Record levels of air pollution in Shanghai delayed or cancelled hundreds of flights on Friday, Reuters reports.

The poor air quality on Dec. 6 also caused the Shanghai government to issue the highest level of public health warning as the Chinese city’s pollution index ranged between 23 times and 31 times the recommended levels.

Chinese authorities advised children, the elderly and the sick to stay at home, and told others reduce their outdoor activities as much as possible, the South China Morning Post reports. Shanghai also ordered vehicles off the road and told factories to cut production, Bloomberg reports.

The incident happened just days before EPA administrator Gina McCarthy arrived in China to discuss US-China cooperation on air quality, climate pollution and environmental issues.

Earlier this month Beijing became the third Chinese city to launch a carbon trading program while the world’s second-biggest carbon market will start this month with the launch of a cap-and-trade system in China’s Guangdong province.

China has a strong policy framework in place to transition to a green economy, but significant challenges — including pollution problems — stand in the way, according to a study sponsored by the United Nations Environment Programme and the Chinese Ministry of Environmental Protection.