Environmental monitoring of static magnetic fields around operating high-voltage direct-current transmission facilities in Japan.

Static magnetic fields are present around high-voltage direct-current transmission facilities, but on-land environmental measurement data under actual operating conditions remain limited. This study measured three-axis magnetic flux densities at 1.0 m above ground at 38 accessible locations around all four operating direct-current transmission links in Japan, including overhead-line and underground-cable sections. Resultant magnetic flux densities ranged from 39.7 to 154.8 μT, while measured local geomagnetic-field levels ranged from 46.6 to 50.6 μT. The maximum value was observed above the Anan-Kihoku Link underground-cable section and was far below the International Commission on Non-Ionizing Radiation Protection (ICNIRP) reference level for general public exposure to static magnetic fields. Lateral-profile measurements showed that values near the lines or cable routes diminished to local geomagnetic-field levels within several tens of metres. Geomagnetic-field-aware calculations were generally consistent with representative measured profiles, although underground-cable comparisons were sensitive to cable-centreline uncertainty. These data support static magnetic-field exposure assessment around operating direct-current transmission facilities.