阅读下面短文，根据所读内容在文后的空格里填上适当的单词或短语。注意：每空不超过三个单词。

　　For years scientists have been worried about the effects of air pollution on the earth’s natural conditions．Some believe the air inside many houses may be more dangerous than the air outside．It may be one hundred times worse．

　　Indoor air pollution can cause a person to feel tired, to suffer eye pain, headache and other problems．Some pollutants(污染物)can cause breathing disorders, diseases of blood and even cancer．Most scientists agree that every modern house has some kind of indoor pollution．

　　People have paid more attention to the pollution now．It is said that when builders began making housses and offices they did not waste energy．To do this they built buildings that limited the flow of air between inside and outside．They also began using man-made building materials．These materials are now known to let out harmful gases．

　　As the problem became more serious,scientists began searching for a way to deal with it．They discovered a natural pollution control system for building-green plants．Scientists do not really know how plants control air pollution．They believe that a plant’s leaves absorb or take in the pollutants．In exchange, the plant lets out oxygen through its leaves and small organizations on its roots．Scientists suggest that all buildings should have one large plant or several small plants inside for every nine square metres(平方米)of space．Studies of different plants show that each absorbs harmful chemicals．So the most effective way to clean the air is to use different kinds of plants．Having green plants inside your house can make it a prettier and healthier place．