Shanghai may have experienced its longest period of June pollution over the past few days since it launched its air quality index in 1997.
Two women are suitably dressed for the weather as they walk along the Bund in Shanghai on Sunday. [Shanghai Daily]
However, officials said the main pollutant was fine particles, which were only monitored in recent years, so any comparison with older figures might not be very accurate.
The city experienced more than 60 hours of pollution, PM2.5 readings confirmed. PM2.5 means particles 2.5 micrometers or less in diameter.
This was due to regional haze in the Yangtze River Delta and poor weather conditions locally, officials from Shanghai Environmental Monitoring Center said, causing the pollutants to concentrate.
Air quality today is expected to improve under better weather conditions.
"The poor air quality was mainly caused by fine PM2.5 particles," said Lin Chenyuan, a forecaster with the Shanghai Environmental Monitoring Center. "The main reason was the poor weather condition, causing the accumulation of local pollutants. Outside impact, straw burning in neighboring Jiangsu Province, also worsened local air quality."
Yangzhou in Jiangsu Province was seriously polluted on Saturday although that improved slightly yesterday.
Its air quality index made it the dirtiest city in the country over the weekend because of straw burning, an agricultural practice that is banned in Shanghai.
"We will do more study on how much straw burning impacted Shanghai," said Lin.
The sweltering and humid weather in Shanghai also raised questions over whether the city had entered the plum rain season. But forecasters said no.
Rain, and possible thunderstorms, is expected today, bringing temperatures down to 28 degrees Celsius. However, cloudy conditions from tomorrow will make the mercury rise again.
Temperatures between Wednesday and Friday could peak at around 31 degrees.
Shanghai's temperature hit 31 degrees yesterday with the city under the influence of a warm and wet air mass.
The resulting humidity, up to 80 percent, and a lack of wind made it feel hotter than the recorded temperature, the Shanghai Meteorological Bureau said.
The lack of wind was also to blame for the poor air quality.
Shanghai Environmental Monitoring Center issued an air quality warning on Saturday morning, advising people with respiratory diseases or heart problems to stay at home.
According to the center's PM2.5 measurements, fine particles exceeded the national standard and even reached a peak of three times the standard on Friday night.
Yesterday, it said, though the air quality rose to good, the concentration of fine particles in the air still remained high. At the Putuo District monitoring spot, the concentration started to rise from Saturday afternoon and reached a peak at night.
The readings dropped from midnight to yesterday morning, but then PM2.5 kept climbing to reach double the national standard.
The readings in the Pudong New Area's Zhangjiang site stayed at between 120 and 160 micrograms per cubic meter from Saturday afternoon to yesterday evening.
The national standard is 75 micrograms per cubic meter.
PM2.5 particles can affect air quality and visibility and pose major health risks as they are small enough to lodge in the lungs and even enter the bloodstream, causing diseases.