The air in Shanghai was heavily polluted yesterday, as the air quality index (AQI) hit 213 at 1pm.

Between June 1 and Wednesday, moderate pollution (when the AQI is between 151 and 200) was reported on 11 days, seven more than in the same period of last year, the Shanghai Environmental Protection Center said.

Ozone was the major pollutant on those days, it said.

"The recent lack of wind was the major cause of the latest round of pollution as there was nothing to diffuse the particles," an official from the center told Shanghai Daily on condition of anonymity.

"We also detected a lot of ozone, which is more common in the summer. Ozone also helps to create a high density of tiny particles like PM2.5, so we advise people to avoid outdoor activities around noon when the ozone level is high," the person said.

Ozone can impair heart function and damage the respiratory system. In extreme cases, long-term exposure can prove fatal, the person said.

Ozone is not only created in the sky, but also by factories.

Between June and August 2013, there were 12 days of moderate or heavy pollution. On 10 of those days, ozone was the major pollutant.

The AQI is expected to be between 145 and 165 this morning, with PM2.5 likely to be the main pollutant. The index should fall to between 105 and 125 in the afternoon, when the main pollutant will be ozone.

The concentration of PM2.5 particles soared to about 160 micrograms per cubic meter yesterday noon, six times the guideline set by the World Health Organization.

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