Missing the Dark: Health Effects of Light Pollution
In 1879, Thomas Edison’s
incandescent light bulbs first illuminated a New York street, and the modern
era of electric lighting began. Since then, the world has become awash in
electric light. Powerful lamps light up streets, yards, parking lots, and
billboards. Sports facilities blaze with light that is visible for tens of
miles. Business and office building windows glow throughout the night.
According to the Tucson, Arizona–based International Dark-Sky Association
(IDA), the sky glow of Los Angeles is visible from an airplane 200 miles away.
In most of the world’s large urban centers, stargazing is something that
happens at a planetarium. Indeed, when a 1994 earthquake knocked out the power
in Los Angeles, many anxious residents called local emergency centers to report
seeing a strange “giant, silvery cloud” in the dark sky. What they were really
seeing—for the first time—was the Milky Way, long obliterated by the urban sky
glow.
None of this is to say that electric lights are inherently bad.
Artificial light has benefited society by, for instance, extending the length
of the productive day, offering more time not just for working but also for
recreational activities that require light. But when artificial outdoor
lighting becomes inefficient, annoying, and unnecessary, it is known as light
pollution. Many environmentalists, naturalists, and medical researchers
consider light pollution to be one of the fastest growing and most pervasive
forms of environmental pollution. And a growing body of scientific research
suggests that light pollution can have lasting adverse effects on both human
and wildlife health.
When does nuisance light become a health hazard? Richard Stevens,
a professor and cancer epidemiologist at the University of Connecticut Health
Center in Farmington, Connecticut, says light photons must hit the retina for
biologic effects to occur. “However, in an environment where there is much
artificial light at night—such as Manhattan or Las Vegas—there is much more
opportunity for exposure of the retina to photons that might disrupt circadian
rhythm,” he says. “So I think it is not only ‘night owls’ who get those
photons. Almost all of us awaken during the night for periods of time, and
unless we have blackout shades there is some electric lighting coming in our
windows. It is not clear how much is too much; that is an important part of the
research now.”
According to “The First World Atlas of the Artificial Night Sky
Brightness,” a report on global light pollution published in volume 328, issue
3 (2001) of the Monthly Notices of the Royal Astronomical Society,
two-thirds of the U.S. population and more than one-half of the European
population have already lost the ability to see the Milky Way with the naked
eye. Moreover, 63% of the world population and 99% of the population of the
European Union and the United States (excluding Alaska and Hawaii) live in
areas where the night sky is brighter than the threshold for light-polluted
status set by the International Astronomical Union—that is, the artificial sky
brightness is greater than 10% of the natural sky brightness above 45° of
elevation.
Light pollution comes in many forms, including
sky glow, light trespass, glare, and over illumination. Sky glow is the bright
halo that appears over urban areas at night, a product of light being scattered
by water droplets or particles in the air. Light trespass occurs when unwanted
artificial light from, for instance, a floodlight or streetlight spills onto an
adjacent property, lighting an area that would otherwise be dark. Glare is
created by light that shines horizontally. Overillumination refers to the use
of artificial light well beyond what is required for a specific activity, such
as keeping the lights on all night in an empty office building.
The ecologic effects of artificial light have been well
documented. Light pollution has been shown to affect both flora and fauna. For
instance, prolonged exposure to artificial light prevents many trees from
adjusting to seasonal variations, according to Winslow Briggs’s chapter on
plant responses in the 2006 book Ecological Consequences of Artificial
Night Lighting. This, in turn, has implications for the wildlife that
depend on trees for their natural habitat. Research on insects, turtles, birds,
fish, reptiles, and other wildlife species shows that light pollution can alter
behaviors, foraging areas, and breeding cycles, and not just in urban centers
but in rural areas as well.
Sea turtles provide one dramatic example of how artificial light
on beaches can disrupt behavior. Many species of sea turtles lay their eggs on
beaches, with females returning for decades to the beaches where they were born
to nest. When these beaches are brightly lit at night, females may be
discouraged from nesting in them; they can also be disoriented by lights and
wander onto nearby roadways, where they risk being struck by vehicles.
Moreover, sea turtle hatchlings normally navigate toward the sea
by orienting away from the elevated, dark silhouette of the landward horizon,
according to a study published by Michael Salmon of Florida Atlantic University
and colleagues in volume 122, number 1–2 (1992) of Behaviour. When
there are artificial bright lights on the beach, newly hatched turtles become
disoriented and navigate toward the artificial light source, never finding the
sea.
Jean Higgins, an environmental specialist with the Florida
Wildlife Conservation Commission Imperiled Species Management Section, says
disorientation also contributes to dehydration and exhaustion in hatchlings.
