A spokeswoman for the mayor's office, Joanna Dovan,says the Pittsburgh area has been working to improve its air quality.

VOA: special.2009.07.13

The Department of Agriculture, for example, provides biologists to help study wildlife that could be a danger to air travel.

VOA: special.2009.04.01

Critics accused European air transport officials of being slow to react, then overreacting to the possible risk to airplanes.

VOA: special.2010.04.23

When melting snow crystals or raindrops fall through very cold air, they freeze to form small particles of ice,called sleet.

VOA: special.2010.01.05

When melting snow-crystals or raindrops fall through very cold air, they freeze to form small particles of ice, called sleet.

VOA: special.2011.01.11

I knew the sky was black because all the heat, which meant light to me, had died out of the air.

VOA: special.2009.11.15

This gives them their color. However,when light travels from air to snow, some light is sent back,or reflected.

VOA: special.2010.01.05

Because of the open-air stage, actors had to shout very loudly and make big motions to be heard and seen by all.

VOA: special.2010.01.06

It says the aim is to bring air guitarists out of the house where they secretly perform and onto the world's stage.

VOA: special.2009.08.14

For this reason,he says, air searches would need to be done in areas where the climate is dry or plants lacking altogether.

VOA: special.2009.11.17

Untreated patients can spread the bacteria from their nose and mouth through the air to people who are near them a lot.

VOA: special.2009.10.14

To play "air guitar" you move to the music as if you are playing an imaginary guitar, maybe like a famous rock musician.

VOA: special.2009.08.14

The effort to stand carried me into the Martian air to the height of about one meter.

VOA: special.2010.07.10

As asthma sufferers struggle to get air into their lungs, they may begin to cough a lot.

VOA: special.2009.07.28