From the article:

In part, the combustion of hydrocarbon fuels is responsible for significant gains in both the quantity and quality of life, and we burn these fuels more cleanly now than at any time in history. However, hydrocarbon fuels do produce trace levels of pollutants, primarily nitrogen oxides (NOx), which society seeks to further reduce. Regulatory bodies all around the world are implementing increasingly strict limitations on a number of pollutants, including NOx.