Cancer Risk for Night Shift Workers Questioned

COLUMBUS, Ohio, Aug. 23 -- The night shift is not so risky after all, according to epidemiologists who studied more than 3.2 million Swedish workers.

Their finding of no link between cancer and the night shift contradicted a range of previous reports that associated working after dark with an increased the risk of breast cancer in women and possibly an increased risk of prostate cancer in men and colon cancer in women.

But among the Swedish workers, followed for nearly 20 years, the only signal was a possible marginal increase in the risk of thyroid cancer, Judith Schwartzbaum, Ph.D., of Ohio State University, and colleagues here and in Sweden, will report in the October issue of the Scandinavian Journal of Work, Environment and Health.

Many of the previous studies included "very specific worker populations" -- such as airline flight attendants -- Dr. Schwartzbaum said.

"But airline workers differ from other shift workers due to their increased exposure to cosmic and solar radiation," she said, "so it's tough to tease out what exactly may contribute to their elevated risk of cancer."

She and colleagues looked at the health and work records of all Swedes who worked at least 20 hours a week in 1970, and who were included in both the 1960 and 1970 population censuses.

All told, the study included 2.1 million men and 1.1 million women.

The researchers categorized occupations -- obtained from the census records -- according to the proportion of people who reported shift work: more than 70% of the time, more than 40%, less than 30%, and none.

The study participants were followed from 1971 to 1989 or until their developed cancer or died, Dr. Schwartzbaum and colleagues said. During the study period, 46,438 workers were interviewed about their exposure to the night shift and other factors.

Over the study period, 201,508 cancer cases occurred among male workers and 99,263 among women. But analysis showed no elevation of the risk for all sites combined or for specific cancers, such as breast and prostate, among night shift workers.

Specifically:

• Among male shift workers, there were 6,524 cancers but the standardized incidence ratio (SIR) was 1.02, with a 95% confidence interval from 1.00 to 1.05.
• There were 1,319 prostate cancers and the SIR was 1.04, with a 95% confidence interval from 0.99 to 1.10.
• Among female shift workers, there were 268 cancers all told, and an SIR of 1.00, with a 95% confidence interval from 0.89 to 1.13.
• There were 70 cases of breast cancer among female shift workers and an SIR of 0.94, with a 95% confidence interval from 0.74 to 1.18.

On the other hand, there was an apparent elevation of the risk of thyroid cancer among men, with 49 cases among shift workers, compared with 36.2 expected, leading to an SIR of 1.35, with a 95% confidence interval from 1.02 to 1.79.

The theoretical basis for thinking that night shift work might increase the risk of cancer is that melatonin -- which inhibits tumor growth -- is suppressed by night time exposure to light, Dr. Schwartzbaum said.

"However, the effects of melatonin on cancer development in humans are not well understood," she said.

The researchers said that their finding should not be considered conclusive, because the null results may have arisen from the method of calculating exposure to night work. Workers were classified as "exposed" to the night shift if a large proportion of people in their industry worked night shifts.

Also, they said, although previous studies may have flaws, "the collective picture they provide is too consistent to be dismissed" easily. The researchers called for more and larger international studies.

"It seems like three million workers ought to be enough to get a firm idea of the risk, but it isn't, especially considering the relatively low percentage of jobs that require shift work," Dr. Schwartzbaum said. "We need studies that include data from multiple countries."