Shanghai suffered its fourth consecutive day of poor air quality yesterday with a sandstorm in north China partly to blame.

Readings of tiny, hazardous PM2.5 particles rose to 120 micrograms per cubic meter at noon and reached 246.3 at 11pm.

A daily PM2.5 reading above 75 micrograms per cubic meter is considered polluted, based on national standards.

The Shanghai Environmental Monitoring Center issued an alert at 3pm, saying the city's pollutants of PM2.5 and PM10 had risen quickly because of dust and sand blown in from the north. It advised the elderly, children and people with respiratory or heart disease to stay indoors. The PM10 figure had risen to 384 by 3pm.

The nation's daily standard for PM10 is 150.

The center said the density of PM2.5 and PM10 would still be at a high level this morning.

The center said sandstorm season in the north of the country usually lasts from late February to mid-April.

As a result, Shanghai should have another hazy day today with grey skies, low visibility and poor air quality, the Shanghai Meteorological Bureau said. Although temperatures are expected to reach 26 degrees Celsius today, outdoor activities may not be advisable, according to forecasters in the city.

The bureau issued a yellow haze alert, lowest of the three-level system, at 5:15pm yesterday warning of moderate haze before noon today and advising city residents to take precautions if going outdoors.