Independent online air quality index monitor AirVisual said on Monday it is under coordinated attack to discredit the company after its data showed Vietnamese cities among the world’s places worst hit by air pollution.

Since Sunday evening, smartphone users in Vietnam have been unable to search for the AirVisual app on both their iOS and Android devices.

The app appears to function normally on devices that already have it installed.

The official Facebook page of AirVisual has also become inaccessible from the Southeast Asian country.

Owing to an episode of serious air pollution in recent weeks, Hanoi has at times topped the list of AirVisual’s ‘major city ranking,’ a live pollution ranking of around 90 major global cities.

“This has helped to raise awareness of air quality issues in Vietnam,” AirVisual said in a statement published on its website on Monday.

“However, AirVisual has also become the target of a coordinated campaign in Vietnam to discredit us.

“AirVisual has received abusive and threatening messages posted on Facebook and on the Apple App Store and Google Play Store.

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

AirVisual had been among the most downloaded apps in Vietnam, and was at one point last week the most-downloaded app on App Store before it was removed, Reuters reported.

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.

The post appears to have been removed or hidden as of Tuesday morning.

“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.

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.

It gives pollution ratings in the form of an air quality index (AQI), which considers up to six main pollutants - PM2.5, PM10, carbon monoxide, sulfur dioxide, nitrogen dioxide and ground level ozone - and translates them into an “easy-to-understand scale” to clearly represent the health risk posed by ambient air pollution, according to AirVisual.

The index ranges from 0 to 500, where high index values indicate higher levels of air pollution and higher potential for adverse health effects.

AirVisual’s data have been frequently cited by Vietnamese news outlets in recent weeks as air quality in Hanoi and Ho Chi Minh City - Vietnam’s two largest cities - has been on a serious decline since September.

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.

Last Wednesday, Vietnamese Prime Minister Nguyen Xuan Phuc called on authorities in Hanoi and Ho Chi Minh City, Vietnam’s main economic hub, to do more to address air pollution problems.

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