Women who work nights regularly may be up to 70 per cent more likely to develop breast cancer than their counterparts in day jobs, according to new research by a Danish specialist in cancer epidemiology.
The findings, by Mr Johnni Hansen of the Copenhagen Institute of Cancer Epidemiology are to be published in Epidemi- ology later this month. They were previewed yesterday at a joint conference of the Danish Cancer Society, the Danish Women Workers' Union (KAD) and Danish General Workers' Union (SiD).
The findings have already led the KAD president, Ms Lillian Knudsen, to call for a rethink of traditional union policies of "insisting that women should work on an equal footing with men - at whatever time of day or night".
She said while it was important to "avoid creating a panic" while awaiting more research, the findings showed steps were needed immediately to minimise risk to women in sectors such as the health services where night work was routine.
Unlike Denmark, there are no figures available for the number of women who work night shifts in Ireland. Nurses are probably the largest occupational group but many women working in manufacturing and service industries work nights regularly.
Mr Hansen's study shows that Danish women aged 30 to 54 are at a 50 per cent greater risk of primary breast cancer if they work nights for at least half a year. Among those working over six years in jobs involving predominantly night work, the risk of breast cancer is 70 per cent higher than among day workers in the same age range.
Although there have been a number of descriptive studies in the past suggesting significantly higher breast cancer rates among night workers, this is the first survey that uses reliable nationwide statistics.
The survey looked at all 7,035 Danish women who had confirmed primary breast cancer, were aged between 30-54 at the time of diagnosis and 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.
Adjustments were made for other factors known to affect rates of breast cancer such as socio-economic status, number of children and alcohol consumption.
In Denmark, as elsewhere, alcohol consumption is higher among night workers than the general population, but the gap does not explain all of the difference in the cancer rates. Mr Hansen argues the most likely culprit is lack of melatonin.
This is produced naturally in the body. 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 it is regulated, via the retina of the eye, by the daily cycle of light and darkness. It is produced mainly at night and a person who stays awake all night under artificial lighting produces less melatonin.