Possible Clues:

• Environmentalist's concern ... or a hint to the circled letters
• Chemical agent potentially causing global warming
• The greatest threat, according to the WHO, to human health this century
• What Congress needs in order to get anything done?
• Environmental concern
• Volatile chemical agent is environmental concern
• Title of puzzle
• Presidential debate topic, or a hint to the starred answers' indicated letters
• Mat crawling with lice, suggesting a worrying trend?
• *Environmentalist's concern