Premature deaths related to PM2.5 pollution in 31 major Chinese cities reached 257,000 in 2013, making it a major killer equivalent to smoking, according to a yearlong study released on Wednesday.

The study, conducted by Greenpeace, the environmental protection group, and Peking University's School of Public Health, took each of the 31 major Chinese cities' average PM2.5 concentration and applied a World Health Organization model to estimate health effects.

It focused mainly on four conditions, including lung cancer and stroke, which have been tied to exposure to the fine particulate matter.