Heavy fog in eastern and central China yesterday harmed Shanghai's air quality, and the effects will continue today with "light pollution" forecast.

The fog also affected flights from Shanghai to some northern cities, airport authorities said.

Yesterday morning, fog hit seven provinces and cities including Beijing, Tianjin and Jiangsu with visibility reduced to below 200 meters in places.

Influenced by the fog, air quality in Shanghai almost reached the "polluted" level, according to the Shanghai Environment Monitoring Center, under the PM10 standard. The content of nitrogen dioxide and sulfur dioxide was also relatively high.

Officials warned that the air quality would be even worse today and people with heart and breathing problems were advised to stay indoors.

The Shanghai Meteorological Bureau said it would be overcast to cloudy today, but tomorrow will see some drizzle, which will help to improve air quality.

Fog may continue to affect some areas of the country this morning.

Last night, Guizhou, Hubei and Henan provinces issued yellow fog alerts.

The National Meteorological Center said eastern China and the Yellow River valley would also be hit by fog, with visibility reduced to below 500 meters.

In northern China yesterday, flights to Beijing and Tianjin were disrupted.

Airport authorities said 98 flights were delayed for an hour and another 34 had been canceled by noon at Beijing because of poor visibility.

Flights from Shanghai to Beijing were also affected with many morning flights not leaving until the afternoon and two flights canceled. Three flights to Tianjin were cut yesterday and about 16 delayed.

Fog is not the only problem facing the country's transport services.

A snow storm was expected to hit western China's Qinghai Province last night, while in some areas of northeast China, in Heilongjiang Province, temperatures were expected to drop to minus 40 degrees Celsius today, authorities said.