Outdoor air pollution leads to millions of premature deaths every year, but it is a tale of two cities. While air pollution is increasing mostly in developing countries, people in the US, China and many parts of Europe are breathing the cleanest air in decades. As countries develop, air quality tends to deteriorate due to burning more fossil fuels but then improves once those countries can afford pollution-reducing technologies and regulations. The graph below focuses on nitrogen oxides (NOx), one of four gaseous pollutants included in the World Health Organization’s Air Quality guidelines. NOx emissions have been declining for decades in the US, the UK and Germany. China has managed to turn the trend in the early 2010s, but India hasn’t yet. Transportation is a major contributor to NOx emissions and stricter regulations for combustion engines have successfully forced carmakers to innovate and reduce emissions.
Source: Hoesty et al., Community Emissions Data System, Our World in Data, February 2025.