

To addres your specific question, lets look at this FAQ #14:

14) How close do you have to be to a power line to be considered exposed?

  'Since no human health hazards from residential exposure to
power-frequency fields have been proven to exist, it is impossible to
rationally define a safe distance or safe exposure level.."

"
If there is a human health hazard from residential exposure to
power-frequency fields it is highly unlikely to depend on anything as
simple as the distance of the residence from the nearest powerline.
Depending of the type of line and its current, magnetic fields from
power lines become less than those produced by the typical residence
at a distance of 20-70 meters (see figure below)."

http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html#14

The search string:  'power line' cancer

 brings many reports of arguments, both pro and con.


Here is summary article, from highly respected journal, on psychology
of this rumor:

Power Lines, Cancer, and Fear
Over the past 18 years, there has been considerable interest in the
possible link between electromagnetic fields and cancer, especially
leukemia. The story of this highly publicized research has been marked
by mystery, contradiction, and confusion. When something as ubiquitous
and misunderstood as extremely-low-frequency electromagnetic fields is
accused of causing cancer in children, people's reactions may be
driven more by passion than by reason.

. [Full Text of this Article]

http://content.nejm.org/cgi/content/extract/337/1/44
Hedgie

Protected from what?

You are surrounded by magnetic fields. Everything with a motor has
one, from your fridge and vacuum to your hair dryer and computer
(fans).

The Earth has a whomping huge one, thankfully. Without it we would
likely not exist.

You can develop a magnetic cage around your home if you are concerned.
Using steel or other magnetic metal, build a cage around the house or
within the walls. Fill the spaces between the steel units with a
magnetic metal mesh. The gauss lines will follow the steel. With minor
modification, this structure/design can protect you from the
electro-magnetic pulse (EMP) of a nuclear blast AND can be set up to
be Tempest approved (an obsolete military shielding standard).

Look, stop smoking, eat right, exercise, see your doctor and you'll be fine.

Magnetic fields travel through trees like they do through air.

Dear McLodio

Whatever sinful cynics like Mark and Stan may say ...

If you donned a pointed hat made of tinfoil (like me) you would obtain
total, year-long, low-cost protection.

In case you wondered, I sleep standing up.

I know a leading UK researcher (no not a GAR) who has studied overhead
power lines and their effect on natural radiation.

His work demonstrates that overhead power lines magnify natural
radiation and that this can adversely affect fauna and flora
immediately within the vicinity of the power lines.

This suggests to me that, being 300 yards away from the power lines,
you should be well outside their area of influence, trees or no.

The magnetic field produced by these power lines drops off rapdily
with distance.  By applying a simplified version of Amperes law, it is
possible to calculate that even with a whopping current of 1000 Amps
(not likely!) travelling through the cables, the magnetic field at
300m would be 0.00000066 Tesla.  For comparisons sake, the magnetic
field of the Earth is around 0.00005 Tesla (although it does vary). 
This is 75 times stronger than that picked up from the cable.

In summary: I really wouldn't worry about it :)

Bryan, is that why power lines are in what appear to be
well-cultivated strips of land? I foolishly thought it was employees
mowing the right of way.

As for tinfoil hats, my "tinfoil" is aluminum. Is that still effective?

A few years ago the British monthly publication "Electronics and
Wireless World" featured prominently an article entitled "The Killing
Fields".

I recall it being said that a statistical corollation exists (in the
UK) between incidences of cancer and proximity to the national grid
(power distribution system at tens/hundreds of thousands of volts).

The link is only statistical, there is no scientific proof of
cause-and-effect.  But the suggestion was made that the cancers might
be caused, not by electrostatic or magnetic fields, but rather by
ozone gas produced by ionization.

I think this is a serious subject which requires much more research. 
The magazine may be found on the Internet and repints of the article
might be available.