A couple wearing a mask and a scarf visits the Bund in front of Pudong Lujiazui financial area on a hazy day in Shanghai Jan 16, 2013. [China Daily]

Shanghai should breathe easier today after dense pollutants pushed the city's air quality index to its highest overnight, officials said.

The AQI surged to 254 at 7pm yesterday, a record high since the index was introduced on November 16.

However, winds along a cold front started to disperse pollutants as of last night, they said.

"Visibility in the city should get better," said Kong Chunyan, a chief service officer of the Shanghai Observatory. "The haze in north China has started to clear, and the situation in Shanghai should get better as well."

Yesterday Shanghai was shrouded in smog and fog as the hourly density of PM2.5 particles was above 200 micrograms per cubic meter all day due to lack of wind.

The hourly readings of the fine particles exceeded 240 micrograms between 9am and midday, officials said.

A warning to people with heart disease and respiratory disorders to stay indoors was issued by the Shanghai Environmental Monitoring Center at 7:30am.

The air quality index, a broader reading, kept rising yesterday and as of 10am exceeded 201, an amount considered heavy pollution.

The major component of the pollutant was PM2.5, which are airborne particles smaller than 2.5 micrometers in diameter.

Daily and hourly AQI readings are based on a composite index of PM10, sulfur dioxide, carbon dioxide, PM2.5, ozone and carbon monoxide.

Air quality is indicated by six levels from excellent to severely polluted, which is an AQI over 300.

Fu Qingyan, chief engineer of the monitoring center, said the city is taking emergency measures on heavily polluted days such as yesterday, with power plants ordered to use high-quality coal and petrochemical and chemical plants told to reduce production.

"The environmental watchdog has tightened inspection of plants and will impose punishment on any factory with excessive emissions, and will order construction sites and storage yards to control flying dirt," she said.