Only 10 countries or territories last year had air quality that met the World Health Organization’s standard for clean air. PM2.5 is a harmful component of air pollution that comes from a variety of sources, including emissions from coal and oil burning as well as dust storms and wildfires. The WHO says PM2.5 can penetrate through the lungs and further enter the body through the blood stream. Central and South Asia contain the top four most polluted countries in the world.