Last month, the Organization for Economic Cooperation and Development, based in Paris, warned that “urban air pollution is set to become the top environmental cause of mortality worldwide by 2050, ahead of dirty water and lack of sanitation.” It estimated that up to 3.6 million people could end up dying prematurely from air pollution each year, mostly in China and India.

There has been growing outrage in Chinese cities over what many say are untenable levels of air pollution. Cities across the north hit record levels in January, and official Chinese newspapers ran front-page articles on the surge — what some foreigners call the “airpocalypse” — despite earlier limits on such discussion by propaganda officials.

In February, the State Council, China’s cabinet, announced a timeline for introducing new fuel standards, but state-owned oil and power companies are known to block or ignore environmental policies to save on costs.

A study released on Thursday said the growth rate of disclosure of pollution information in 113 Chinese cities had slowed. The groups doing the study, the Institute of Public and Environmental Affairs, based in Beijing, and the Natural Resources Defense Council, based in Washington, said that “faced with the current situation of severe air, water and soil pollution, we must make changes to pollution source information disclosure so that information is no longer patchy, out of date and difficult to obtain.”

Chinese officials have made some progress in disclosing crucial air pollution statistics. Official news reports have said 74 cities are now required to release data on levels of particulate matter 2.5 micrometers in diameter or smaller, which penetrate the body’s tissues most deeply. For years, Chinese officials had been collecting the data but refusing to release it, until they came under pressure from Chinese who saw that the United States Embassy in Beijing was measuring the levels hourly and posting the data in a Twitter feed, @BeijingAir.

Last week, an official Chinese news report said the cost of environmental degradation in China was about $230 billion in 2010, or 3.5 percent of the gross domestic product. The estimate, said to be partial, came from a research institute under the Ministry of Environmental Protection, and was three times the amount in 2004, in local currency terms. It was unclear to what extent those numbers took into account the costs of health care and premature deaths because of pollution.