Chinese authorities have shut down nine factories and detained some executives after reports that toxic industrial waste was used to make running tracks widely used at schools, official media said Thursday.

Smelly synthetic sports fields and athletics circuits, along with students falling sick from exposure to them, have regularly made headlines in China in recent years.

Parents of pupils at an elite elementary school in Beijing have been protesting, saying that their children suffered from nose bleeds and allergic reactions after using running tracks, the latest health scare in a country where safety standards are frequently compromised for profits.

Incidents in Beijing are seen as particularly unsettling as many Chinese believe regulations are more strictly enforced in the capital than elsewhere.

State broadcaster China Central Television (CCTV) reported this week that dozens of companies in Cangzhou and Baoding in Hebei province, which surrounds Beijing, had been producing running track materials from recycled industrial waste such as automobile tyres and electrical wires, which were believed to contain toxic chemical substances and heavy metals.

The Cangzhou government set up an investigation team and local authorities have shut down nine plants, sealing their machines, materials and semi-finished products and putting "related personnel" in custody, said hebnews.cn, the Hebei provincial government's news portal, on Thursday.

Polluted air and contaminated food regularly worry Chinese parents, many of whom have only one child due to the country's family planning policies.

Dozens of parents at the Beijing Number Two Experimental Primary School gathered to protest Wednesday after commissioning a private survey which found high levels of pollutants in the running track, a parent surnamed Ge told AFP.

School officials refused to meet them and around 50 took their protest to Beijing's main thoroughfare Changan Street, she said, adding: "Parents are angry."

In April, reports said almost 500 students were sickened after a top middle school in the eastern city of Changzhou relocated to a site close to decommissioned chemical factories.