Power Frequency Magnetic Fields in Computer Classes in Schools

Exposure to power frequency (50 Hz) magnetic fields (MFs), and especially children`s exposure, is of major public concern in Israel. The objective of this study was to measure the MFs in computer classes in schools. Measurements were performed near 222 computers in 3 schools (12 classes, 12-28 computers in each class). The MFs were measured using an EMDEX II meter (detection limit 0.1mG), at four measurement points (MPs) near each computer: legs, torso, head, and hands (few centimeters above the keyboard). Three operation conditions (OCs) were defined: computers and monitors are disconnected from the mains (i.e. background field), connected to the mains (yet turned off), and turned on. Measurements were conducted while all computers in the class were set to the same OC, yielding 12 measurements for each computer, and a total of over 2,500 measurements. For each computer and OC we calculated the whole body average (WBA, i.e. the average of the 4 MPs), and the differences between each MP and the corresponding WBA. For each class we calculated the medians, the averages and the STD of all these values.

It was found that background levels can be divided into two groups: in 9 classes, the WBA MFs were negligible (averages and medians <0.25mG, STD<0.2mG), and in three classes (#1, #5, #12) they were higher, but yet very small (averages and medians in the range 0.7-1.2mG, probably due to "net currents"); in two of these classes (#1, #12), the average background diminished to 0.1-0.2mG when the lights were turned off. For comparison, we found in a previous study that the average 24-hour exposure of adolescents in Israel was 0.73±0.67mG, and their average exposure at school was 0.5±0.52mG, quiet similar to the situation in other countries. In 11 classes, the average of the differences between the measured MPs and their corresponding calculated WBA was ≤0.1mG (STD<0.2mG), indicating very homogenous fields. Only in one class (#1), the average of the differences between the head exposure and the WBA was +0.1mG, while the average legs - WBA difference was -0.3mG.

In 10 classes, both connecting the computers to the mains and turning them on resulted in negligible changes in the medians of the WBA (≤0.1mG), and the fields remained homogenous (averages and medians of the differences between all MPs and the WBA ≤0.1mG). In one class (#12), connecting the computers to the mains doubled the median WBA from 0.68mG to 1.35mG, while the medians at the legs and the hands increased by 1.4mG and 0.5mG respectively. These increases resulted from AC adaptors located in a drawer behind the monitors. In another class (#5), turning on the computers increased the median WBA slightly (+0.18mG), mainly due to an unexplained increase at the hands and the legs.