Ninety-nine percent of Americans never see a truly dark sky. One tenth of the world’s population live where it’s never dark enough for eyes to adjust to night vision. In an age where we pay to repair our carbon footprints halfway around the world, order organic produce delivered to our front doors, and Gavin has banned plastic bags, we ignore the light pollution problem right outside our front doors.

Fly into a city past sunset and the view is dazzling: a panoply of colored lights spreads out below. But light sources should not be visible from overhead. Any light with a directly visible source, such as a bulb, that sends light streaming directly upward, is a polluter.

Light pollution is everywhere, and it isn’t just fewer visible stars. When light escapes from our cities, up to 90 percent of it stays in our atmosphere, brightening our entire globe. Increased exposure to this trapped artificial light has been linked to serious health problems, including higher cancer rates, and the destruction of ecosystems. It also contributes to air pollution, global warming, and the costs the United States an estimated one billion annually in wasted energy costs. It also happens to be one of the easiest polluters for the public to stop.

The authors of the first extensive study of light pollution, as related to where people live, Dr. Pierantonio Cinzano and Fabio Falchi (both of the University of Padua, Italy) and Dr. Chris Elvidge (NOAA National Geophysical Data Center, Boulder, Colorado) found the effects of light pollution were a global problem, not one specific to heavily lit urban areas, as many thought. In the opening of their groundbreaking publication, they state that their study “provides a nearly global picture of how mankind is proceeding to envelope itself in a luminous fog.”

Lights are supposed to illuminate their target, and their target only. Your neighbor’s garage lights should not shine through your bedroom window, and the corner streetlight should illuminate only the road beneath it. Poorly aimed light streams, wasted, into the atmosphere where up to 90 percent of it remains, bouncing off particulate matter and creating the luminous fog Cinzano, Falchi and Elvidge mentioned.

Light fixture designers and installers rarely think about this when they create lighting for customers. Near-horizontal light, the city lights you see when you are driving across the Golden Gate Bridge and into San Francisco, is far more harmful than more vertical light spills, such as upward aimed landscape and building façade lighting. Just like stars directly overhead are brighter because their light travels few less atmosphere to reach our eyes, the distance artificial light travels upward before escaping the atmosphere is shorter. It is during the long, horizontal distances that little of the light makes it out. It stays, bouncing off the particulate matter in our air.

Those particulate pollutants scatter natural light just as well as they scatter artificial light, keeping both types in the atmosphere.

Light pollution does not end with obscured stars and glaring billboards. The energy required to power the light into our atmosphere contributes to the wide range of pollution caused by wasted light. The United States spends a conservative one billion dollars per year in energy to power that unused light.

The 175-watt mercury vapor light bulb, which gives off a familiar blue-white tint, is commonly used for yard, security and street lighting. Most of these fixtures are controlled by photocells, meaning they turn on at dusk and off at dawn. The mercury vapor actually uses 210 watts, when all elements are accounted for, and left to its photocells they burn approximately 4,100 hours a year. That is over five and half months of being on non-stop.

This translates to 860 kilowatt-hours used per year, per bulb. On a national average one kilowatt-hour costs eight cents, which means the operating cost for one dawn-to-dusk mercury light per year is about $70. In California during the 1999 energy crisis, the residential price went up to 10.71 cents per kilowatt-hour, making mercury vapor lights even less efficient.

The city of Tuscon, Arizona has roughly 1/5th the population of the United States. In the late 1990s it cost them an average of $1.4 million to power an about twenty-thousand mercury vapor lights within their city. Thus the national average of running mercury vapor light bulbs, only one of multiple types used in the U.S., cost seven hundred million dollars per year. If those mercury vapor bulbs were replaced with low-pressure sodium bulbs, which use less energy and give off better light, the cost would drop two hundred million dollars. The leading authority on light pollution, the International Dark Sky Association, conservatively estimates the national amount of wasted light to cost one billion dollars, five times that of mercury vapor.

