The main gas emitted by vehicles is carbon dioxide (CO2), which is produced during the combustion of fossil fuels such as gasoline and diesel. When fuel is burned, the carbon combines with oxygen from the air to create CO2, which contributes to greenhouse gas emissions and is a significant factor in global warming and climate change. In addition to carbon dioxide, vehicles also emit nitrogen oxides (NOx), particulate matter, and unburned hydrocarbons, which can lead to poor air quality and various health issues. Efforts to reduce these emissions include the development of electric vehicles and stricter emissions standards.