The amount of air pollutants in China’s bustling commercial center of Shanghai were reportedly ten times higher on Monday than the level considered safe by the World Health Organization, leading officials to issue a warning telling the elderly and children to say indoors.

According to a report in Bloomberg, the official warning comes one day after 35,000 athletes participated in the annual Shanghai International Marathon. Some runners reportedly wore face-masks during the race, while others complained of lung pain after finishing the grueling event.

While China has been commended for decades of explosive growth that has pulled hundreds of millions out of poverty, experts warn that the government must address ongoing environmental damage caused by the country’s economic revival.