“It’s hard to say if the ones that have made it into the water aren’t more
susceptible to predation at this later point,” she says.
Bright electric lights can also disrupt the behavior of birds.
About 200 species of birds fly their migration patterns at night over North
America, and especially during inclement weather with low cloud cover, they
routinely are confused during passage by brightly lit buildings, communication
towers, and other structures. “Light attracts birds and disorients them,”
explains Michael Mesure, executive director of the Toronto-based Fatal Light
Awareness Program (FLAP), which works to safeguard migratory birds in the urban
environment. “It is a serious situation because many species that collide
frequently are known to be in long-term decline and some are already designated
officially as threatened.”
Each year in New York City alone, about 10,000 migratory birds are
injured or killed crashing into skyscrapers and high-rise buildings, says Glenn
Phillips, executive director of the New York City Audubon Society. The
estimates as to the number of birds dying from collisions across North America
annually range from 98 million to close to a billion. The U.S. Fish and
Wildlife Service estimates 5–50 million birds die each year from collisions
with communication towers.
Turtles and birds are not the only wildlife affected by artificial
nighttime lighting. Frogs have been found to inhibit their mating calls when
they are exposed to excessive light at night, reducing their reproductive
capacity. The feeding behavior of bats also is altered by artificial light.
Researchers have blamed light pollution for declines in populations of North
American moths, according to Ecological Consequences of Artificial
Night Lighting. Almost all small rodents and carnivores, 80% of marsupials,
and 20% of primates are nocturnal. “We are just now understanding the
nocturnality of many creatures,” says Chad Moore, Night Sky Program manager
with the National Park Service. “Not protecting the night will destroy the
habitat of many animals.”
Resetting the Circadian Clock
The health effects of light pollution have not been as well
defined for humans as for wildlife, although a compelling amount of epidemiologic
evidence points to a consistent association between exposure to indoor
artificial nighttime light and health problems such as breast cancer, says
George Brainard, a professor of neurology at Jefferson Medical College, Thomas
Jefferson University in Philadelphia. “That association does not prove that
artificial light causes the problem. On the other hand, controlled laboratory
studies do show that exposure to light during the night can disrupt circadian
and neuroendocrine physiology, thereby accelerating tumor growth.”
The 24-hour day/night cycle, known as the circadian clock, affects
physiologic processes in almost all organisms. These processes include brain
wave patterns, hormone production, cell regulation, and other biologic
activities. Disruption of the circadian clock is linked to several medical
disorders in humans, including depression, insomnia, cardiovascular disease,
and cancer, says Paolo Sassone-Corsi, chairman of the Pharmacology Department
at the University of California, Irvine, who has done extensive research on the
circadian clock. “Studies show that the circadian cycle controls from ten to
fifteen percent of our genes,” he explains. “So the disruption of the circadian
cycle can cause a lot of health problems.”
On 14–15 September 2006 the National Institute of Environmental
Health Sciences (NIEHS) sponsored a meeting that focused on how best to conduct
research on possible connections between artificial lighting and human health.
A report of that meeting in the September 2007 issue of EHP stated,
“One of the defining characteristics of life in the modern world is the altered
patterns of light and dark in the built environment made possible by use of
electric power.” The meeting report authors noted it may not be entirely
coincidental that dramatic increases in the risk of breast and prostate
cancers, obesity, and early-onset diabetes have mirrored the dramatic changes
in the amount and pattern of artificial light generated during the night and
day in modern societies over recent decades. “The science underlying these
hypotheses has a solid base,” they wrote, “and is currently moving forward
rapidly.”
The connection between artificial light and sleep disorders is a
fairly intuitive one. Difficulties with adjusting the circadian clock can lead
to a number of sleep disorders, including shift-work sleep disorder, which
affects people who rotate shifts or work at night, and delayed sleep–phase
syndrome, in which people tend to fall asleep very late at night and have
difficulty waking up in time for work, school, or social engagements.
The sleep pattern that was the norm before the invention of
electric lights is no longer the norm in countries where artificial light
extends the day. In the 2005 book At Day’s Close: Night in Times Past,
historian Roger Ekirch of Virginia Polytechnic Institute described how before
the Industrial Age people slept in two 4-hour shifts (“first sleep” and “second
sleep”) separated by a late-night period of quiet wakefulness.
Thomas A. Wehr, a psychiatrist at the National Institute of Mental
Health, has studied whether humans would revert back to the two-shift sleep
pattern if they were not exposed to the longer photoperiod afforded by
artificial lighting. In the June 1992Journal of Sleep Research, Wehr
reported his findings on eight healthy men, whose light/dark schedule was
shifted from their customary 16 hours of light and 8 hours of dark to a
schedule in which they were exposed to natural and electric light for 10 hours,
then darkness for 14 hours to simulate natural durations of day and night in
winter. The subjects did indeed revert to the two-shift pattern, sleeping in
two sessions of about 4 hours each separated by 1–3 hours of quiet wakefulness.
