China has unveiled a string of tough new measures to curb pollution if the stubbornly persistent haze hovering over Beijing does not clear in time for the Olympic Games.

More cars could be removed from the roads and hundreds more factories closed if forecasts show insufficient improvement in air cleanliness over an upcoming 48-hour period, the Ministry of Environmental Protection disclosed.

Since July 20, Beijing has already implemented a traffic system that allows cars onto the roads on alternate days, depending on their odd or even licence plate. Now this system could be extended so that vehicles whose plates' final digit matches the last number of the date would be banned under the contingency plan.

And the odd-and-even rules could be extended to the nearby port city of Tianjin and to four urban areas in Hebei province, which surrounds Beijing.

The ministry said 105 electronic, chemical, furniture and construction material factories in the capital would suspend production or close the part of their processing that emits pollutants. In Tianjin, 56 coal-fired plants and other factories could be turned off and in Hebei small steel plants would be ordered to slash production significantly.

Officials said the measures implemented 11 days ago were finally starting to have an effect on the air quality. Beijing's pollution level has dropped today to less than half that of yesterday to show the lowest reading since half the city's cars were pulled off the roads. A cooling wind and overnight rain helped to sweep away pollutants and brought relief from high temperatures and debilitating humidity.

The air pollution index dropped to 44 yesterday (Wednesday), less than half the level a day earlier. A reading below 50 is considered good and between 51 and 100 is moderate while anything over that fails to meet standards. From Thursday to Sunday, the city recorded readings of 113, 110, 118 and 113 are classified as unhealthy for sensitive people.

Du Shaozong, deputy director of the city Environmental Protection Bureau, said levels were improving.

"The daily data since July 20 shows an improvement in air quality. It reflects the results since we restricted traffic and stopped heavy-polluting factories and construction. That's why we say the measures have been effective," he said.

Mr Du has said people should not judge the pollution levels from merely looking at the haze shrouding the capital but rather based on scientific data. He said a mix of heat, high humidity and little wind created conditions that made it difficult to disperse major airborne pollutants such as construction dust, vehicle exhaust, factory emissions and power plant fumes.

The polluted skies have been one of the biggest worries for Olympics organisers and the IOC has said some events could be rescheduled if need be in response to the quality of the air.