Taipei, July 29 (CNA) The Hsinchu City government said Wednesday it has expanded testing of locally produced rice noodle products after several made by Hsinchu manufacturers were found to contain formaldehyde, a substance prohibited for use as a food additive and classified as a human carcinogen. Hsinchu Mayor Kao Hung-an (高虹安) said at a news conference that the city collected 28 samples from 16 rice noodle and rice vermicelli manufacturers from July 16-27 and sent them to the Food Industry Research and Development Institute (FIRDI) for formaldehyde testing on Tuesday. Kao said the city was first notified by the Tainan City government on May 27 that steamed rice noodles produced by Hsinchu-based Ming Hao Enterprise (明好企業社) tested positive for formaldehyde. The bureau said formaldehyde is not included in the central government's routine testing program for rice noodle products, which instead focuses on sulfur dioxide, hydrogen peroxide, preservatives, microorganisms and heavy metals. It said the contamination came to light only because Tainan authorities had expanded their testing of noodle products, and urged the central government to add formaldehyde to routine risk monitoring and self-inspection programs for rice noodle products.