OSAKA -- Middle-aged men working 11 hours or more a day are 1.63 times more likely to have a heart attack than those on the job for 7 to 9 hours, according to research by a team at Osaka University and other institutions.
The team began tracking some 15,000 men aged 40 to 59 years old in 1993. The subjects hailed from Mito, Ibaraki Prefecture; Nagaoka, Niigata Prefecture; Kami, Kochi Prefecture; Shinkamigoto, Nagasaki Prefecture; and Miyakojima, Okinawa Prefecture.
The team considered those working at least 7 hours but less than 9 hours per day as the standard group, and compared them with three other groups: those working less than 7 hours, those working at least 9 hours but less than 11 hours, and those working for 11 hours or more. They then examined whether spending long days on the job was associated with increased risk of heart attack or stroke.
This was apparently the first ever large-scale longitudinal study conducted in Japan examining the relation between long working hours and the risk of developing the cardiovascular diseases. A total of 212 men had heart attacks, and 745 men had strokes during the approximately 20-year study.
The ratio of participants who had heart attacks and were working at least 11 hours per day was 1.63 times that of those who worked at least 7 hours but less than 9 hours a day, after taking into account the effects of smoking, drinking, diabetes, and other possible causes. Moreover, the ratio among employed workers who had heart attacks or strokes was 2.11 times that of those working at least 7 hours but less than 9 hours, while there was little gap among the self-employed. In the other two groups, there was no major difference in heart attack risk, and their working hours showed no connection with strokes.
The government standards on work-related illnesses and accidents recognize 80 hours of overtime per month as the "karoshi" overwork death risk line. "Our study, with such a large sample size, supports how long working hours impact our bodies," said Hiroyasu Iso, a professor at Osaka University's Graduate School of Medicine.
The research results were published on March 6, in the Japanese Circulation Society's international online Circulation Journal.
(Japanese original by Ryo Watanabe, Osaka Science & Environment News Department)