They found that over a period of six years, planting three rows of trees reduced total dust and ammonia by more than half.

VOA: special.2009.09.08

The rock formations there are the result of deposits made by ancient lakes and rivers over a period of about twenty million years.

VOA: special.2009.04.15

Volunteers received vaccinations over a period of six months and were tested for H.I.V.

VOA: special.2009.09.30

"The Lost Symbol" takes place in Washington,D.C., over a period of just twelve hours.

VOA: special.2009.10.09

Instead,slow earthquakes release their energy over a period of hours or even days.

VOA: special.2009.07.07

It also can develop slowly, over a period of days.

VOA: special.2010.07.27

And over a period of time her grades fell completely.

VOA: special.2010.04.22

Volunteers received vaccinations over a period of six months.

VOA: special.2009.10.13

over a period of three years.

VOA: special.2009.07.17

The Iraq surge involved about 20,000 U.S.combat troops over a period of a year and a half.

VOA: standard.2009.11.04

But in a recent study they declared that forty percent of the tweets captured over a two-week period were "pointless babble."

VOA: special.2009.09.14

Medical experts say small amounts of carbon monoxide over a long period of time can greatly harm an unborn baby.

VOA: special.2010.02.02

Researchers in the United States say drinking a lot of alcohol over a short period may influence genes involved in bone formation.

VOA: special.2010.08.31

The researchers studied the diets of thirty-one thousand children ages two to eighteen over a thirty-year period.

VOA: special.2010.03.17