KANSAS CITY, Mo. -- Stargazers have complained for years because cities shine so much light into the night sky that stars disappear behind the gray glow.

Now researchers are beginning to worry that light pollution -- the artificial brightening of our nights -- is creating problems for wildlife and possibly even human health.

Indeed, a series of studies in the past decade has led some scientists to investigate whether there may be an association between light pollution and cancer, especially breast cancer.

"It could potentially be a problem," said David E. Blask, professor of structural and cellular biology at Tulane University. "All of us are exposed to some light at night, so it really is an issue for everybody."

Robert Wagner of Kansas City, who has become a leader in fighting local light pollution, said most people are not aware of the problem.

"Ninety percent of any law we pass will be education," Wagner said. "When people realize there is better lighting that won't annoy your neighbor or impair your driving on a road, they will want it."

Wagner is part of an international dark-sky movement that has been pushing state and local leaders to pass laws to control light pollution.

Some cities have taken dramatic steps. As part of a broader development ordinance, Kansas City could pass light pollution restrictions as early as next month.

One side benefit, advocates say: Correcting light pollution could save billions of dollars a year in energy costs and also help slow climate change.

The most confounding part of light pollution is that it is so simple to fix, says Peter Strasser, senior technical adviser in Arizona for the International Dark-Sky Association.

It is as easy as designing light fixtures that are hooded so they cast light downward -- not horizontally or upward. In addition, businesses and homeowners should turn off lights when they are not being used.

"People say, 'Is that all there is to it?' " Strasser said. "Pretty much, that's all there is to it. But, boy, there is all kinds of reluctance."

Wagner and others have met several times with Kansas City staff about lighting. But public safety officials are concerned that limiting lights at night would increase crime and traffic accidents.

"We would have to approach that very carefully so that we wouldn't have unintended consequences," said Dennis Murphey, the city's chief environmental officer, who was at the meetings.

Milky Way cast shadow in Galileo's time

To get a sense for the magnitude of modern light pollution, consider: When Galileo studied the Milky Way 400 years ago, it was so bright that it cast shadows on the ground. Today most Americans no longer live in a place where they can see it with the naked eye.

And that much light creates some concerns:

• Cancer: Stray light from streetlamps has caused many a sleepless night, but studies in the past decade have raised questions about a link between nighttime lighting and certain forms of cancer.

The studies have prompted further research by both the federal government and Great Britain.

The most recent study, released last summer, showed that Israeli women living in neighborhoods with light bright enough at midnight to read a book had a 73 percent greater chance of getting breast cancer than women who live in areas that receive light from the stars.

One thing seems certain, according to other studies: Artificial lighting, especially for people working the night shift, interferes with the production of melatonin. That is the hormone that controls our circadian rhythm, the day-night cycle.

Melatonin also has been found to ward off cancer, and people with decreased levels of melatonin are more susceptible to cancer, the studies say

Now the question is whether light pollution is intense enough to affect melatonin levels, said Blask, who has been studying the issue for years.

More studies are needed, he and other scientists say.

"I can see the logic," said Mark Rea, director of the lighting research center at Rensselaer Polytechnic Institute in New York. "But the jury is still out."

• Wildlife: Electric lights take their biggest toll on birds, experts say. During migration, birds use the stars for navigation. Thousands of birds have become disoriented by the bright lights of buildings, crashing into them.

In Chicago and Toronto, officials dim or turn off what they consider unnecessary lights twice a year during migration to prevent birds from being killed.

Why turn them back on?

"The thing that is so curious, if you deemed them not necessary and turned them off, why after that brief period of time do you turn them back on?" Strasser said.

The problem exists in the Kansas City area, too, said Jill DeWitt of Audubon Missouri.

"Taking this issue seriously will save the lives of thousands of birds annually while reducing global warming pollution," DeWitt said.

Studies try to measure the effect

Numerous field studies are being conducted to find out how to protect wildlife, bats, fish and reptiles from light pollution. Scientists also are questioning why fireflies have largely disappeared in recent years -- pesticides or light?

Kansas City, with 90,000 street lights, has installed 25,000 flat-lens lamps that project downward. The city requires new developments to use those lights. The city is gradually changing out the others or retrofitting hoods.

Kansas City considers light rules

The ordinance would:

• Prohibit flashing or revolving exterior lights from being visible outside the property line.

• Require all outdoor lighting to be reflected away from homes and streets.

• Require light fixtures under gas station canopies to be recessed.

Missouri legislators will consider a light pollution bill in the next session, and one is being discussed in Kansas.

Some cities control light pollution

Several North American cities and states, including Arizona, California and Calgary, Alberta, in Canada, have rules to control light pollution. Such steps include installing hooded streetlights, limiting lights at residences and turning off business lights.

In the 1970s, Tucson, Ariz., officials saw that their tall mountains and dark sky were disappearing behind "glare bombs." So they took action.

"We have a million people here, but I can see the Milky Way from my driveway every evening," Strasser said.

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On the Web: International Dark-Sky Association: www.darksky.org