Air pollution in northern Thailand has been ranked among the worst in the world by a Swiss air quality monitor.

The air quality index (AQI) for Chiang Mai reached 161 or 'unhealthy' levels today, with a PM2.5 level of 75.4 micrograms per cubic metre, Swiss company IQAir reported this morning.

Chiang Mai had placed eighth in IQAir's hourly rankings, with Baghdad in Iraq topping the list at an AQI of 241.

IQAir said on its website that the AQI 'is a measurement of air pollutant concentrations in ambient air pollution and their associated health risks.'

The index measures six air pollutants, including PM2.5, PM10, carbon monoxide, sulfur dioxide, nitrogen dioxide, and ground-level ozone.

Thai media reported that the air quality in Chiang Mai had slightly improved last week after rain hit the province, but were still beyond the 'safe' threshold.

Continued agricultural burning and forest fires caused the fine dust levels to spike once again.

Many of Thailand's northern provinces, including Phayao, Phetchabun, Uttaradit, Chiang Rai, and Phrae are seeing 'unhealthy' AQI levels, according to IQAir rankings.

Three new hotspots have also been detected in Chiang Mai today.

Scientists found that the smog is at its worst in the country when farmers burn waste crops to cheaply clear their land between late October and April. This is combined with high numbers of polluting cars on the road and large construction projects across the capital.

The seasonal lack of wind and rain, combined with a winter low-pressure system in which cold air is trapped closer to the ground by warm air above – prevent the pollution from being dispersed naturally.

The Thai government has been criticised for failing to tackle the country's air pollution crisis, with bizarre measures such as spraying water into the air and giant purifiers having no impact.

Data scientist engineer Worasom Kundhikanjana said: 'PM 2.5 pollution has a seasonal trend in Bangkok and the northern provinces, but has only recently received public attention. During this winter haze, the harmful particle pollution level is high throughout the day, including on weekends.

'Unfortunately, spraying water into the air does not appear to be effective, since the volume of water is minuscule compared to actual rain.

'With the start of the monsoon season, the weather in Bangkok will get better, and public attention on this issue will likely fade. However, without immediate policy measures, the problem will come back again next winter.'