More than 80 per cent of people living in urban areas are exposed to air quality levels that exceed the World Health Organization limit. While all regions of the world are affected, populations in low-income cities are the most impacted. About 98 per cent of cities in low- and-middle-income countries with more than 100,000 inhabitants do not meet WHO's air quality guidelines. However, in high-income countries, that percentage decreases to 56 per cent.