The city’s Air Quality Index peaked at 261 at 10am — well above the level of 200 which is considered heavily polluted by the national standard.
The density of tiny PM2.5 particles was more than 200 micrograms per cubic meter, reported the Shanghai Environmental Monitoring Center.
The World Health Organization recommends exposure to no more than 25 micrograms per cubic meters of PM2.5 on average over a 24-hour period.
China recommends exposure to no more than 75 micrograms per cubic meters of PM2.5 on average over the same period.
Tiny PM2.5 pollutants are especially harmful as they can get deep into the respiratory system.
Pollutions levels eased in the afternoon and air quality is forecast to be better today as sea breezes arrive.
The density of PM2.5 began rising in Sunday afternoon, with most city observation stations recording slightly polluted levels. By around 4am yesterday the situation had worsened, peaking at 10am before falling back, said center officials.
They said pollutants were carried to Shanghai by a cold front from the north, which affected most cities in the Yangtze River Delta Region.
As yesterday’s morning rush hour got under way, pollutants from vehicles added to the problem, they added.
A pall of pollutants hung over the city at less than 500 meters from the ground.
But by 1pm, the average AQI in the city was 149, just within the slightly polluted range.
A change in wind direction yesterday evening was set to bring sea winds and cleaner air to Shanghai, said officials.
According to the environmental monitoring center, the AQI for yesterday evening fell to between 85 and 105 — slightly polluted.
And by this morning the figure should be between 70 and 90 — good according to the six-tier national standard.
Other cities in the Yangtze River Delta Region also suffered from poor air quality yesterday.
AQI readings were reported over 190 in cities like Suzhou, Nantong, Taizhou, Wuxi in Jiangsu Province as well as Huzhou in Zhejiang Province.
Today through Thursday is forecasted to be sunny with some cloud, before showers arrive on Friday.
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