Seoul was blanketed with toxic haze on Tuesday morning, with air pollution at one point reaching the second worst in the world.

The density of ultrafine or PM2.5 particles reached over 100 ㎍/㎥ per hour in the capital. The air quality in Seoul has hovered between 51 and 100 ㎍/㎥, or even worse for four days. Skies in most parts of Gyeonggi Province were also obscured by toxic haze all morning.

Seoul's air quality index at one point reached 179, the second worst in the world after New Delhi, according to AirVisual, a website that measures and compares pollutant levels in major cities around the world.

A total of 85 ultrafine dust advisories have been issued across the country so far this year, far more than 51 in 2015 and 41 in 2016, according to the National Institute of Environmental Research. Advisories are issued when ultrafine dust levels in the air are higher than 90㎍/㎥ for more than two hours.

The government started issuing advisories in 2015 to raise awareness of ultrafine dust, which the World Health Organization has classified as a first-degree carcinogen.