This flood of unused light turns into an annual waste of at least six million tons of coal, or ± 23 million barrels of oil. The U.S. receives most of its energy from coal, which is a prolific polluter in its own right and a primary contributor to global warming and air pollution. When burned, one ton of coal creates enough energy to power five dusk-to-day lights, and releases a surge of greenhouse gasses: 6,600 pounds of carbon, 50 pounds of sulfur dioxide, 30 pounds of nitrogen oxide, and trace amounts of mercury and arsenic into the air. These air pollutants contribute to the already heated battle to improve global air quality.

A 2002 American Cancer Society study linked air pollution — which the wasted six million tons of coal contributes heavily to - to rising lung cancer rates and concluded, “every reduction in air pollution will likely lower death rates.” Cancer is the second leading cause of deaths in the U.S, and lung cancer claims the highest percentile, 30 percent, of that number.

Beyond air quality, light pollution has been found to contribute to a number of other health problems. Researchers found that increased contact with man-made light is dangerous to the body’s natural defenses. Over-exposure to artificial light disrupts melatonin production, decreasing the amount of the hormone in the body. Decreased levels pose a serious problem because the hormone has been found to counter the growth of breast cancer by 70 percent, by putting the cancerous cells to sleep. Researchers at the Fred Hutchinson Cancer Research Center, and another, un-connected study conducted by the Danish Cancer Society, found that there was a 60 percent increased risk of breast cancer among workers who held graveyard night shifts, especially if they worked during the hours of one and two in the morning. This was explained when lab mice with human breast cancers were exposed to constant light and researchers at the Basset Research Institute of New York found their tumor growth skyrocketed. This was attributed to the disruption of melatonin production, which regulates the body’s sleep-wake cycles, and is highest during the body’s nocturnal hours.

“With constant light, tumors grow seven times faster and soak up incredible amounts of linoleic acid,” their study stated, citing an acid which aids the growth of breast cancer cells, but which melatonin counters the effects of. “During the day, the cancer cells are awake and the linoleic acid stimulates their growth, but at night the cancer cells go to sleep. When we turn on lights at night for a long time, we oppress melatonin and revert back to the daytime condition.”

In another study it was found that children under the age of two who slept with a night light on are most susceptible to near-sightedness later in their lives than those children not exposed to light.

As is usually the case, the environmental issues that create problems for humans also create problems for in the animal world. For hundreds of migrating birds that fly after the sun sets the dangers of light pollution are a matter of life and death. Birds become confused and disoriented by the lights of manmade structures and cities, known to crash into windows, fly into floodlights or buildings when lights lure them off course and fly, exhausted and lost, until they die of exhaustion. According to research by biologist Sidney Gauthreux of Clemson University, “birds are attracted to the light much like moths are to a flame, but the reasons are unclear. They may use it as a reference and home in on it.”

The precise number of birds killed due to uncontrolled light is unknown, but the Fatal Light Awareness Program estimates that at least ten thousand birds are killed or injured annually in the core of Toronto’s downtown from such incidents.

The U.S. Fish and Wildlife Service has concluded that too much artificial light changes the habits of pelicans: fewer chicks hatch in areas with compromised darkness. Along the shores of Florida, five species of sea turtle hatchlings rely on an instinctive attraction to light to guide them to the water. Many adult female turtles will not come to shore to lay eggs once they see it is bathed in artificial light, and those who do, leave their hatchlings to chose between the bright lights of civilization and the moon- death or life.

Beyond the physical dangers of light pollution—polluted air; wasted money, energy and oil; rising cancer statistics, and baby sea turtles crawling toward interstates—there are far less tangible ones. At night, standing far from the intoxicating lives of the city, about two thousand stars are visible scattered across the sky. On a clear San Franciscan night you should consider yourself lucky if you glimpse Jupiter, Saturn, and a few of the brightest stars overhead. More than 80 percent of Americans live where light pollution makes the night sky brighter than it is during a full moon on a clear, cool night.

The night sky is an inheritance few have considered failing to pass on. Most haven’t realized that at the current rate, one day our children will only know stars as the reproductions on domed planetarium walls. The loss of something as humbling, compelling and eternal as the Milky Way parading across the night is incalculable.

What distances this from other pollutants is, in the words of the American Planning Association, “Unlike the problems of global warming and destruction of the planet’s rain forests, light pollution can be easily addressed with existing strategies and reasonably priced light equipment.” It’s as simple as going to the hardware store and altering your lighting lifestyle.