2014, Lanzhou officials advised residents not to drink tap water, because benzene levels were 20 times the national limit of 10 micrograms per liter. The city water supply suspected industrial chemical production to be the culprit, similar to what happened in the 2005 Jilin chemical plant explosions. A brucellosis outbreak infecting humans took place in Lanzhou in 2020 after the Lanzhou Biopharmaceutical Plant involved in vaccine production accidentally pumped out the bacteria into the atmosphere in exhaust air, due to use of expired desinfectant, and affecting over 3,000 people. Earthquakes Lanzhou experiences earthquakes regularly, although usually at low intensities. In 1920 a large earthquake was experienced killing more than 100,000 people in Ningxia and Eastern Gansu province, although only 42 were killed in Lanzhou itself, the low number being attributed to the strong yet flexible nature of the wooden buildings in the city. Lanzhou was also affected by 2008 Sichuan earthquake in 2008. Demographics As of the 2020 Chinese census, Lanzhou surpassed 4 million population for the first time, with a total population of 4,359,446 inhabitants and 3,042,863 in the built-up (or metro) area of 1,112.9 square kilometres (429.7 sq mi) made of the 4 central urban districts, Lanzhou Xin, near the airport, growing very quickly and Honnggu Districts not yet conurbated. This marked a significant growth after a period of slower growth. The strong growth has been attributed to parents seeking better education opportunities for their children, retired migrant workers returning to Gansu, improved services in the city and government policies. Sport The 14,000-capacity Northwest Minzu University Stadium is one of the main sports venues in the city. It is mostly used for football games. A new sports center complex, including a stadium with a capacity of 60,000 spectators and a swimming hall, is under development. Lanzhou previously had a professional soccer team named Gansu Tianma from 1999 to 2003.The team played in Chinese Football Association Yi League from 1999 to 2001 and bought a position in the Jia League from Tianjin Lifei. The team relocated to Ningbo, Zhejiang and changed their name to Ningbo Yaoma in 2003. The team later relegated to China League Two in 2004 and became Dongguan Dongcheng after moving to Dongguan, Guangdong. Former England international Paul Gascoigne played four games in both a playing and coaching role for Gansu in 2003, scoring two goals, before returning to England after falling out with the club,