May 23, 2017

Light at Night and Breast Cancer Risk

While in Maui recently, we drove to the Haleakala Summit at 10,000 feet to watch the sunrise. We miscalculated driving times and reached the top three hours before the sun was supposed to rise. Yet we all agreed our mistake was worth it because we got to see the brightest star-studded sky any of us could remember. There wasn’t a cloud in the sky when we arrived, but by sunrise the summit was enveloped in a whiteout fog so that we never did see the sun come up or any color in the sky. I was reminded of this experience recently as I worked with Heather Wright to review a study by Portnov et al for NMJ on how exposure to light at night may affect risk of cancer.

It seems that an easy and perhaps effective way to lower risk of breast cancer may be to reduce or avoid exposure to artificial lighting at night. It’s that simple. A handful of recent studies confirm what we’ve long suspected….light pollution is a significant risk for breast cancer. Artificial light is a carcinogen.

The Portnov study reported that light at night was correlated with incidence of breast cancer in women in Connecticut.1 The researchers reported a significant association between light at night and breast cancer risk. Women living in areas with the greatest light at night had a 63% greater risk of breast cancer compared to women living in areas with the lowest light.

Blind women, because they use less electric lighting, have a lower risk of breast cancer.2 Women who work night shifts are at greater risk for breast cancer. A 2015 meta-analysis of 16 prospective cohort studies reported that increased morbidity and all-cause mortality in breast cancer corresponded with number of years engaged in night work.3 In a recently published French case-control study the risk for ER+ and PR+ breast cancer doubled, and risk for HER2+ cancers nearly tripled in women working nights shifts.4

The first time I noticed this idea of measuring light at night was in Kloog’s 2008 study. They used NASA nighttime satellite photos to estimate the LAN levels in Israel and reported a strong association with breast cancer rates, a 73% higher breast cancer incidence in the bright communities compared to dimmer communities.5

Something as simple as closing shutters or drawing the blinds at bedtime may reduce breast cancer risk by 18%.6

Given the weight of the evidence we need to start treating light at night as a carcinogenic environmental pollutant and strive to reduce exposure. There is a fascinating website devoted to this mission. The International Dark Sky Association advocates for preserving darkness at night and encourages communities to preserve and protect dark sites through responsible lighting policies and public education.

They also provide some fascinating images of the world as seen from space at night. If you are not familiar with this group, take a moment to locate your home and see what it looks like at night.

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