Source: Xinhua | 2021-06-21 16:30:11 | Editor: huaxia BEIJING, June 21 (Xinhua) -- A Chinese research team has recently revealed the emerging risks of declining oxygen levels in large cities around the world, raising concerns over people's health and the potential for sustainable development in major cities. A research team from Lanzhou University investigated oxygen balances and related risks in 391 cities worldwide with a population of more than 1 million. They studied the oxygen index, the ra...