NOx, an abbreviation for oxides of nitrogen, is a type of emission produced by internal combustion engines that leads to the formation of ground-level ozone. Ground level ozone is a component of what is commonly called smog. Ground-level ozone forms when NOx interacts with volatile organic compounds (VOCs), also a product of combustion engine emissions.
The U.S. Environmental Protection Agency (EPA) has more information on ground-level ozone and emissions sources on its website. The EPA regulates the levels of production of NOx emissions produced by different sources.
The EPA's statistics show that motor vehicles emitted 6,491,821 tons of ground-level ozone in 2005. Off-road equipment emitted 4,162,872 tons that year. The next highest sources were electricity generation, at 3,783,659 tons, and fossil fuel combustion (outside of vehicles) at 2,384,297 tons. VOCs from motor vehicle emissions and off-road equipment are the second and third-highest sources, according to EPA statistics, at 4,112,147 tons and 2,843,213 tons in 2005, respectively. Solvents were number one, at 4,245,897 tons.