根据以下资料，回答以下题。   Many people use wood stoves and fireplaces to heat their homes．Scientists have become worried about the smoke that they give off．Harmful chemicals are in the smoke．The smoke is causing air pollution．The pieces of pollution, called soot, are floating in the air．They are too tiny to see．Scientists must use a microscope to see them.
  Small amounts of soot are safe, but large amounts can be dangerous．The govenment wants to limit this kind of pollution．It may stop people from using their fireplaces.
  The air is tested every day．When soot levels are high, more than 65 micrograms of soot per cubic meter, factories must stop making smoke．The government thinks that limit should be much lower.
Factories produce the most smoke and soot．But the government thinks that all types of burning should be limited．The soot levels from factories and homes could be limited．Home owners may have to install new wood stoves that they comply with the new law．Or, they may not be able to bum on days when the air quality is bad.
  Residents of some towns are fined if they violate the burning ban．Scientists hope these new regulations will make the air cleaner and less harmful to breathe.