Extremely low frequency electric Electromagnetic fields (ELF-EMF) are those generated by household electric power lines at 60 Hz. They have been linked to childhood leukemia, but there is still no conclusive evidence on the effects.

At Electro Magnetic Field (EMF) - Hazardous to Our Health? at mercola.com, a health products company, they say:
"Can electromagnetic fields (EMF) from power lines, home wiring, airport and military radar, substations, transformers, computers and appliances (e.g. hair dryers) cause brain tumors, leukemia, birth defects, miscarriages, chronic fatigue, headaches, cataracts, heart problems, stress. nausea, chest pain, forgetfulness, cancer and other health problems?

Numerous studies have produced contradictory results, yet some experts are convinced that the threat is real. "

"Electric blankets create a magnetic field that penetrates about 6-7 inches into the body. Thus it is not surprising that an epidemiological study has linked electric blankets with miscarriages and childhood leukemia."

It's best to turn on the blanket to warm up your bed then turn if off before you get into bed.

Note: There are now low EMF electric blankets on the market.

The Electric Blanket Institute says:
"The United States government became involved and studied over 500 peer reviewed papers written on this potential problem. The National Research Council spent around 65 million dollars and finally stated that there was no conclusive evidence that residential EMF's play any role in the development of cancer, in reproductive issues or in learning/behavioral issues. Nevertheless Sunbeam, which was then the only major United States manufacturer of electric blankets, altered the design of its products to reduce EMF's. Redesigned blankets produced after 1992 had much weaker electrical fields than their predecessors. The argument over EMF's has never gone away completely although very few scientists and medical experts continue to believe that they can cause problems."

A 2003 study at Stony Brook University, documented at the NIH, concluded there was no relation between electric blanket use and breast cancer. At NIEHS report on the Health Effects from Exposure to Power-Line Frequency Electric and Magnetic Fields they say:
"A small, non-significant association between use of electric blankets and the risk for breast cancer was observed in one, large U.S. study, but not in another."

EMF Readings From Various Devices We Use Every Day at NaturalNews.com
Many experts recommend avoiding levels of 2.0 milligauss or higher.

Source	Distance	milligauss
Background		0.5 to 2.0
Electric toothbrush		<2mG
Hair dryer, 1200 watts	6 in f/ nozzle	5-10mG
Hair dryer handle	contact	>70mG
Personal hair trimmer (1 AA battery)	contact cutting tip	10-20mG
Refrigerator	1 ft	3-5mG
Microwave oven	1 ft	40-50 mG
TV, 21 inch, CRT	2 ft	4-5mG.
Laptop computer LCD	6 in	<2mG.
Electric blanket, (99.98% EMF Cut)	contact	<2mG
Electric blanket cord		>70mG
Electric clock radio	6 in	<2mG
Sunlamp, 800 watt	6 in	5mG
Electric power distribution lines	10 ft	3-4mG
Electric power transmission line 250 kV *	100 ft	7mG
Electric power transmission line 500 kV	100 ft	12.6mG
Electric power transmission line 500 kV	200 ft	3.2mG
Cell Phone	contact	3 to >70mG

* Power transmission line data from niehs.nih.gov/health/docs/emf-02.pdf

An article ELECTROMAGNETIC-POLLUTION at Health and Spirality says:

"Long-term exposure may aggravate any existing health problems or diseases and may cause or intensify especially lack of energy or fatigue, irritability, aggression, hyperactivity, sleep disorders and emotional instability. Increasing numbers of individuals are becoming hypersensitive to EMR."

It has no scientific references except a link to www.emrsafety.8m.net/ which wasn't useful.

Cordless and mobile telephones and wireless local area networks (WLANs/Wi-Fi) have resulted in increased exposure to RF energy.
To date (2006), the only health effect from RF fields identified in scientific reviews has been related to an increase in body temperature (> 1 °C) from exposure at very high field intensity found only in certain industrial facilities.

Base stations, those large mobil phone towers, have been a concern to some. Recent surveys have shown that the RF exposures from base stations range from 0.002% to 2% of the levels of international exposure guidelines. This is lower or comparable to RF exposures from radio or television broadcast transmitters.
In fact, due to their lower frequency, at similar RF exposure levels, the body absorbs up to five times more of the signal from FM radio and television than from base stations. This is because the frequencies used in FM radio (around 100 MHz) and in TV broadcasting (around 300 to 400 MHz) are lower than those employed in mobile telephony (900 MHz and 1800 MHz) and because a person's height (1/2-1 wavelength for VHF) makes the body an efficient receiving antenna.
Wi-FI is even higher frequency (2.5 GHz).
Source: Electromagnetic fields and public health at the World Health Organization (WHO)

Links:
Electric & Magnetic Fields at the National Institute of Environmental Health Sciences.
EMF Safe Appliances - Low EMF Products at Electromagnetic Health Hazard Control, Inc
Are electric Blankets harmful ?? - discussion of physiological effects.
Gauss Electrosensor EMF Detector $30
Electromagnetic Fields at HealthGoods.com
Electromagnetic fields and public health at the World Health Organization (WHO)
Electromagnetic Spectrum