BANGKOK, March 22 (Xinhua) -- Northern Thailand has been blanked by hazardous dust recently leading to a decline of visitors.

The PM 2.5 pollutants in the region have reached extremely hazardous levels for almost a week. The PM 2.5 readings in the north stood at 76 to more than 300 micrograms per cubic meter. The maximum safe threshold is 59 micrograms per cubic meter.

The worst reading of air quality was recorded in Chiang Rai on Friday. The air quality index of the province stood at 316.

Local officials said the number of visitors to Tham Luang cave complex, one of Chiang Rai's famous destinations, has decreased by half from around 3,000 daily to 1,500 following reports about the haze.

Forest fires have sent air quality in nine provinces to a level considered harmful to people's health, as cities in the north like Chiang Mai registered air quality that was among the world's worst last week.

Provincial authorities have been strictly controlling the burning of rubbish, farm waste and clearing of scrubland to ease the air pollution.