The blue light emitted by street lights including LEDs, and commercial outdoor lighting such as advertising, may significantly increase the risk of breast and prostate cancer, a study has warned.
Researchers from the Barcelona Institute for Global Health (ISGlobal) and University of Exeter in the UK found that participants living in large cities with heavy exposure to blue lights at night had double the risk of prostate cancer and 1.5 times higher risk of breast cancer.
This was compared to populations with less exposure to blue light.
Older lighting schemes emit a glow within the 'orange' spectrum, but new modern lighting creates a bright 'blue' light emission.
The researchers found the bluer the light emission that people in large cities were exposed to, the higher the risk of cancer. The study also found that people who lived in homes with darker rooms, by using window shutters for example, had lower risk than those who did not.
The study, published in the journal Environmental Health Perspectives, included medical and epidemiological data of more than 4,000 people between 20 and 85 years of age in 11 Spanish regions.
Indoor exposure to artificial light was determined through personal questionnaires. In the first study of its kind, outdoor levels of artificial light were evaluated based on night-time images taken by astronauts aboard the International Space Station.
"Humans have evolved to need light during the day and darkness at night. As towns and cities replace older lighting, we're all exposed to higher levels of blue lights, which can disrupt our biological clocks," said Alejandro Sanchez de Miguel, from the University of Exeter.
"It is imperative that we know for sure whether this increases our risk of cancer. Scientists have long suspected this may be the case - now our innovative findings indicate a strong link," said de Miguel.
"We must also investigate whether night-time exposure to the blue light emitted by smartphones and tablets increases our risk of cancer," he said.
"Currently, the images taken by astronauts on the International Space Station (ISS) are our only way of determining the spread of blue light-emitting white LEDs in our cities," he added.
Little is known about how environmental factors affect breast and prostate cancer. The researchers hypothesise that LED lights may disrupt the body's 24-hour cycle known as the circadian rhythm, in turn affecting hormones. Both breast and prostate cancer are hormone-related.