Air pollution in Hanoi and other major cities in Vietnam has hit its worst-ever levels, with the government’s Environment Administration last week warning people to limit outdoor activities.

Pollution could become a key political issue in the Southeast Asian country, where protests have been held against the degradation of the environment.

“AirVisual has received abusive and threatening messages posted on Facebook and on the Apple App Store and Google Play Store,” it said in a statement on Monday.

“Consequently, the AirVisual apps and Facebook page are currently no longer accessible in Vietnam,” it said.

AirVisual had been among the most downloaded apps in Vietnam, and was at one point last week the most-downloaded app on Apple Inc’s App Store store before it was removed.

The attack came after Vietnamese Facebook user Vu Khac Ngoc, an online chemistry teacher with almost 350,000 followers on the website, said in a lengthy post that AirVisual was manipulating its data in order to sell air purifiers made by its parent company, IQair.

Ngoc did not offer any evidence to back up his claims, but the post - which said the AirVisual ranking would harm tourism to Vietnam and urged people to leave negative reviews of the app - quickly received thousands of shares and likes.

AirVisual 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.

AirVisual said its data has helped raise awareness of air quality issues in Vietnam, but this also made it the target of a campaign to discredit the company.

By Khanh Vu & James Pearson - Reuters - October 7, 2019