Cassia seeds used as a substitute for sand in school play pits could pose a health risk to children, a doctor has warned.

"If a seed were to get lodged in a child's ear or nose, it could be difficult to extract," said Dr Ni Kun from the Shanghai Children's Hospital.

"If that happens, the child should be taken to hospital. In extreme cases, surgery might be required to remove the seed," the doctor said.

The comments were made in response to posts circulated on weibo.com that the seeds, which have replaced sand in play pits in many malls, parks and schools across the city, could be dangerous.

Local parents said they were surprised by the rumors.

"I was happy about having cassia seeds for kids to play in because they look clean," said the mother of a 6-year-old boy.

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