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Outdoor air quality has improved since the 1990s, but many challenges remain in protecting Americans from air quality problems. Ground-level ozone, the main part of smog, and particle pollution are just two of the many threats to air quality and public health in the United States.

Join experts from CDC, EPA, NOAA, and the National Park Service Thursday, April 30, at 1:00 pm EDT for a TwitterChat about air quality, physical activity, and health.

Use the hashtag #AirQualityChat in your messages during the chat to join the conversation.