Electromagnetic fields are emitted by power lines, fluorescent lights, microwaves and other household appliances including hair dryers.
Researchers at Kaiser Permanente in Oakland followed more than 600 pregnant women and had them wear a meter for one day that measured their average exposure. Children whose mothers had the highest exposure were three-and-a-half times more likely to develop asthma.
Critics say measuring exposure for only one day is not long enough to establish such a link and many factors influence asthma risk.
