The number of ozone alert days in Seoul posted a ten-fold increase over the past decade. 

According to the city government on Wednesday, the number of days when the ozone alerts were issued in the city jumped to 31 through August this year, up from just three days in 2015.

The total number of ozone alerts issued in the city also soared from four to 109 over the period. 

The yearly average ozone concentration level rose to zero-point-037 parts per million(ppm) this year, up from zero-point-024 ppm in 2015.

During the high-concentration period from May and August, the average ozone level in Seoul reached zero-point-044 ppm this year, the highest since records began in 2000. 

High ozone concentrations can affect the senses and respiratory system. The elderly, children, and patients with respiratory diseases are advised to refrain from going outside when the alert is issued.