The large cylindrical containers used by street vendors to carry baked sweet potatoes, a snack popular in China in winter, are the same ones that were previously used to store petrol and other chemicals.

The containers may still contain petrol traces, and these could be transferred to the sweet potatoes, which when consumed could cause cancer in the alimentary canal, according to a senior nutritionist at the Shenzhen No. 2 People's Hospital. The names of petrochemical plants are clearly visible on many of the containers.

Sweet potatoes baked in these containers are becoming increasingly popular as winter settles in, and the aroma given out by the baked food attracts a number of people in the city's busy shopping areas and commuters outside Metro stations and near bus stops.

A street vendor surnamed Xu said he bought his container from a petrochemical plant. Xu said most containers cost between 60 yuan (US$7.5) and 80 yuan.

Xu said he washed his container before using it to bake sweet potatoes. He added that the containers were also available in scrap heaps outside the Special Economic Zone.

Nutritionist Zhang Yiyun said the containers which weren't thoroughly disinfected carried the risks of cancer. Toxic materials cannot be eliminated by water, Zhang said.

It is hard to kick the habit of eating baked sweet potato after work when the weather is cold, said a woman who identified herself as Lin. Lin said she hoped that the food supervision department could provide an authoritative report on the dangers posed by the baked sweet potatoes.

However, the Shenzhen Food and Drug Administration has not yet investigated the issue or presented any report.

Editor: Yan