Related Clues:

• It's found in stacks
• Smudge on Santa
• Urban pollution
• Haze
• Industrial pollutant
• Visibility problem
• Chimney buildup
• Santa's laundry problem
• Air pollutant
• Cough cause
• Flue blackener
• Big city haze
• Flue dirt
• It might hang over your head
• It might make you short of breath