More than half of the nation's junior high school teachers have worked more than 20 hours overtime per week — a level that could cause serious health problems, a government survey showed Friday.

Some 57.7 percent of junior high school teachers and 33.5 percent of elementary school teachers said their weekly labor exceeded 60 hours, including over 20 hours of overtime, the survey by the education ministry showed.

More than 80 hours of overtime a month over is considered the threshold for increased risk of karoshi, or death from overwork.