Four of the top 10 most air polluted cities in the world are located in Iran, with Ahwaz, Sanandaj, and Kermanshah ranking among the top 6. High levels of air pollution in Iran are largely caused by coal burning and vehicle emissions, especially in Tehran. Air pollution has negatively impacted public health in Iran, resulting in school cancellations and increased carbon monoxide levels. Sanctions have also made it difficult for Iran to improve air quality.