BEIJING (Reuters) - A dust storm enveloped a swathe of northern China on Thursday (May 4) dragging down air quality and visibility and prompting warnings for children and old people to stay indoors.

Spring is northern China's dust storm season, when winds whip across the vast Gobi Desert picking up fine sand and dust particles and dumping them along a belt of heavily populated land further south.

The official People's Daily said the worst of the storm would be concentrated on a remote area along the Mongolian border, but that Beijing and locations as far away as the remote north-east of China would be affected for at least 24 hours.

Official data from the Beijing government showed average readings of small breathable particles known as PM 2.5, a major component of China's air quality index, had risen to 630 micrograms per cubic metre in parts of the city by Thursday morning.

The World Health Organisation recommends concentrations of just 10 micrograms.

State television said old people and children should stay indoors to avoid the worst effects.

The dust storm underlines the environmental challenge China faces, added to existing concerns about choking smog from coal powered power plants and factories that also periodically covers much of northern China.

The government has spent billions of dollars on projects to rein in the spread of deserts, planting trees and trying to protect what plant cover remains in marginal areas.