# Evaluation of Sub-Lethal Toxicity of Benzethonium Chloride in _Cyprimus carpio_ [PERSON]
## 1 Introduction  
Nowadays, cationic surfactants are widely used for various household and industrial applications, in the composition of personal care products, but also in pharmacy, chemistry, biocide (antimicrobials), metallurgy, and petrochemistry products [1]. Therefore, large amounts of surfactants, as well as their by-products, are discharged into sewage with the final destination being the surface aquatic systems. Many surfactants, along with their degradation outcomes, have been found all over the world in fresh and sea waters [2]. These compounds are able to block the normal biogeochemical cycles and thus, significantly affect the ecosystems. Previous studies performed on surfactants for the last decades [3; 4; 5; 6] have focused mainly on anionic and nonionic surfactants; therefore, there is only a little information about the toxicity of cationic surfactants. In natural waters, the concentration of surfactants rangesfrom 0.001 to 10 mg/L and rarely is higher than 0.5 mg/L [7]. The toxicity of surfactants for aquatic organisms (fish, invertebrates, algae) was reviewed to be between 0.08 and 156 mg/L [8], depending on each organism and various factors [9]. Most surfactants have an ionic or polar head group connected to a hydrophobic tail with a straight or branched hydrocarbon chain. The cationic surfactants contain a minimum of one alkyl chain linked to a positively charged nitrogen atom, and other alkyl groups which are mostly short-chain substituents such as methyl or benzyl groups. This structure gives them the capacity to interact with sewage sludge, soil, and sediment, which are predominantly negatively charged [2,10]. Some studies showed significant concentrations of cationic surfactants in the sewage water 20-5600 ug/L [11; 12; 13], sludge 0.2-1640 ug/L, and surface waters 0.01-3900 ug/L [11; 14]. These surfactants are aerobically biodegradable in activated sludge systems, but their degradation capacity depends on various factors such as chemical structure, the presence of anionic compounds, microbial systems, and their adaption, with their removal rates estimated up to 90% [14]. The toxic actions of these surfactants are different according to their structure, lipophilicity, membrane permeability, and biodegradability/persistence, metabolites occurrence, the interaction with other pollutants, synergism phenomena, etc. [15]. Ecotoxicological studies reported toxic effects of some cationic surfactants on aquatic organisms (fish, Daphnis, rotifers, algae, protozoa, and bacteria) for concentrations ranging from 0.07 to 42 mg/L depending on various abiotic and biotic factors [16]. Benzenthonium chloride (BEC, Hyamine 1622) is one of the quaternary ammonium salts with cationic properties, mainly used in different concentrations depending on the application (0.1-1.6% in detergent products and \(>\)50% in sanitation products). Moreover, it is used in the manufacture of pharmaceuticals for human and animal use, and for obtaining personal care products (such as moisturizers, foot odor control sprays, hand sanitizers, antiperspirants, conditioners, scalp treatment masks, etc.) in concentrations under 1% according to the Cosmetic Ingredient Review [17]. Due to its biocidal characteristics and uses, BEC is under the incidence of Biocide Regulation no. 528/2012, Detergent Regulation no. 648/2004, and also Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) Regulation no. 1907/2006. The aquatic environmental risk for BEC (Hyamine 1622) was estimated to be high due to the inhibitory effects on small aquatic organisms such as algae and planktonic crustacean, at very low concentrations, below 10 ug/L, which were detected in the Danube river in the period 2010-2011 [18]. [PERSON] et al. [13] reported damage effects of BEC (LC50 30.5 mg/L) for nematode _C. elegans_ after 24 h. Additionally, at 5 mg/L, this caused 52% mortality of zebrafish embryos [13]. Generally, a higher concentration of surfactants can cause a suppression of metabolic processes in the cells.