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A while ago, I was hiking near the Hollywood Sign in Los Angeles. When I got to the sign, I found out it was fenced off; with several signs and a security camera promising prompt enforcement. As I was pondering the likelihood it was a bluff, I ran into this sign.

Sign

Radio waves aren't ionizing like X-rays or Gamma-rays. So assuming an arbitrarily large dose, in what manner do the radio waves interact with the human-body?

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Just to be clear this warning sign isn't for the Hollywood Sign. It just happens that the closest spot to the sign is an access road for a large radio tower. –  David Jun 5 '11 at 19:49
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up vote 7 down vote accepted

They heat it, by different degrees depending on the polarization of molecules in the tissues and liquids. The molecules try to re-align after the radio-wave field and the movement dissipates as general heat. Think microwaves.. a consumer-grade microwave oven operates at the same radio wave spectrum as your home WiFi network (2.4 GHz) but much stronger.

The SAR, Specific Absorption Rate measures the amount of RF-energy deposited in the body in a well specified way (in W/kg) from a radio product. Cellphones have to be rated with this to be sold. I don't think the Hollywood Sign is rated with a SAR though... :)

Actually, the SAR for cellphones is measured using a dummy human head with similar dielectric properties as a real human head. This can pose problems when engineers try to bring these dummies as carry-on luggage through airport security screenings, a friend who works at a cellphone company mentioned ;)

There have been reports now and then on RF-waves doing more complicated, specific things with tissues like opening the blood-brain barrier etc. As far as I know none of these have been conclusively proved (in a human model) but this area of course features incredible pressure from health organizations, governments and huge industrial companies.

There usually is an "old story" from people in the military service where they heat water-bottles by hanging them in front of their mobile radar station.. I don't know if that is true in particular, but it can't be very good to hang out in front of strong omnidirectional radio-emitters even for no other reason than the tissue-heating :)

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The reason that Raytheon named their first microwave oven the "Radarange" was that the inventor, their employee Percy Spencer, had a chocolate bar melt in his pocket in 1945 when servicing an active radar set. –  Bill Slugg Jun 5 '11 at 13:14
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And because chocolate contains almost no water, it is much less (almost not) affected by microwaves, The tissue in his limb would have been heated much more! Last not least chocolate bars were wrapped in aluminium foil (or tin foil). The story is not good. (Or lets say, its good for laymen) –  Georg Jun 6 '11 at 12:04
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The SAR as defined in in the Wiki link is not a measure at frequencies of 2 and more GHz! The heating by microwaves is not dependent on conductivity. It is the relaxation of orientational polarisation which works in this domain, nothing else. BTW electrolytic conductivity ceases at some MHz already, because ions are too heavy to follow such rapid changing fields. This fact and the skin effect make all ideas on cancer and radio waves speculation. –  Georg Jun 6 '11 at 12:18
    
Funny, I once had a chocolate bar melt in my pocket while driving. I guess cars are dangerous for human health. ;) –  Emilio Pisanty Mar 15 '13 at 0:08
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