Nitrogen dioxide (NO₂) is a reddish-brown gas with a characteristic sharp, acrid smell. It is a significant air pollutant, primarily produced from burning fossil fuels, such as in vehicles and power plants. NO₂ can contribute to the formation of ground-level ozone and fine particulate matter, which can harm human health and the environment. In the atmosphere, nitrogen dioxide plays a role in the formation of acid rain and can lead to respiratory problems in humans. It is monitored by environmental agencies to ensure air quality standards are met, as prolonged exposure can cause serious health issues, particularly for those with pre-existing conditions.