A number of students from a Hangzhou-based middle school were sickened recently, and synthetic material used for the running tracks and artificial grass at the school were the suspected culprits.

When a number of samples were sent to a lab, one of the grass samples was found to be emitting toxic benzene more than 14 times above the national safety limit. A second grass sample was found to be above the limit by about 27 percent.

No figures were given for samples of the track surface, but the school reported that they were all found to be safe.

The students at a new campus of Hangzhou Foreign Languages School in Hangzhou, Zhejiang province, suffered illnesses including nosebleeds, coughs and delayed menstruation since the end of May, South Metropolis Daily reported on Wednesday.

"After participating in sports classes, some students felt uncomfortable physically, and they believed this was caused by the rubber material used for the running tracks," according to a statement at the school's website.

The school sent samples from the tracks and artificial grass to a Shanghai-based institute for testing. The samples were collected in the presence of people representing students' families.

"There was no problem with the rubber, but the artificial grass was found to emit toxic ingredients more than expected," the school said. "China has no standards for the toxic ingredients of artificial grass, so we can only judge whether the material is safe according to the standards for synthetic running tracks."

Two of the seven artificial grass samples were reported as having benzene concentrations above national standards. One sample measured 63.8 mg/kg, about 27 percent over the national upper safety limit of 50 mg/kg. A second material used in the grass－a compound of methylbenzene and xylene－had benzene emissions at a whopping 733.8 mg/kg.

Previous tests conducted in October, after the playground was finished, show that the materials used on the ground were qualified.

"The high temperature might be the cause of the rise of toxic ingredients," the school said.

The school dismissed all students for an early summer vacation on Monday, though half of them will return to the campus for three-days of final exams next week.

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