By Michael Bristow
BBC News, Beijing
Air pollution in Beijing will not significantly improve before next year's Olympic Games, a United Nations report suggests.
Beijing is often shrouded in smog
In some cases, pollution is said to be more than three times the safe limits set by the World Health Organization.
The report seems to contradict claims from Beijing Olympic officials that air quality will not be a problem.
International Olympic Committee President Jacques Rogge said events may be postponed if pollution was too bad.
The report, written by the UN Environment Programme, reviews Beijing's environmental commitments related to next year's Olympics.
While UNEP officials praise the Chinese government for achievements already made, they say air pollution remains a "legitimate concern".
The 163-page report states that, between 2000 and 2006, the Chinese authorities reduced the concentration of some key air pollutants such as sulphur dioxide and carbon monoxide.
But that downward trend came to a halt in 2006 when the levels of four major pollutants rose or ceased to decline, it adds.
The report also has grim findings about the concentration of particulate matter, which comes from construction sites, coal-burning boilers and dust storms.
This pollutant is at about the same concentration level as it was in 2000, and at certain periods is three times above the WHO safe limit.
UNEP spokesman Eric Falt said Olympic organisers, athletes, spectators and Beijing residents had a right to be worried.
But he refused to spell out more clearly exactly what the UNEP figures mean for next year's Olympics.
"We have said it has been a concern for a long time, but I do not want to go beyond what has been said," he told the BBC.
But it seems clear from the report that air pollution will not significantly improve before the Olympics begins next August.
"It may take years to determine significant changes in air quality. Relevant progress may be evident only in the medium to long term," the report says.
This point was reinforced by Mr Falt, who said only long-term planning and proper enforcement could solve the problem.
The UNEP report seems to contradict comments made by Beijing officials.
Du Shaozhong, Beijing's head of environmental protection, said in August: "I am sure we will be able to ensure good air quality during the Olympic Games."