ARE WE RUNNING OUT OF ROOM?

THE POPULATION HITS 6 BILLION, FUELING A DEBATE OVER JUST HOW MANY MORE PEOPLE THE EARTH CAN SUPPORT.

Heard about the Y2K computer crisis? Get ready for Y6B. For months, ominous-looking posters have appeared on buses around Washington, D.C., with the message "Y6B." Sponsored by the group Zero Population Growth, the message stands for "Year of 6 Billion."

On October 12, the world population hit 6 billion, according to United Nations estimates. It took a million years of human history for the global population to reach the 1 billion mark in 1804, but just 12 years to go from 5 billion to 6 billion.

"As more people are crowded onto the planet, far more are becoming alarmed about the potentially disastrous consequences," says Lester Brown, president of the environmental group Worldwatch Institute.

But whether continued population growth will bring catastrophe remains subject to debate. Some experts believe Y6B poses far more danger to far more people than the so-called Y2K problem, which may cause massive computer malfunctions in the year 2000. As the world's population continues to multiply, so, too, will problems such as food shortages, species extinction, and environmental destruction, they say. Some scientists say the Earth has a finite limit to the number of people it can support, and we're fast approaching it.

But others see things as far less dire. They say advances in technology, medicine, and education hold out the promise of a better quality of life for all the world's people. While the Earth may not have much room to spare, technology continues to devise new ways of supporting more people.

Which side proves to be right depends largely on the planet's young people. There are more than 1 billion people between 15 and 24 years of age today. Most will go on to have their own families. Their decisions about how many children to have will determine whether population becomes a larger or lesser issue.

EDUCATION AND BABIES

The key factor in their decisions, experts say, will be the education they receive. Studies show that the more education ,people get--particularly women--the fewer children they have. As a result, many nations now view education as the antidote to a population crisis. At the International Conference on Population and Development in 1994, 179 nations agreed that giving women equal rights and education helps economic development. Since then, two thirds of all nations have introduced policies empowering women.

With improving education levels and the spread of information about birth control, the rate of world population increase has declined, even as the number of people has grown. In the 1950s, the average woman had about six children; that figure has now been halved to an average of 2.9 per woman. In the United States, the average is 2.03 children per woman, while in Europe and Japan, the average is 1.4--a figure so low that more people are dying than being born.

Declining birthrates result in more opportunities for fewer people, allowing countries a chance to develop and prosper. The trend has evoked sighs of relief from some experts.

"The population explosion is over," says Ben Wattenberg, a senior fellow at the American Enterprise Institute. "Never before have birthrates fallen so far, so fast, so low, for so long, all around the world."

But a recent UN report on population warns against being complacent about overpopulation. "The danger now is that we will declare victory and go home," the report says.

A BILLION HUNGRY PEOPLE

Most of today's population growth occurs in the world's poorest areas, such as sub-Saharan Africa and southern and western Asia, the UN says. These areas are the least prepared to deal with the results of overpopulation. In recent years, famine has killed millions of people in Ethiopia, Somalia, Sudan, and North Korea. More than a billion people suffer from severe nutritional deficiencies, according to the UN.

A growing population strains more than a country's efforts to feed its people. It also crimps government's ability to provide basic services. Nearly three fifths of people living in developing nations lack basic sanitation; almost one third have no access to clean water; one fourth do not have adequate housing; one fifth have no access to modern health services, and one fifth get no schooling past fifth grade.

But the problem isn't limited to developing nations. The industrialized West is the big culprit when it comes to other population-related problems, such as the overconsumption of nonrenewable resources.

In fact, people's choices can be as crucial as their numbers. "How many people the Earth can support depends in part ... on how many will eat beef and how many bean sprouts," says Joel Cohen, a professor of populations at Columbia and Rockefeller universities in New York City. Lands devoted to cattle yield only about one seventh as much food per acre as croplands.

The rapid growth of cities also puts pressure on the environment, displacing natural habitats and polluting the air and water. At a recent meeting of the American Anthropological Association, Boulder, Colorado, physician Warren Hern compared cities to "a cancer on the planet." When seen from the air, he said, they even look like cancer.

"Some people ask: How many people can the Earth sustain?" says Brian Halweil of the Worldwatch Institute. "The more pertinent question is, what level of human suffering and what level of ecological destruction are we satisfied with?"

IF THE WORLD WERE A CLASSROOM

If your class of 25 students were an exact representation of the world's population:

* 5 students would control 75 percent of the wealth

* 5 would survive on less than $1 a day

* 2 would have cars

* 16 would have access to clean drinking water.

* 20 would live in substandard housing

* 8 wouldn't be able to read

* Probably no one would go to college (only 1 in 100 people worldwide has a college degree)

* No one would own a computer (only 1 in 1,000 people worldwide has one)

* Odds are, you have no doctor (worldwide there's only 1 doctor per 1,000 people)

WORLD POPULATION ROCKETS

From 1804, when the world's population passed the 1 billion mark, it took 123 years to reach 2 billion. It took only 12 years to go from 5 to 6 billion. In the next centuries, growth is expected to slow gradually as fertility rates drop and populations age. The world s population is expected to level off at just under 10 billion around the year 2220.

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With reporting by Malcolm W. Browne, senior science writer for The New York Times.