UNITED NATIONS (AP) _ The United Nations is using this year's World Population Day to draw attention to the challenges the world faces as the earth's population passes the 6 billion mark.

The U.N. Population Fund is urging governments, schools, foundations and organizations to use Sunday's commemoration to build awareness for the Day of 6 Billion, which will be marked Oct. 12.

``Reaching 6 billion marks a success,'' said the population fund's executive director, Nafis Sadik. ``People today live longer and healthier lives than any generation in history.''

But the 6 billion mark will also be a challenge, particularly for the billion people between ages 15 and 24 who must have the choice to determine the size of their families, she said.

``Their decisions will also help determine how they live _ in poverty or prosperity; on a green and healthy planet or in a world devastated by human activities,'' she said in a statement.

World population growth has slowed, but the globe is still crowded by 78 million more people every year.