Alteration of the circadian clock can branch into other effects
besides sleep disorders. A team of Vanderbilt University researchers considered
the possibility that constant artificial light exposure in neonatal intensive
care units could impair the developing circadian rhythm of premature babies. In
a study published in the August 2006 issue ofPediatric Research, they
exposed newborn mice (comparable in development to 13-week-old human fetuses)
to constant artificial light for several weeks. The exposed mice were were
unable to maintain a coherent circadian cycle at age 3 weeks (comparable to a
full-term human neonate). Mice exposed for an additional 4 weeks were unable to
establish a regular activity cycle. The researchers concluded that excessive
artificial light exposure early in life might contribute to an increased risk
of depression and other mood disorders in humans. Lead researcher Douglas
McMahon notes, “All this is speculative at this time, but certainly the data
would indicate that human infants benefit from the synchronizing effect of a
normal light/dark cycle.”
Since 1995, studies in such journals as Epidemiology,
Cancer Causes and Control, theJournal of the National Cancer Institute,
and Aviation Space Environmental Medicine, among others, have
examined female employees working a rotating night shift and found that an
elevated breast cancer risk is associated with occupational exposure to
artificial light at night. Mariana Figueiro, program director at the Lighting
Research Center of Rensselaer Polytechnic Institute in Troy, New York, notes
that permanent shift workers may be less likely to be disrupted by night work
because their circadian rhythm can readjust to the night work as long as
light/dark patterns are controlled.
In a study published in the 17 October 2001 Journal of the
National Cancer Institute, Harvard University epidemiologist Eva S.
Schernhammer and colleagues from Brigham and Women’s Hospital in Boston used
data from the 1988 Nurses’ Health Study (NHS), which surveyed 121,701
registered female nurses on a range of health issues. Schernhammer and her
colleagues found an association between breast cancer and shift work that was
restricted to women who had worked 30 or more years on rotating night shifts
(0.5% of the study population).
In another study of the NHS cohort, Schernhammer and colleagues
also found elevated breast cancer risk associated with rotating night shift
work. Discussing this finding in the January 2006 issue of Epidemiology,
they wrote that shift work was associated with only a modest increased breast
cancer risk among the women studied. The researchers further wrote, however,
that their study’s findings “in combination with the results of earlier work,
reduce the likelihood that this association is due solely to chance.”
Schernhammer and her colleagues have also used their NHS cohort to
investigate the connection between artificial light, night work, and colorectal
cancer. In the 4 June 2003 issue of the Journal of the National Cancer
Institute, they reported that nurses who worked night shifts at least 3
times a month for 15 years or more had a 35% increased risk of colorectal
cancer. This is the first significant evidence so far linking night work and
colorectal cancer, so it’s too early to draw conclusions about a causal
association. “There is even less evidence about colorectal cancer and the
larger subject of light pollution,” explains Stevens. “That does not mean there
is no effect, but rather, there is not enough evidence to render a verdict at
this time.”
The research on the shift work/cancer relationship is not
conclusive, but it was enough for the International Agency for Research on
Cancer (IARC) to classify shift work as a probable human carcinogen in 2007.
“The IARC didn’t definitely call night shift work a carcinogen,” Brainard says.
“It’s still too soon to go there, but there is enough evidence to raise the
flag. That’s why more research is still needed.”
Brainard and a growing number of researchers believe that
melatonin may be the key to understanding the shift work/breast cancer risk
association. Melatonin, a hormone produced by the pineal gland, is secreted at
night and is known for helping to regulate the body’s biologic clock. Melatonin
triggers a host of biologic activities, possibly including a nocturnal
reduction in the body’s production of estrogen. The body produces melatonin at
night, and melatonin levels drop precipitously in the presence of artificial or
natural light. Numerous studies suggest that decreasing nocturnal melatonin
production levels increases an individual’s risk of developing cancer. [For
more information on melatonin, see “Benefits of Sunlight: A Bright Spot for
Human Health,”EHP 116:A160–A167 (2008).]
One groundbreaking study published in the 1 December 2005 issue of Cancer
Researchimplicated melatonin deficiency in what the report authors called a
rational biologic explanation for the increased breast cancer risk in female
night shift workers. The study involved female volunteers whose blood was
collected under three different conditions: during daylight hours, during the
night after 2 hours of complete darkness, and during the night after exposure
to 90 minutes of artificial light. The blood was injected into human breast
tumors that were transplanted into rats. The tumors infused with
melatonin-deficient blood collected after exposure to light during the night
were found to grow at the same speed as those infused with daytime blood. The
blood collected after exposure to darkness slowed tumor growth.
