Most of China's big cities failed to adequately make pollution data public last year, a new study indicates.

The Pollution Information Transparency Index, an annual ranking of 113 Chinese mainland cities, indicates that a total of only 19 of the 113 cities achieved a passing score of 60 out of 100 points for 2011.

More than 10 Chinese cities scored less than 20.

While the average transparency score was 40, it is an improvement compared to an average of 31 in 2008 when the study -- jointly developed by Beijing's Institute of Public and Environmental Affairs and U.S. environmental group Natural Resources Defense Council -- was first conducted. In 2009 and 2010, the average score was 36.

More than 500 enterprises provide information to environmental NGOs about their pollution monitoring and disclosure systems, up from none five years ago, the study shows.

Still, the authors say, thousands of companies have yet to take the necessary steps.

"There is plenty of room to improve but we are seeing progress every year," Bernadette Brennan of the National Resources Defense Council told The Guardian newspaper.

"On the whole the trend is toward open information. More people realize this is good for society and good for business."

China, the world's top emitter of greenhouse gases, passed the first national regulation on information disclosure in May 2008.

But Zhu Xiao, an associate professor of law at Renmin University of China, who was involved in designing the index, said it was "appalling" that most cities are still not following the information disclosure law more than three years after it was passed.

"The so-called progress so far has been way too slow," he said in a South China Morning Post report.

Beijing was ranked seventh in the transparency index, moving up from 31st place in 2010.

Ningbo in Zhejiang province and Shenzhen in Guangdong both scored over 80.

But some major polluting provinces, such as Shandong and Sichuan and the Inner Mongolia autonomous region, were found to have reneged on their commitment to disclose the pollution data. For example, the study indicates that the ranking for Shandong, the country's biggest emitter of sulfur dioxide and nitrogen oxide, dropped from 12th in 2008 to 17th last year.

"We strongly urge major cities in Shandong to strike a balance between economic growth and environmental protection," said Ma Jun, director of the Institute of Public and Environmental Affairs.