The financial hub Shanghai was enveloped in a heavy smog on Thursday, the first heavily polluted day in the city this fall.

According to the Shanghai Environmental Monitoring Center, the city's air quality index had surged to 253 as of 3 pm on Thursday, or seriously polluted.

The main pollutant is PM2.5, or particles less than 2.5 micrometers in diameter, believed to be blown south from northern China.

Responding to the push by the public to take emergency measures against pollution, the city's environmental authorities reiterated the conditions to launch an emergency plan for smoggy weather on Thursday afternoon.

The Shanghai Environment Protection Bureau said it will launch an emergency plan or higher level measures when the AQI exceeds 200 for 12 consecutive hours, the level of PM2.5 reaches 150 micrograms per cubic meters and the condition will not improve in the next 24 hours.

Under those conditions, authorities will launch measures including canceling large-scale outdoor activities and sports events, and banning vehicles that give off a high amount of emissions. Also, 30 percent of government vehicles will be taken off the road.