“We now know that light suppresses melatonin, but we are not
saying it is the only risk factor,” says first author David Blask, a research
scientist at the Bassett Healthcare Research Institute in Cooperstown, New
York. “But light is a risk factor that may explain [previously unexplainable
phenomena]. So we need to seriously consider it.”
The National Cancer Institute estimates that 1 in 8 women will be
diagnosed with breast cancer at some time during her life. We can attribute
only about half of all breast cancer cases to known risk factors, says
Brainard. Meanwhile, he says, the breast cancer rate keeps climbing—incidence
increased by more than 40% between 1973 and 1998, according to the Breast
Cancer Fund—and “we need to understand what’s going on as soon as possible.”
Linking Light Pollution to Human Health
The evidence that indoor artificial light at night influences
human health is fairly strong, but how does this relate to light pollution? The
work in this area has just begun, but two studies in Israel have yielded some
intriguing findings. Stevens was part of a study team that used satellite
photos to gauge the level of nighttime artificial light in 147 communities in Israel,
then overlaid the photos with a map detailing the distribution of breast cancer
cases. The results showed a statistically significant correlation between
outdoor artificial light at night and breast cancer, even when controlling for
population density, affluence, and air pollution. Women living in neighborhoods
where it was bright enough to read a book outside at midnight had a 73% higher
risk of developing breast cancer than those residing in areas with the least
outdoor artificial lighting. However, lung cancer risk was not affected. The
findings appeared in the January 2008 issue ofChronobiology International.
“It may turn out that artificial light exposure at night increases
risk, but not entirely by the melatonin mechanism, so we need to do more studies
of ‘clock’ genes—nine have so far been identified—and light exposure in rodent
models and humans,” Stevens says. Clock genes carry the genetic instructions to
produce protein products that control circadian rhythm. Research needs to be
done not just on the light pollution–cancer connection but also on several
other diseases that may be influenced by light and dark.
Travis Longcore, co-editor of Ecological Consequences of
Artificial Night Lighting and a research associate professor at the
University of Southern California Center for Sustainable Cities, suggests two
ways outdoor light pollution may contribute to artificial light–associated
health effects in humans. “From a human health perspective, it seems that we
are concerned with whatever increases artificial light exposure indoors at
night,” he says. “The effect of outdoor lighting on indoor exposure could be
either direct or indirect. In the direct impact scenario, the artificial light
from outside reaches people inside at night at levels that affect production of
hormones. In an indirect impact it would disturb people inside, who then turn
on lights and expose themselves to more light.”
“The public needs to know about the factors causing [light
pollution], but research is not going at the pace it should,” Blask says. Susan
Golden, distinguished professor at the Center for Research on Biological Clocks
of Texas A&M University in College Station, Texas, agrees. She says, “Light
pollution is still way down the list of important environmental issues needing
study. That’s why it’s so hard to get funds to research the issue.”
“The policy implications of unnecessary light at night are
enormous,” says Stevens in reference to the health and energy ramifications
[for more on the energy impact of light pollution, see “Switch On the Night:
Policies for Smarter Lighting,” p. A28 this issue]. “It is fully as important
an issue as global warming.” Moreover, he says, artificial light is a
ubiquitous environmental agent. “Almost everyone in modern society uses electric
light to reduce the natural daily dark period by extending light into the
evening or before sunrise in the morning,” he says. “On that basis, we are all
exposed to electric light at night, whereas before electricity, and still in
much of the developing world, people get twelve hours of dark whether they are
asleep or not.”
Sources believe that the meeting at the NIEHS in September 2006
was a promising beginning for moving forward on the light pollution issue. “Ten
years ago, scientists thought something was there, but couldn’t put a finger on
it,” says Leslie Reinlib, a program director at the NIEHS who helped organize
the meeting. “Now we are really just at the tip of the iceberg, but we do have
something that’s scientific and can be measured.”
The 23 participants at the NIEHS-sponsored meeting identified a
research agenda for further study that included the functioning of the
circadian clock, epidemiologic studies to define the artificial light
exposure/disease relationship, the role of melatonin in artificial
light–induced disease, and development of interventions and treatments to
reduce the impact of light pollution on disease. “It was a very significant
meeting,” Brainard says. “It’s the first time the National Institutes of Health
sponsored a broad multidisciplinary look at the light-environmental question
with the intent of moving to the next step.”
