It can hurt kids' lungs and cause heart attacks in adults. According to one study, the risk of cancer in Los Angeles is still hundreds of times higher than it would be if the air met government standards. Another big problem partly caused by cars is global warming. The burning of fuel in all cars, even new ones that don't pollute very much, puts a gas called carbon dioxide in the air. Carbon dioxide isn't pollution.
Plants use it for photosynthesis.
But the millions of cars, trucks, buses, factories and powerplants running all over the world are making much more carbon dioxide than would be in the air normally. The extra carbon dioxide helps trap the sun's heat on earth. The whole planet is heating up because of it. A few scientists say that global warming isn't a big problem. But most think it's very serious.