School uniforms made by a Shanghai company have been found to contain cancer-causing dyes, according to a local consumer watchdog.

A total of 21 primary and middle schools in Shanghai that purchased student uniforms from Shanghai Ouxia Clothing Co. Ltd. have notified students not to wear the clothes after carcinogenic azo compounds were found in the material.

The Pudong Education Bureau, on Monday, also issued the list via its official Sina Weibo account of schools that were to turn in student uniforms to be sent in for testing.

The uniforms were made from a fiber sourced from a company in neighboring Zhejiang Province, which used dyes containing the banned compounds, a representative of the company, surnamed Wang, told the official Global Times.

Wang said that the company only produced the batch of 50 uniforms as samples and they were not meant for sale, according to the paper.

The Shanghai Daily reported that the uniforms had appeared on quality blacklists over the past three years for failing production indicators or having a high pH index, which can cause skin allergies.

The company’s qualification rating has dropped from about 89 percent in 2011 to 72.7 percent during the most recent round of testing, according to the Global Times.

The Shanghai Bureau of Quality and Technical Supervision has launched an investigation into the factory and ordered it to stop production of the products in question.