Citation: Siddiqui, S.I.; Allehyani, E.S.; Al-Harbi, S.A.; Hasan, Z.; Abomuti, M.A.; Rajor, H.K.; Oh, S. Investigation of Congo Red Toxicity towards Different Living Organisms: A Review. Processes 2023, 11, 807. https://doi.org/10.3390/pr11030807 Academic Editor: Ángeles Alonso- Moraga Received: 14 December 2022 Revised: 19 February 2023 Accepted: 21 February 2023 Published: 8 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Review Investigation of Congo Red Toxicity towards Different Living Organisms: A Review Sharf Ilahi Siddiqui 1,2 , Esam S. Allehyani 3 , Sami A. Al-Harbi 3 , Ziaul Hasan 4 , May Abdullah Abomuti 5 , Hament Kumar Rajor 1, * and Seungdae Oh 6, * 1 Department of Chemistry, Ramjas College, University of Delhi, New Delhi 110007, India; sharf_9793@rediff.com 2 Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India 3 Department of Chemistry, University College in Al-Jamoum, Umm Al-Qura University, Makkah 24382, Saudi Arabia 4 Department of Biosciences, Jamia Millia Islamia, New Delhi 110025, India 5 Department of Chemistry, Faculty of Science and Humanities, Shaqra University, Dawadmi 17472, Saudi Arabia 6 Department of Civil Engineering, College of Engineering, Kyung Hee University, Yongin-si 17104, Gyeonggi-do, Republic of Korea * Correspondence: rajorh@gmail.com (H.K.R.); soh@khu.ac.kr (S.O.) Abstract: The use of dyes is widespread across almost all industries. Consequently, these dyes are found in various sources of water and food that humans, animals, and plants consume directly or indirectly. Most of these dyes are comprised of complex aromatic structures that have proven harmful. Congo red dye, a complex aromatic azo dye based on benzidine, is most commonly used in these dyes; its metabolites (benzidine and analogs) can be toxic, but Congo red dye itself is not always harmful. The present review summarizes the toxicity of Congo red dye towards different living forms. Herein, the primary emphasis has been given to the mutagenic, teratogenic, and carcinogenic consequences of Congo red and its metabolites. The mechanisms of azo dyes’ carcinogenicity have also been discussed. This review will undoubtedly be beneficial for researchers to understand the harmful effects of Congo red in genotoxic, teratogenic, mutagenic, and carcinogenic factors. Keywords: dyes; Congo red; benzidine; toxicity; reduction 1. Introduction A dye is known for its coloring effects (a good factor) and its toxicity (a bad factor) [1]. Properties such as mutagenic, carcinogenic, and teratogenic define how toxic a dye is in nature [2]. The evaluation of these properties for a particular dye helps approve the dye’s production and use. The paper and textile industries run vastly throughout the world. They are the source of employment for many workers and produce colored compounds for coloring materials. These coloring compounds are applied to the materials to enhance aesthetic value [3]. As a coloring compound, azo dyes are the most suitable option. These are a crucial class of dyes, responsible for exceeding 50% of yearly global manufacturing and about two-thirds of all synthetic dyes [4–6]. Azo dyes may be water-soluble or water-insoluble and are considered toxic, mutagenic, and carcinogenic agents. The carcinogen may be a dye itself or its metabolites. In water-soluble dyes, the metabolite is the carcinogen readily absorbed by the body [7]. The adverse effects of azo dyes on the ecosystem account for a longer duration. The complex aromatic structure of dyes, which depend on aromatic rings, is a major factor in their toxicity [8]. The acute toxicity of azo dyes is determined to be very low, with 50% lethal dose (LD 50 ) values down to 250–2000 mg kg −1 body weight, as mentioned by European union guidelines for the classification of hazardous compounds [9]. Processes 2023, 11, 807. https://doi.org/10.3390/pr11030807 https://www.mdpi.com/journal/processes