Doctors say that over time the damage interferes with the natural exchange of oxygen and carbon dioxide in the lungs.

VOA: special.2010.06.23

Natural gas is considered the cleanest of the fossil fuels, the fuels created by plant and animal remains over millions of years.

VOA: special.2009.10.23

UNESCO's World Heritage list includes over nine hundred natural or cultural places considered to have universal value.

VOA: special.2010.11.03

A blue-gray haze from natural and now manmade sources hangs over them like smoke.

VOA: special.2009.09.28

"I'm Cari Corrigan. I'm a geologist over at the Natural History Museum.

VOA: special.2010.07.28

The Russian national budget relies heavily on taxes from the sale of oil and other natural resources, and oil prices have dropped sharply over the past year.

VOA: standard.2009.05.25

"It's natural that in parliament people agree or disagree and negotiate and bargain, especially over election laws, which are the most major laws that a parliament passes."

VOA: standard.2009.10.26

Energy has become a major concern in several Western countries, after a dispute between Ukraine and Russia over prices led to major natural gas shortages in Europe earlier this year.

VOA: standard.2009.10.23