The effects of air pollution on natural conditions have been a disturbing problem for many years. Some scientists hold the view that the air inside houses may be more dangerous than the air outside. It may even be one hundred times worse. Most scientists agree that every modern house has some kind of indoor pollution, which can cause a person to feel tired, to suffer eye pain, headache and other problems. Believe it or not, some pollutants can cause breathing diseases of blood and even cancer. What made us feel relieved is that there's growing concern about the problem now. It is true that when builders began constructing houses and offices they did not want to waste energy. they built buildings that prevented or limited the flow of air between inside and outside. For the same purpose, man-made building materials were employed to build houses, which are now known to let out some harmful gases. Facing the serious challenge, scientists have been searching for a way to handle the problem. To everyone's delight, they discovered a natural pollution control system for building--green plants, though they do not really know how plants control air pollution. Scientists suppose that a plant's leaves absorb or take in the pollutants and in exchange gives out oxygen through its leaves and tiny organizations on its roots. Therefore they recommend that all buildings should have one large plant or several small plants inside for every nine square meters of space. Because studies indicate that different plants absorb different chemicals, the most effective way to clean the air is to use different kinds of plants. In conclusion, having green plants inside your house can make it a prettier and healthier place. Why not try? According to scientists, some kind of indoor pollution can be found in    _   .
every old building every modern house all kinds of houses all kinds of offices