Shedding Light on a Cause of Breast Cancer

The malignancy is more common in well-lit communities than in naturally dark ones.

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Brightly lit communities have high rates of breast cancer, according to a new study of cancer data and satellite images of light pollution.
Brightly lit communities have high rates of breast cancer, according to a new study of cancer data and satellite images of light pollution.

When Edison invented the light bulb, did he accidentally spawn a cancer epidemic? It's certainly starting to look that way. In study after recent study, exposure to artificial light has been linked to certain kinds of tumors, especially those in the breast.

Consider some of the evidence: Blind women have low rates of breast cancer. So do women in underdeveloped countries, where artificial lighting is an uncommon luxury. By contrast, female nurses and other women who frequently work night shifts have high breast cancer rates. The reason, experts believe, is that their schedules expose them to illumination during what should be the darkest hours of their days, and that disrupts the body's production of the cancer-suppressing hormone melatonin. In lab experiments, human breast tumors have been found to grow relatively quickly when fed by the blood of women who have been in a brightly lit room in the middle of the night. When blood is drawn from women who've been sitting in darkness, it's richer in melatonin and less nourishing to the cancer.

Based on those and other observations, a unit of the World Health Organization announced in December that shift work is a "probable human carcinogen." But shift work may be merely the tip of Edison's epidemic.

In fact, any woman whose community is filled with streetlamps and other light sources may face an unnaturally high risk of breast cancer. A new study, slated to appear in the journal Chronobiology International, finds that breast cancer incidence is about 73 percent higher in communities with the greatest amount of artificial light at night than in communities with the least. The researchers assessed different communities' nocturnal light levels by analyzing satellite images of how much illumination escapes into space. (You can see this Washington Post article for details.)

Light pollution seems to have other untoward consequences, including harmful effects on animals like migratory birds and sea turtles. But the apparently carcinogenic effects of light pollution have received—and arguably deserve—the lion's share of scientists' attention. No one has paid more notice to the light-cancer connection than Richard Stevens, the University of Connecticut Health Center epidemiologist who first proposed a possible link more than two decades ago. Stevens collaborated on the new study with four colleagues in Israel, and I asked him to comment on its significance.

He was quick to say that the study falls short of proving cause and effect. But it's consistent, he said, with the hypothesis that light at night accounts for a "substantial fraction of breast cancer."

"Lighting the night is as important an ecological issue for the planet as global warming," he added. "In addition to its effects on all life forms, unnecessary lighting of the night accounts for a lot of fossil fuel consumption and also contributes to global warming."