Insiders said thick straws, such as those used for bubble tea, tend to be more at risk because they use more materials and are more expensive to make.

"The price of a qualified straw is around 0.1 yuan ($0.016), which is double or even triple the price of a substandard one," said Han Ying, business department manager of Shanghai Guanxuan, a company that makes household products.

"There is no threshold for producing inferior drinking straws as it's easy to attain recycled plastics, and anyone could start a business with several machines at home," she added.

There are five straw producers in Zhejiang province's Yiwu, an international trade hub that produces at least 90 percent of the straws in the Chinese market, but the number of unlicensed producers in the country is more than 200, insiders said.

The health supervision department in Changsha, Central China's Hunan province, issued a warning in May that some drinking straws might contain toxic substances after the findings of Lou's investigation.

zhouwenting@chinadaily.com.cn