ICEM UPDATE

No. 2/2001

10 January 2001

The following is from the International Federation of Chemical, Energy, Mine and General Workers' Unions (ICEM):

Night Work Linked To Breast Cancer

New Danish Study Shows 50-70 Percent Higher Risk

Disturbing new evidence of a link between breast cancer and night work is to be published soon by a Danish researcher.

The findings were previewed at a conference held jointly last Thursday by the Danish Cancer Society (Kraeftens Bekaempelse), the Danish Women Workers' Union (KAD) and the Danish General Workers' Union (SiD).

They show a 50 percent higher risk of primary breast cancer among 30- to 54-year-old women who work night shifts in selected trades for at least half a year.

The risk tends to increase somewhat in women who work nights long-term, researcher Johnni Hansen told the conference. Among women with over six years of employment in trades with predominantly night work, the risk of breast cancer is 70 percent higher than among women daytime workers in the same age range. However, the group of women night workers in this category is small.

Hansen is a specialist at Copenhagen's Institute of Cancer Epidemiology. He has just completed a study of breast cancer among women who work at night. Although a number of descriptive studies in the past have shown significantly higher breast cancer rates among night workers, this is thought to be the first survey that uses reliable nationwide statistics.

His findings will be published in EPIDEMIOLOGY, January 2001, Vol 12, No. 1, pp 74-77.

The increased risks revealed by the new survey are probably not statistical flukes - especially as the study was conducted on a very large sample and was based on Denmark's detailed registry of employment records, health records and population.

The study looked at all 7,035 Danish women who fulfilled all of the following conditions: they had confirmed primary breast cancer, they were born in the period 1935-59, they were aged 30-54 at the time of diagnosis and they had been in employment.

Each case was then matched against a "control", who was a woman drawn at random from the files of the central population registry but whose year of birth was the same as the corresponding case. "Controls" were women who were alive, without cancer, and who had been in employment before the date of diagnosis of the corresponding case.

Women were considered to work predominantly at night if they had been employed for at least half a year in one or more of the trades in which at least 60 percent of the female respondents had night-time schedules.

Appropriate adjustments were made for other factors known to affect rates of breast cancer, such as socio-economic status, the number of children to which the women had given birth and the women's age at the time of the births. The study also looked at alcohol consumption, which is a known risk factor for breast cancer. In Denmark, as in many countries, alcohol consumption is higher among night workers than in the population as a whole - but the gap would probably not explain all of the difference in the breast cancer rates.

So the explanation must be sought elsewhere, and by far the most likely candidate is a lack of melatonin.

Melatonin is produced naturally in the human body, where it helps to regulate body rhythms, sleep and, possibly, tumour growth. In women, it may also help to prevent overproduction of oestrogen. Concentrations of melatonin have been shown to inhibit the growth of human breast cancer cells.

The body's production of melatonin is regulated, via the retina of the eye, by the daily cycle of light and darkness. It is produced mainly at night. Thus, a person who regularly stays awake all night under artificial lighting will produce less melatonin.

For complex reasons linked to the light/dark cycle, this melatonin deficit is unlikely to be made up by sleeping during daylight hours. Totally blind women are known to have an approximately 50 percent lower relative risk of breast cancer than sighted women.

But whatever the causes of the higher breast cancer rates among night workers, the socio-economic effects could be very serious. Trade unions have long fought for full equality of opportunity at work, and women's status in the workplace has improved significantly in most parts of the world as a result.

In recent years, these equality drives have included the opening of night work to women in a growing number of sectors, and the lifting of regulations that previously limited their access to these higher-paying jobs. For many women workers around the world, night work is both an economic necessity and a step up the promotion ladder.

Breast cancer is by no means the only physical and psychological health risk faced by night workers - whether women or men. And accident rates are significantly higher on the night shift than during the day. Also, it may be wondered if a melatonin deficit may not produce other ill-effects in men and women alike.

So tackling the problem of breast cancer among night workers will, in part, depend on negotiation and on attitude - particularly the attitude of the employers. Trade unions have always insisted that jobs should be designed for people, rather than people for jobs. It may also be even more necessary in future for collective agreements to state that biological factors must not impede equality of pay and opportunity.

Lillian Knudsen is the President of the Danish Women Workers' Union (KAD). She is also on the executive of the 20-million-strong International Federation of Chemical, Energy, Mine and General Workers' Unions (ICEM), to which both the KAD and the SiD are affiliated at the global level.

"Internationally, I have always insisted that women should work on an equal footing with men - at whatever time of day or night," Knudsen says. "But this new report has made me nervous, because it would seem that, if we women can sleep in darkness, we produce a hormone that helps to protect us against breast cancer. But we're not stupid. We know full well that some sectors - hospitals and care homes for instance - will always have to work right around the clock. And we must avoid creating a panic. Clearl though, more research is needed into this mechanism, so that we can find out how to design jobs better and minimise the risk of breast cancer."

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Editor: Ian Graham, Information Officer

Publisher: Fred Higgs, General Secretary.

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