An open letter signed by 23 NGOs urged environmental agencies at all levels on Thursday to disclose more information to the public about sources of pollution.

"So far, 2013 has seen a leap forward in open environmental information, as 74 cities are providing real-time air quality readings so people can protect themselves from airborne pollution, especially from fine particles," the letter said.

"But to make a real change, we need to actively reduce emissions rather than simply covering ourselves up."