Dental decay, also known as dental caries, begins first inside the tooth. A white spot appears on the enamel where the tooth has started to weaken inside. At this stage, the tooth can repair the weakened area with the help of fluoride and minerals in saliva.
Beijing, also Peking, is the capital of the People's Republic of China and one of the most populous cities in the world. Its population in 2013 was 21,150,000. The city proper is the 3rd largest in the world.
This is greater than the highest density major urban area today, Dhaka (Bangladesh) at 90,000 per square mile or 35,000 per square kilometer. Today's urban Beijing may have an overall population density one-eighth that of the 1950s.