Researchers are raising several possible health concerns related to nighttime light exposure, among them a higher risk of cancer.
I usually think of light pollution as astronomers’ concern. Who else would mind if the sky glow is so bright that it washes out Orion? (When I can’t see Orion, I feel jilted — yes, even in the months when it’s below the horizon at night.) But the issue has a broader reach than my petulance. Fighting light pollution isn’t merely about seeing stars; it’s about being sensible in our usage and reducing waste.
Still, I was surprised to see that the American Medical Association recently released a report entitled “Light Pollution: Adverse Health Effects of Nighttime Lighting.” It’s a review of some of the available research literature on nighttime lighting’s effect on people; it doesn’t present new research done by the AMA, although many of the results considered come from the authors' own work. The report covers a lot of ground, but it’s unclear what the review’s effect will ultimately be.
The document’s first human concern is glare, which report coauthor Dr. Mario Motta (Tufts Medical School) outlined for S&T readers back in 2009. Glare’s a pretty standard discussion topic in light-pollution conversations, in part because, as drivers age, their eyes become less able to cope with poorly directed light that scatters inside the eye itself. In 2009 the AMA passed a resolution submitted by Motta supporting the use of fully shielded lights, such as the flat-bottomed street lights. The new report reaffirms that resolution.
Vision researcher Gary Rubin (University College London) agrees with the report’s concern, saying the conclusions are “balanced, well-reasoned and thoroughly researched.” Disability glare — as opposed to “discomfort glare,” which differs from person to person — is definitely a problem for drivers, he says, noting that some cataract patients have had second surgeries to replace their new intraocular lenses with another kind that causes less nighttime glare. And as many of us know from experience, modern halogen and LED headlamps can make nighttime driving downright painful. (I can’t tell you how many times I’ve looked away from an oncoming car’s bright bluish headlights and thought, with scathing condescension, “Is that really necessary?”) Blue-rich light’s destructive effect on the molecule rhodopsin (a.k.a. “visual purple”) in the retina is what makes these headlights hurt so much.
But what really struck me was how much attention the reports’ authors give to serious health effects, particularly cancer. Several studies have concluded that women who mess up their natural day-night cycle by working night shifts on a long-term basis (such as nurses in a brightly lit hospital) were more likely to develop breast cancer. This natural day-night cycle, called the circadian rhythm, is a roughly 24-hour cycle our bodies follow to regulate things such as cellular processes and hormone release. It’s generally reset to match our environment by light that reaches specific photoreceptors in our retinas, signals that are then passed on to our brains. Usually that light comes from the Sun, but more frequently our bodies are taking their cues from manmade light sources.
The evidence for shiftwork’s long-term risk is convincing enough that in 2007 the U.N.’s International Agency for Research on Cancer classified shiftwork that disrupts the circadian rhythm as “probably carcinogenic to humans.” The new documentary The City Dark also highlights the possible consequences of shiftwork in an interview with a nighttime shopping show host who later developed cancer.
One marker for circadian rhythm is a hormone called melatonin. (You may have seen it in the pharmacy’s sleep aids section.) Our bodies naturally make more melatonin at night and less in the daytime, but when exposed to enough light we damp down melatonin production, regardless of the “real” time.
As the new report acknowledges, what constitutes “enough light” is debatable. One 2011 study by AMA report coauthor and neurology specialist George Brainard (Jefferson Medical College) and his colleagues suggested that 90 minutes of nighttime exposure to 200 lux of fluorescent white light (equivalent to a moderately lit office) could drop melatonin blood levels by 30% — but so could only a few lux of blue light over the same period of time (melatonin production is particularly sensitive to the 450-480 nanometer range). Other studies have found a variety of effects, including a lack of notable effect. Mark Rea (Rensselaer Polytechnic Institute), director of RPI’s Lighting Research Center, suggests that 30 lux (think “modest twilight”) for 30 minutes is probably a good benchmark for when light exposure will start to affect melatonin levels.
Studies done by report coauthor Dr. David Blask (Tulane University School of Medicine) and others also suggest that melatonin fights tumor growth in rodents. But extrapolating that result from a nocturnal creature such as a mouse to the diurnal human is tricky. Melatonin levels and day-night ranges also differ significantly from person to person, Rea notes. He stresses that it’s still unknown how much of a melatonin drop might be needed to have serious health consequences. “That information just doesn’t exist,” he says. “I believe that melatonin is something you don’t want to squander, but extrapolating that to everybody’s daily life is unnecessarily dire.”
One of Rea’s major concerns with the AMA report is that it presents no specific measurements and therefore sets no threshold for what is and isn’t safe nighttime exposure. “You can say the sky is falling, but how much does it weigh?” he quips. He stresses that he’s convinced by Blask’s tumor work and the evidence for higher breast cancer risk in nighttime shift workers, but he wants to be careful about connecting the dots. “I’m against light pollution, I’m against breast cancer, I’m in favor of melatonin. Let’s just put them in the right order.”
Motta counters that there is now enough evidence to suggest that long-term melatonin suppression could lead to an increased risk for hormonally-induced cancer in people. “I’m not saying we go back to the Stone Age and not use nightlights,” he says. It’s not the brief exposures to light he’s worried about, either, or even delaying bedtime by a couple of hours. The problem arises when people are in “constant jetlag,” he says — living with confused circadian cycles and consistently suppressed melatonin. He hopes that the new report will push awareness of the importance of proper light usage.
The AMA review doesn’t distinguish between outdoor and indoor night lighting, but from poking through the research I get the sense that it’s the indoor light pollution that researchers are focusing on, not the pollution that keeps me from seeing Orion at night. On the other hand, Motta is also an amateur astronomer, and both he and Rea have successfully fought to limit outdoor lighting in their home neighborhoods. Inside vs. outside nighttime lighting is no doubt one of the issues that need further exploration.
Work is ongoing to expand our understanding of how light — particularly bluish light, like that from LCD screens — affects our melatonin levels. Some of the AMA report’s authors are among the leaders in this field. And a team led by Rea’s LRC colleague, Mariana Figueiro, is looking at iPads’ effect on melatonin and hopes to have those results out soon.
At Rea’s suggestion, I downloaded an iPhone app that measures the illuminance in your environment (yes, there’s an app for that) and tested evening conditions at home. I don’t know which wavelengths my iPhone camera is most sensitive to, but I figured a rough estimate was better than none. I held my phone beside my face and pointed its camera at my laptop, the overhead light turned on and reflecting off the bright s