The World Health Organization’s recommended exposure 25 micrograms per cubic meter while the levels in China's capital, Beijing, have reached 300 micrograms per cubic meter. PM2.5 particles lodge deep inside the lungs and are considered the most dangerous kind of air pollution to human health. China has for years been hit by heavy air pollution, caused by enormous use of coal to generate electricity to power a booming economy, and more vehicles on the roads. Environmental groups are also increasing the pressure on the authorities in Beijing. Greenpeace Asia projected a huge message reading “Blue Sky now!” on to the front of Drum Tower, a historic building to the north of the historic center of Beijing, the Forbidden City.