Car and coal-fired power plants account for all of the state's ozone causing nitrogen oxide. To remedy this, the North Carolina General Assembly passed the Ambient Air Quality Improvement Act of 1999. The law expands the current automobile emissions testing program and the number of counties using the program. By 2006, forty-eight counties having 80 percent of all the cars and trucks on the state's roads will be required to use this emissions test. The law is not without its detractors, however.