The most important components of air pollution are particulate matter, typically measured as PM2.5, nitrogen oxides or NOx, sulfur oxides, primarily sulfur dioxide (SO2), and carbon dioxide (CO2). The combustion of wood and other non-coal solid fuels are responsible for more than half of current PM2.5 emissions. Coal combustion is the largest source of global emissions of SO2. Diesel oil and gasoline combustion in vehicles are the dominant source of NOx. Natural gas combustion produces little SO2 and PM2.5 emissions, and about 10% of global emissions of nitrogen oxides which is less than coal’s contribution. Coal is the largest source of CO2 emissions followed closely by oil product combustion for transportation. Natural gas is responsible for significant greenhouse gas emissions but is a cleaner source of energy than coal even when methane loss from production and distribution, estimated at 1.7% of global natural gas production, is included.