The capital city of Seoul saw its first ozone warning of the year take effect on Thursday.

The Seoul Metropolitan Government said the warning was issued at 3 p.m., as ozone density in the northern part of the city averaged 0.131 ppm per hour.

The warning is issued when the density of the powerful oxidants is more than 0.120 ppm.

“We measure the density in five regions of the city, and even one area detected to exceed that point leads to the issuance (of the warning) for the whole city,” said Kim Hyun-sik, chief of the city’s climate department.

“Patients with respiratory diseases, the elderly and children are advised not to go outside, and people on the whole are asked to use public transportation rather than driving,” he added.

Thursday marks the hottest day in Seoul so far for the year, with its daily high soaring above 30.2 degrees Celsius. (Yonhap News)