Pregnant mothers' exposure to strong electromagnetic fields may increase the risk of developing asthma for their baby according a new study conducted by US researchers.

Many studies in the past have shown that long exposure to electromagnetic fields can be harmful and dangerous to human health. Children whose mothers had a fair amount of exposure had a 74 percent higher risk, compared with a group that had less contact to certain electronic devices, according to research in the Archives of Pediatrics and Adolescent Medicine published today.

The study is the first to find a link between exposure to magnetic fields during pregnancy and asthma in children, lead study author De-Kun Li said. Li is a senior research scientist as well as reproductive and perinatal epidemiologist at the Kaiser Permanente in Oakland, California.

Li designed a so-called prospective study in which 801 pregnant women wore monitors that measured their exposure to magnetic fields for 24 hours.

These monitors measured their exposure to low-frequency magnetic fields from electronics such as microwave ovens, vacuum cleaners, hair dryers, coffee grinders, fans, fluorescent lights and electric blankets that are used at average household. However, it did not measure the exposure to higher frequency electromagnetic fields produced by cellphones or cellphone towers.

The researchers then followed the children from these women for 13 years using electronic medical records to see which ones developed asthma. After all, 130 children, or 20.8 percent of study participants, developed asthma and majority were diagnosed by 5 years old.

The author said he is not sure how exposure to these man-made magnetic fields during pregnancy can cause asthma in children but he said there are several possibilities.

Magnetic fields may affect the immune system and response of the unborn child or they could hinder cell communication while the fetus is developing. Asthma is the most common chronic condition in U.S. children, affecting about 13 percent of those younger than age of 18, or 9.4 million U.S. children.

Li said the result of the study offers a strong reason for studying how magnetic fields may affect humans. "This really needs to be studied," Li said.

However, he also mentioned that his discoveries should be replicated by other researchers.

He gave a tip for reducing exposure to electromagnetic fields.

"The problem with EMF is that you can't see, smell it, you can't touch it," he said. "The great thing about EMF is that distance really helps,"

"When you turn the microwave on, don't stand right next to it. Try to, when you use a hair dryer, try to use it far away from your tummy as much as you can."

Overall, about 13 percent of U.S. people younger than age of 19, or 9.4 million U.S children have asthma, according to the study, while prevalence has risen 74 percent from 1980 to 1996. 

The study was published online in the Archives of Pediatrics and Adolescent Medicine.