Studies link shift work to cancer risk, increased injuries

Shift workers have a greater chance of being injured at work than those on working on a regular day shift, according to several studies compiled by Toronto-based research firm the Institute for Work and Health (IWH).

According to a recent Issue Briefing published by the IWH, the risk of accidents and injuries are higher for afternoon and night shift workers, compared to daytime or morning shift workers. Issue Briefing is a regular publication by IWH that summarizes research findings on various topics.

“For at least 50 years, researchers have been exploring the question of whether working shifts poses a health hazard,” the IWH wrote.

In 2005, 25.5 per cent of full-time workers aged 19 to 64 worked shifts. Shift workers comprise about 45 per cent of health care workers, 66 per cent of those in protective services (police, security guards), 40 per cent of those in sales and services, and 42 per cent of those in occupations unique to primary industries such as agriculture, forestry and mining.

For night shift workers, the risk of injury or accidents rises about 20 per cent from the first to the second hour of the night shift, but then falls steadily throughout the shift, except between 3 a.m. and 4 a.m., the IWH said.

Incident rates also increase during successive night shifts. “On average, the incident rate on the fourth night is 36 per cent higher than on the first night.” On the other hand, the increase in incident rates is significantly smaller for successive morning or day shifts. Studies also concluded that the risk increases significantly after more than eight hours of working.

Health effects
Numerous studies also explored the adverse effects of shift work to the workers’ health.  One theory that has emerged, according to IWH, was that working nights increases the risk of cancer due to disrupted melatonin levels, which affect tumor growth.

“Melatonin secretion is normally at its peak at night, but production of this hormone is reduced as a result of light exposure during night hours,” IWH said, indicated previous experimental studies on rodents support a link between melatonin and tumor suppression, particularly breast tumors.

In 2003, researchers Schernhammer and Hankinson reviewed studies that link light exposure at night and breast cancer among shift workers. For instance, a study conducted in Finland reported significantly higher incidence of breast cancer among female flight attendants who have been working in the airline industry for 15 years or more. Similar results were found among female flight attendants in Iceland. In Norway, a study of female radio and telegraph operators with potential exposure to light at night also found a higher risk of breast cancer.

These studies are pointing to light exposure at night, as a possible culprit for breast cancer. The IWH cautioned, however, that these studies contained only a few controls for other possible causes of breast cancer.

In a 2006 study, Shernhammer’s team studied breast cancer incidence among nurses, and found that those who work in rotating night shifts for more than 20 years had a 79 per cent higher risk of developing breast cancer, than nurses who did not work nights.

Earlier in 2003, Shernhammer’s team found evidence of increased risk of colorectal cancer among female nurses who had been on rotating night shifts for more than 15 years, the IWH briefing said.

It’s not only among female workers that shift work seems to be having an adverse effect on. A 2006 study of among male workers in Japan revealed that those with rotating shifts had a significantly higher risk of prostate cancer than day workers. The risk seemed to be focused only on rotating shift workers, however, as the study found no significant increase in risk for those on fixed night shifts.

Some of the studies covered in the IWH Issue Briefing also pointed out the effects of shift work on pregnancy, gastrointestinal disorders, sleep disorders, heart disease, diabetes and psychological distress.

Mitigating risks
The IWH also included some of the researchers’ recommendation on how to prevent or mitigate the adverse effects of shift work. Some of these recommendations include:
•    restricting successive evening shift to three shifts;
•    avoiding permanent night work;
•    using forward or clockwise rotation in rotating shift systems (e.g. morning to evening to night, rather than backward or counterclockwise rotation);
•    providing adequate resting time (greater than 11 hours) between shifts; and
•    limiting weekend work

Some researchers stressed that the approach for mitigating the risks needs to be tailored to the specific work context. “In addition, there is evidence for gains from designing the shift system in a participatory way, with engagement of workers, unions, supervisors, etc., and with monitoring and evaluation impacts,” the IWH briefing said.