Heavy smog again blanketed many parts of China on Wednesday, causing chaos and disruption as it forced the cancellation of flights and highway closures.

Thousands of passengers were stranded in Hubei's Tianhe Airport when poor visibility delayed the arrival and departure of 30 flights.  Jinan and Qingdao airports also reported the delay of 10 flights.

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Over in Shandong, 120 highway toll stations were forced to shut down due to the smog, which began to set in early last Monday.

On Wednesday the meteorological center raised the yellow alert, the second highest level in China's air pollution gauge, saying the smog will persist until Thursday night.

Moderate to heavy smog will be experienced in Beijing, Hebei, Tianjin and Shandong, the center said, adding that a cold front on Friday may disperse the smog.

As early as Monday, Beijing's Air Quality Index (AQI) dropped to 294 at 2:30PM, signifying the onset of another rise in air pollution.

At 294, breathable air becomes "Very Unhealthy" under the international AQI range of 201-300.  At these levels, health alerts would indicate that everyone may experience more serious health effects.

Under China's meteorological alert standards, air quality conditions are categorized as blue, yellow, orange and red as the severity ascends. The affected regions were placed under yellow alert conditions.

Ahead of the center's smog warning for this week, residents were feverishly buying face masks, which have become a necessity these days.

Consumer groups, however, have been scrutinizing the effectiveness of the many face masks that are flooding the market.

Among the 37 types of face masks examined by the China Consumers Association, only nine have met the requirement for filtering air particulates and easy breathing.

"The vast majority of face masks on the market give no protection against PM2.5, even if the manufacturers claim they do," Lei Limin, vice chairman of the China Textile Commerce Association disclosed. 

Despite the surge in demand, the government has yet to standardize the protective quality of face masks in the country for consumer protection. 

Lei and his group are pushing for a national standard for anti-smog face masks, echoing calls made last week by the China Consumers Association.