On iOS devices, AirVisual ranked second among the most downloaded apps by Vietnamese people in the free app category, and 41st on the Android platform as of Sunday.

In the weather app category in Vietnamese, AirVisual topped the list of most downloaded apps last week on both iOS and Androi platforms.

Data from analytics website SimilarWeb, updated until last Wednesday, showed that AirVisual ranked 18th among top 20 most downloaded iPhone apps by Vietnamese online users with 6,000 downloads.

Worsening air quality, manifest in recent days in Hanoi and Ho Chi Minh City as a thick haze, has become a matter of great worry for their residents.

"Every day I have to open this app before I go out. Thanks to AirVisual, I can adjust my outdoor activities, depending on the air quality and will be proactive in protecting my health," said one user, Hong Quang.

Many Vietnamese users have also said that downloading the app gives them information on air quality in their neighborhood so that they can take necessary measures to protect their health and that of their relatives, especially the elderly and young children.

According to the Switzerland-based air quality monitoring facility IQAir AirVisual, all air quality indexes displayed on the app are collected based on real time. Pollutants are measured and based on the U.S. Air Quality Index value.

The application collects data from 14 air control stations in Hanoi and seven stations in Ho Chi Minh City.

At 9 a.m. on Tuesday, HCMC's air quality index (AQI) was measured at an "unhealthy" 159, while Hanoi's index was 254, a level ranked "very unhealthy" where people with respiratory or heart diseases will be significantly affected and will experience reduced endurance in activities.

On Monday morning, Hanoi took the top spot for the city with worst air quality in the world in a ranking of more than 10,000 cities worldwide, when its AQI stood at 213. 

An AQI level above 100 is considered polluted or unhealthy for humans.

The city's level of PM2.5 or super fine particles, a fraction of the width of a human hair released from vehicles, industry and natural sources like dust, was 102.2 microgram per cubic meters (μg/m3) of air by Friday morning. The World Health Organization Air Quality Guideline recommends an annual mean exposure threshold of 10 μg/m3 to minimize health risks. Vietnam's national PM2.5 threshold is 25 μg/m3.

Louise Watt, a spokeswoman of IQAir AirVisual, said it provides real-time air quality indices to help the public protect themselves and raise awareness over the matter.

When people are aware that the air quality is bad for their health, they will demand actions to fix the problem, as it has happened in other cities like Beijing, she said.