A total of 30 subway stations in the capital city of Seoul contained higher-than-allowed levels of cancer-causing substances over the last decade, the municipal government said Monday.

The city’s regular checks between 1998 and 2004 found that the level of radon in 30 out of 285 stations in Seoul from Line No. 1 through 7, including Gwanghwamun Station in downtown Seoul, exceeded the standard of 4 picocuries at least once, according to the Seoul Metropolitan Government.

A picocurie is one trillionth of a curie, which is a standard measure for the intensity of radioactivity contained in a sample of radioactive material.

Radon is a radioactive gas that is chemically inert and naturally occurring. It is known as one of the main sources of ionizing radiation, causing tens of thousands of deaths from lung cancer each year globally, according to the World Health Organization.

In the wake of the revelation, the city took measures and was able to bring the concentration under control, its officials said.

“We put the 30 stations on the list of special management, and took such measures as implementing required facilities to prevent the inflow of the gas into the stations from outside as well as boosting ventilation,” said Choi Young-soo, a city official in charge of the air quality management.

The city has also increased monitoring of the 30 stations in order to keep the radon at a safe level, along with 24 other stations whose radon levels are close to exceeding the safe level, he added.

Around 6.67 million citizens used the city’s subway per day in 2013, according to government data.

(Yonhap)