The city experienced medium to heavy air pollution levels for 11 days during the first quarter of the year, two day more than last year's total.

"The situation is very serious," Ji said. It's a challenge to meet the target of this years air quality controls.

Sand, road construction and pollutant discharges are the main reasons for the worsened air quality. More than 50 percent of construction sites don't reach environmentally friendly standards.

A survey by the Beijing Environmental Protection Bureau said that a big part of the pollutants came from construction sites, which produce large levels of dust and sand.

The bureau plans to control pollutant emissions from vehicles, construction sites and roads by issuing more frequent checks. Any construction site that can't reach the standard will be shut down.

It will also launch anticipatory projects to decrease the amount of construction sand storms and will be notifying residents prior to onset projects.

Beijing needs to build stadiums and gymnasiums to prepare for the 2008 Olympic Games, so the increasing construction sites reduce the air condition. However, it has to hold a "green" Olympic Games, which demands it to keep the sky clear the air clean.