China's National Stadium, known as the Bird's Nest, and the National Aquatics Centre, known as the Water Cube, left, are seen through pollution in Beijing, Thursday July 24, 2008. (Greg Baker/Associated Press)Officials in Beijing have assured athletes that the skies over the city will be clear for the Olympic Games next month, but they say efforts to cut smog need more time to be effective.

The deputy director of the Beijing Municipal Environment Protection Bureau told reporters on Saturday that air quality has been deemed acceptable for four days since measures to reduce air pollution were implemented last Sunday.

Du Shaozhong said, however, that air pollution has been serious enough in the last two days to warrant health alerts for seniors, children and those with breathing ailments such as asthma.

Beijing has closed factories and limited car use in order cut to down on smog.

Du said there is still time for the emission-reduction measures to have a significant impact and he guaranteed the air quality would be acceptable for the duration of the Olympics, which begin Aug. 8.

Visibility in the city remained poor on Saturday, CBC journalist Kas Roussy said, despite efforts to cut traffic in half to reduce emissions.