text: The attack came after Vietnamese Facebook user Vu Khac Ngoc, an online chemistry teacher with almost 350,000 followers, said in a lengthy post that AirVisual was manipulating its data in order to sell air purifiers made by its parent company, IQAir, Reuters said.
  Ngoc did not offer any evidence to back up his claims, but the post - which said the AirVisual ranking would harm tourism in Vietnam and urged people to leave negative reviews of the app - quickly received thousands of shares and likes.
  “We would like to give a heartfelt thank you to the people in Vietnam who have taken the time to share with us their kind support during this time,” AirVisual said.
  The company said it is working with Apple, Google and Facebook to confirm that it has been unfairly attacked, and to make the AirVisual apps available again in Vietnam.
  The Air Quality Index ranking of the world's cities as seen on the AirVisual website on the morning October 7, 2019.
  AirVisual is a Swiss company that tracks air quality in major cities around the world using input data from on-site monitors as well as satellites and remote sensors.
  The government’s Environment Administration last week warned people to limit outdoor activities as the government blamed the pollution on low rain levels and farmers burning rice crop remnants after the harvest to prepare for new plantings. Coal is also widely used for power generation in the country.