Blue Light From Street Lamps Linked to Colorectal Cancer

Exposure to blue light from outdoors has been linked to an increased risk of developing colorectal cancer in a study.

Scientists found a positive association between the amount of light in the blue spectrum shining near people's homes at night and rates of colorectal cancer. They found no link between overall visual light and the disease, however, and the pattern only appeared among those who had smoked at least once in their lifetime.

The team looked at data on 661 people living in the Spanish cities of Madrid and Barcelona, who had colorectal cancer between 2007 to 2013. They compared them to 1,322 people without a history of cancer and who were matched to the 661 by sex and age, to act as the control group. Participants' exposure to outdoor artificial light at night across the red, green and blue spectra was measured using images taken from the International Space Station.

Researchers also asked participants questions including where they had lived and worked and for how long, as well as cancer risk factors, like their age, education and socioeconomic status, BMI, family history of cancer, and whether they smoked, drank alcohol, exercised, what they ate, and how they slept. On average, those with cancer were 67 years old and more likely to have a family history of cancer.

Co-author Manolis Kogevinas, scientific director of the Severo Ochoa Distinction at the Barcelona Institute for Global Health, told Newsweek via email the team wanted to build on several past studies that found links between night shift work and light that disrupts our body clock, known as the circadian rhythm. The researchers also previously found a link between blue light and breast and prostate cancer.

Writing in the journal Epidemiology, the team said the circadian rhythm is closely related to our hormone system, and evidence suggests being exposed to light at night may change the production of certain types of these chemicals. In addition, colon cancers have in the past been tied to circadian disruption in animal experiments.

It is thought that encountering artificial light at night may lessen and delay the release of the hormone melatonin, which is linked to tumor growth and made in the dark phase of the body's 24-hour cycle.

The team said the study was limited because they were unable to measure the amount of light that reaches people's homes.

Kogevinas said: "I would have liked to have a few thousand people wearing a personal light sensor for a few years! This would have given a much more accurate estimate of light. We measure environmental light and particularly blue light accurately but we are not really measuring what reaches the eyes of the person."

He said the study reinforces past findings that colorectal cancer is associated with circadian disruption. "The precise mechanisms, i.e. whether it is only sleep-related or whether there are hormonal or immune pathways involved, is to be seen."

Asked what readers should take from the study, Kogevinas said he would avoid giving public health recommendations off the back of one study.

"In any case, the main message of this and other studies is that light at night and light spectrum is not a neutral factor, it can affect body functions and biological pathways related to health and disease," he said.

"Exposing ourselves to unnecessary levels, particularly of blue light, is not necessary. Part of the blue light exposures comes from tablets and smartphones. Companies should develop more their technologies to reduce even further the light emissions."