Houston is well known for its oil and petrochemical industries, which are leading contributors to the city's economy. The industries located along the ship channel, coupled with a growing population, have caused a considerable increase in air pollution for the city each year. Houston has excessive ozone levels and is ranked among the most ozone-polluted cities in the United States. Ground-level ozone, or smog, is Houston's predominant air pollution problem. In 2011, Houston was ranked as the 17th most polluted city in the US, according to the American Lung Association. A 2007 assessment found the following twelve air pollutants to be definite risks to health in Houston: