Air pollution increased in Beijing on Tuesday, a day after an Olympic pollution prevention test took over a million cars off the roads, environmental officials said, indicating the measure works.

To see if the city's poor quality air could be improved at least temporarily during next August's Games, Beijing ordered 1.3 million cars off the roads by alternately banning odd and even number plates from Friday to Monday.

"The test has clearly demonstrated we are capable of providing good air quality for the Beijing Olympic Games," Beijing Environmental Protection Bureau (BEPB) deputy chief Du Shaozhong told reporters.

Official air quality reports over the four days of car restrictions came in at Grade 2 on 1-5 scale. This worsened to Grade 3 by midday on Tuesday.

Du said the car restrictions had produced good results in improving the air quality, reducing carbon monoxide and other air-borne pollutants by between 15 to 20 percent.

"The accumulation of emissions clearly slowed over the four-day test. With these weather conditions, we would not have met air quality targets over four consecutive days without the restrictions," Du said.

International Olympic Committee (IOC) President Jacques Rogge said at the one year countdown to the 2008 Beijing Games earlier this month that some endurance sports like cycling might have to be postponed if air quality was not improved.

"If the air quality is as good as Grade 2, it would be suitable for activities including the (Olympic) sporting events," said Du.

Beijing regards Grade 1 and 2 air pollution ratings as acceptable "blue sky days".

"I think four days are too short to improve the air quality. It should be longer. Certainly I hope the progress of cleaning up the air could last forever," he said.