An air quality monitor shows pollution figures on a street in Hanoi, Vietnam May 18, 2018. Picture taken May 18, 2018. REUTERS/Kham “I usually joke with my friends, the more polluted the air is, the more prosperous I get,” said Cao Xuan Trung, a Hanoi dealer in air purifiers, who expects monthly revenue to double by 2020, from 3 billion dong ($131,199) now, a value…