Grass and vegetation fires have added to air pollution which is still blighting parts of Thailand.

The hillside blaze spread through the mountainous area of Chiang Mai, northern Thailand on Thursday (February 19) afternoon.

The provincial governor Charoen Sanguansat arranged 150 firemen and locals to battle the fire. A helicopter later arrived to help quell the flames.

The two mile long line fire was successfully extinguished on Friday (February 20) afternoon but smoke smothered the area.

It increased the micro dust concentration in the area until it reach unhealthy level of 199 microgram per cubic on Saturday (February 21) morning.

The provincial governor warned the locals to avoid outdoor activity. He said: "The fire that was extinguished directly effected the air quality in the area.

"Now the micro dust concentration has increased to the unhealthy level the locals should avoid outdoor activities until the pollution problem will be better."

Meanwhile, Bangkok was ranked the 24th worst city in the world for air quality as of 7.20am on Saturday (February 22).

Reading on the application ‘Air Visual’ showed the Thai capital's average pollution level at 144. The air situation is much worse in Chiang Mai, with average pollution level of 157, making it the 12th worst city for air quality. 

The amount of particulate matter less than 2.5 microns in diameter (PM2.5) in Bangkok and Chiang Mai was at 41 and 66.7 micrograms per cubic metre respectively. 

According to the Air Quality Index (AQI), air quality between 0 to 50 is healthy, 51 to 100 is moderate, 101 to 150 is unhealthy for sensitive groups, 151 to 200 is unhealthy, 201 to 300 is very unhealthy, and 301 to 500 is dangerous.