Does working nights cause breast cancer?

Whether breast cancer should be labeled as an occupational disease is still unclear, researchers behind a new study from China suggest.

Breast cancer is the most common cancer in women worldwide, and so far only one country (Denmark) has awarded compensation to shift workers who develop the disease. If more were to follow suit, it could have far-reaching economic consequences.

The new report, however, showed no sign of a connection between night-shift work and breast cancer.

"We basically found no association, even among women who had more than 25 years of shift work," said Wong-Ho Chow, a researcher with the National Cancer Institute in Bethesda, Maryland, who worked on the study, published in the American Journal of Epidemiology.

With colleagues from US and Chinese universities, Chow followed more than 70,000 women in Shanghai, inquiring about their work and cancer status.

Earlier research in animals had suggested that disturbances in the light-dark cycle could knock certain chemicals in the body - for instance, melatonin and estrogen - off balance, and as a result increase cancer risk.

"We wanted to explore this hypothesis," Chow told Reuters Health.

Over a decade, about one in a hundred women in the study developed breast cancer. But the risk didn't increase for women who worked nights, regardless of how long they had done so.

"There is really not much that you can criticize from reading the paper," said Dr. Kurt Straif, an expert in occupational medicine, who was not involved in the research.

But, he added, "One has to keep in mind that this is the first study from a non-Western country."

In 2007, a team headed by Straif at the World Health Organization's International Agency for Research on Cancer had concluded that night-shift work was a probable cause of cancer.

Based on animal research as well as studies of nurses and flight attendants, the team listed shift work in the second-highest cancer-risk category, group 2A, which includes exposure to tanning beds, crude-oil refinement and hairdressing agents.

Although the evidence wasn't clearcut, Straif said, six out of eight studies found an increased risk of at least 50 percent.

Denmark reacted to the team's conclusion by granting breast cancer the status of occupational disease. In 2009, the Scandinavian country began awarding compensation to affected women who had no other obvious risk factors.

"We have a pretty expansive attitude toward recognizing occupational hazards," said Johnni Hansen of the Danish Cancer Society, who has spent a decade studying shift work and breast cancer.

"In most other countries," he told Reuters Health, "you need to have a more solid connection."