It functions by filtering exhaust from a combustion engine in a vehicle for potentially harmful gases and catalyzes reactions to transform them into less harmful chemicals such as water, carbon dioxide, and nitrogen.[9] Also, reducing the amount of lead in gasoline did directly correlate with a decrease in blood lead content during a four year period in the 1970s.[8] Although continuous measures have been taken to reduce the emissions content due to cars, they still rank unfavorably compared with other forms of travel.[10]