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Exposure to electromagnetic fields has been linked to a number of health problems, including cancer and immune system and reproductive abnormalities, and now the latest research adds another concern to the list: childhood asthma.

In the first study of its kind, scientists strapped magnetic field monitors on pregnant women to determine their level of exposure, and studied whether it was associated with the risk of asthma in their children. They found that children born to women with the highest levels of exposure to electromagnetic fields (EMF) — including from microwaves, hair dryers and power lines — had a more than three-fold higher rate of asthma compared to those whose moms had the lowest exposure.

The monitors specifically measured low-frequency magnetic fields, which often co-exist with electromagnetic energy, says lead author Dr. De-Kun Li, a reproductive and perinatal epidemiologist at Kaiser Permanente Division of Research.

They study builds on previous work Li and his group have done looking at the effect of EMF exposure on miscarriage. In that investigation, Li found that women with high levels of exposure had double the risk of miscarrying than women with lower levels of EMF exposure.

Li’s team then followed-up with the women who delivered and recorded the number of cases of asthma among their children 13 years later. The researchers focused on asthma because the recent, rapid increase in cases of the disorder suggests that an environmental trigger may be at play; there are genetic components to the immune system disorder as well, but genes simply don’t change that fast in populations.

Li’s team looked at the link between asthma and EMF levels because people are much more likely to be exposed to these fields now than ever before. “EMF is really increasing partially because electricity use is increasing, through electronic devices and wireless networks,” he says.

Li asked the women wearing the monitors to record where they spent their days. Their time could be divided into one of five place categories: home but not in bed, home and in bed, at work, traveling, or everywhere else. The women were not able to see their level of magnetic field exposure, so as not to bias their behavior.

Li found that exposure levels were about the same in all locations, but he attributes the most exposure to EMF to the use of common household appliances such as the microwave oven, vacuum cleaner and hair dryer. Pretty much anything that uses electricity, he says, including refrigerators, stoves, automobiles and power lines, generates low-frequency EMF.

What the devices were not designed to measure, however, was exposure to higher-frequency magnetic fields such as those emitted by cell phones and wireless networks, which are ubiquitous in homes and offices. These may be nearly impossible to avoid, but Li suggests that pregnant women can at least reduce some of their exposure to the low-frequency fields. Don’t stand in front of the microwave when it’s heating food, for example, and hold the hair dryer as far away from your belly as possible, or switch to a dryer that’s battery operated.

“Pregnancy is the most sensitive time for the fetus,” Li says. “Animal studies show that EMF can impact the immune system, and the latest research suggests that cells use magnetic fields to communicate with each other. If an external EMF comes into interfere with that, cell communication needed for normal development can be disrupted.”

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.