Cellphones, Wi-Fi computers, cordless telephones and other communication devices send and receive signals through microwaves, a part of the electromagnetic spectrum that's less powerful than infrared light but more powerful than radio waves.
The electrons zipping back and forth in a power line -- or in any wire, for that matter -- create a magnetic field.
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Such fields also surround any appliance that uses a lot of power, including ovens, electric blankets and hair dryers.
Here's a closer look at some common sources of electromagnetic fields:
Fortunately, cell phones don't produce enough microwaves to actually cook your brain. But how many waves are you getting? The amount of radiation that enters the body during talk mode is called the specific absorption rate, or SAR. According to a recent report from the Environmental Working Group, a public health advocacy organization, the SAR for current models ranges from 0.15 to 0.35 watts per kilogram (the Samsung Impression) on the low end to 1.56 w/kg (the Palm Pixi) on the high end. The Federal Communications Commission considers any SAR under 1.6 w/kg to be safe because it's not enough to actually heat tissue.
Towers emit microwave radiation, but the strength of the field drops dramatically with distance. According to the FCC, a person would likely have to climb the tower and stand directly in front of an antenna to receive a potentially harmful dose of radiation. At ground level, the field is generally thousands of times weaker than the FCC's safety limits.
Like cellphones, Wi-Fi hot spots use microwaves to carry signals. But even in a coffee shop or airplane terminal full of laptops, the signal isn't especially strong. A 2007 survey of 55 hot spots in four countries found that the EMF exposures were at least 250 times weaker than established safety standards.
Many appliances that use a lot of electricity -- including microwave ovens and hair dryers -- create significant magnetic fields when turned on. Because we get close to these appliances, the field we encounter could easily be 100 times stronger than anything coming from a distant power line. The magnetic field one inch away from an electric stove or microwave oven could be as high as 200 microTeslas, the units by which magnetic fields are measured, but it drops to less than 10 microteslas just a foot away. But because they're only on briefly, appliances likely account for a trivial part of a person's day-to-day exposure to electromagnetic fields.
The current running through power lines creates a magnetic field that reaches anyone living nearby. But like all magnetic fields, the power of the field plummets the farther away you get from the source.
The official safety limit for power line fields is 100 microteslas. By comparison, a large power line 100 feet from a house might create a field of about 0.4 microteslas. The strength of Earth's natural magnetic field ranges from about 30 to 60 microteslas.
For more information about EMFs, their sources and their possible health effects, see the World Health Organization's report at www.who.int/peh-emf/about/WhatisEMF/en/.