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Hong Kong (Chinese: ??, Cantonese: hœ???.k????), officially the Hong Kong Special Administrative Region of the People’s Republic of China, is an autonomous territory on the eastern side of the Pearl River estuaryin East Asia, south of the mainland Chinese province of Guangdong, and east of the special administrative region of Macau. With over 7.3 million Hongkongers of various nationalitiesnote 1 in a territory of 1,104 km2, Hong Kong is the fourth-most densely populated region in the world.Hong Kong was formerly a colony of the British Empire, after the perpetual cession of Hong Kong Island from Qing China at the conclusion of the First Opium War in 1842. The colony expanded to the Kowloon Peninsula in 1860 after the Second Opium War, and was further extended when Britain acquired a 99-year lease of the New Territories starting in 1898. The Empire of Japan attacked the city as part of a co-ordinated offensive against the Allied Powers at the start of the Pacific War and held it under military occupation for the duration of the war, until the British resumed control of the colony in 1945. The entire territory, including ceded areas, was returned to China under the framework of the Sino-British Joint Declaration, signed by the United Kingdom and China in 1984 and marked with the transfer of sovereignty over Hong Kong at the end of the New Territories lease in 1997, when it became a special administrative region of the People’s Republic of China.16Under the principle of “one country, two systems”,1718 Hong Kong maintains a separate political and economic system apart from China. Except in military defence and foreign affairs, Hong Kong retains independent executive, legislative, and judiciary powers.19 Nevertheless, Hong Kong does directly develop relations with foreign states and international organizations in a broad range of “appropriate fields”,20 being actively and independently involved in institutions such as the Asia-Pacific Economic Cooperation forum and the World Trade Organization.2122Hong Kong is one of the world’s most significant financial centres, holding the highest Financisal Development Index score and consistently ranking as the world’s most competitive and freest economic areas.2324 As the seventh-largest trading entity in the world,2526 its legal tender, the Hong Kong dollar, is the 13th-most traded currency.27 Hong Kong’s tertiary sector dominated economy is characterised by competitive simple taxationand supported by its independent judiciary system.28 Although the city boasts one of the highest per capita incomes in the world, it suffers from severe income inequality.29The territory features the most skyscrapers in the world, surrounding Victoria Harbour, which lies in the centre of the city’s dense urban region.3031 It has a very high Human Development Index ranking and the world’s longest life expectancy.32 Over 90% of its population makes use of well-developed public transportation.33 Seasonal air pollution with origins from neighbouring industrial areas of mainland China, which adopts loose emissions standards, has resulted in a high level of atmospheric particulates in winter.343536