HENAN/SHIJIAZHUANG, China - More than 400 students in China's Henan province were forced to sit for their exam in an outdoor field amid choking smog, sparking public anger after an image of the exam was circulated on social media.

The 480 pupils were eighth-graders from a middle school in Linzhou, in China's Henan province, who had to endure the heavily-polluted air when they sat for their exam on Monday (Dec 19). Severe pollution had caused local authorities to order the closure of all schools, the South China Morning Post reported.

The image showed rows of students hunched over their desks on a field, with the pupils seated further away appearing only as silhouettes shrouded in the thick haze.

According to the school's principal, the exam was conducted by the school staff, with the staff deciding to proceed with the exam since it had already been organised.

The principal has reportedly been suspended.

The smog started to enshroud Beijing, Tianjin and the provinces of Hebei, Henan and Shandong over the weekend. It is forecast to clear on Thursday.

The oppressive haze had triggered concern over the slow response to the threat to children's health, AFP reported.

Nowhere has been hit as hard as Shijiazhuang, the capital of Hebei province.

Shijiazhuang was one of the more than 20 cities which went on red alert last Friday evening, triggering an emergency plan to reduce pollution by closing polluting factories and taking cars off the road, among other measures.

A decision by the city's education department to wait until Tuesday evening to announce it was closing elementary schools and kindergartens, provoked anger on social media.

The announcement had said middle and high schools could close on a voluntary basis.

"Are middle school students' bodies' air purifiers?" one incredulous commentator asked, adding: "Are you going to wait for us all to become sick before you step up to fix this?"

Shijiazhuang has seen 10 bouts of serious air pollution so far this winter, according to the China Daily newspaper, putting it at the top of the environmental ministry's list of cities with the worst air quality.

Over the last 48 hours, levels of PM 10 - a measure of particulates in the atmosphere - have been literally off the charts in the city, repeatedly maxing out at 999.

Levels of the smaller PM 2.5 particles, tiny enough to be absorbed into the bloodstream and thought to be a major contributor to respiratory and cardiovascular disease, reached as high as 733, more than 29 times the World Health Organisation's daily recommended maximum exposure of 25.

Shijiazhuang's smog and its government's reticence to act have tested the patience of not just the public but even state media.

On Tuesday, the official Xinhua news agency published an article scolding public officials in the city for waiting to cancel schools even though smog was "off the charts".

"If (officials) turn a deaf ear or act indifferent, and the people, especially minors, are exposed to potential health risks, this is undoubtedly a dereliction of duty," it said.