Int. J. Environ. Res. Public Health 2015, 12, 5657-5684; doi:10.3390/ijerph120505657 OPEN ACCESS International Journal of Environmental Research and Public Health ISSN 1660-4601 www.mdpi.com/journal/ijerph Review Triclosan: Current Status, Occurrence, Environmental Risks and Bioaccumulation Potential Gurpreet Singh Dhillon 1, Surinder Kaur 1,2, Rama Pulicharla 1, Satinder Kaur Brar 1,*, Maximiliano Cledón 1,3, Mausam Verma 4 and Rao Y. Surampalli 5 1 2 3 4 5 INRS-ETE, Université du Québec, 490, Rue de la Couronne, Québec, QC G1K 9A9, Canada; E-Mails: garrydhillons9@gmail.com (G.S.D.); surinder_dhillons@yahoo.ca (S.K.); Pulicharla.Rama@ete.inrs.ca (R.P.); Maximiliano.Cledon@ete.inrs.ca (M.C.) Department of Mycology & Plant Pathology, Institute of Agricultural Sciences, Banaras Hindu University (BHU), Varanasi-221005, India CONICET-IIMyC, National Council of Scientific and Technical Research, C1033AAJ Buenos Aires, Argentina CO2 Solutions Inc., 2300, Rue Jean-Perrin, Québec, QC G2C 1T9, Canada; E-Mail: mausamverma@yahoo.com Department of Civil Engineering, University of Nebraska-Lincoln, N104 SEC P.O. Box 886105, Lincoln, NE 68588, USA; E-Mail: surampalli.rao@giees.org * Author to whom correspondence should be addressed; E-Mail: satinder.brar@ete.inrs.ca; Tel.: +1-418-654-3116. Academic Editor: Paul B. Tchounwou Received: 29 December 2014 / Accepted: 18 May 2015 / Published: 22 May 2015 Abstract: Triclosan (TCS) is a multi-purpose antimicrobial agent used as a common ingredient in everyday household personal care and consumer products. The expanded use of TCS provides a number of pathways for the compound to enter the environment and it has been detected in sewage treatment plant effluents; surface; ground and drinking water. The physico-chemical properties indicate the bioaccumulation and persistence potential of TCS in the environment. Hence, there is an increasing concern about the presence of TCS in the environment and its potential negative effects on human and animal health. Nevertheless, scarce monitoring data could be one reason for not prioritizing TCS as emerging contaminant. Conventional water and wastewater treatment processes are unable to completely remove the TCS and even form toxic intermediates. Considering the worldwide Int. J. Environ. Res. Public Health 2015, 12 5658 application of personal care products containing TCS and inefficient removal and its toxic effects on aquatic organisms, the compound should be considered on the priority list of emerging contaminants and its utilization in all products should be regulated. Keywords: degradation by-products; dioxins; emerging contaminants; personal care products; triclosan; toxicity 1. Introduction Triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy) phenol) is a synthetic, broad-spectrum antimicrobial agent. It has antibiotic and antimycotic properties [1]. Triclosan also blocks fatty acid synthesis by inhibiting enoyl reductase enzyme. TCS is categorized as a halogenated aromatic hydrocarbon having phenolic, diphenyl ether and polychlorinated biphenyl (PCB) substructures [2]. Its chemical structure is a halogenated biphenyl ether which confers it chemical properties related to many toxic compounds such as PCBs, polybrominated diphenyl ethers, bispenol A and dioxins [3]. The worldwide annual production of TCS in 1998 was approximately 1500 tonnes, out of which about 350 tonnes and more than 450 tonnes were utilized in Europe and USA, respectively [4,5]. The main release of TCS into the environment is due to personal care products containing around 0.1% to 0.3% (w/w) TCS [6,7]. Such products are externally applied to the human body, thus TCS is generally not subjected to metabolic alteration. Moreover, it is usually released into the domestic wastewater, thus ending up in local wastewater treatment plants (WWTP). Poor solubility and high adsorption of TCS to solids results in its removal from WWTP effluent up to 99%. [8,9]. The high log Kow value of 4.76 for TCS suggests high sorption potential and it adsorbs onto the settled sewage sludge [10,11] which may be amended to agricultural soils [12,13]. Thus, the most important sources of TCS in the environment are use of biosolids as agro-fertilizers [14]. The chemical properties of TCS suggest its possible bioaccumulation and further environmental persistence (Table 1). Currently, TCS and its degraded byproducts are found throughout the environment, including soil, surface waters, and human breast milk [14–18]. The continuous detection of TCS and its degradation products has led to debate on safety, effectiveness and regulation of TCS usage. Various studies shed light on the emerging health concerns related to the use of TCS, such as microbial resistance, dermal irritations, endocrine disruption, higher incidence of allergies, altered thyroid hormone metabolism and tumors development due to TCS and its by-products [19–21]. Unlike other emerging contaminants (ECs), such as organochlorine compounds, pharmaceutically active compounds (PhACs) and endocrine disrupting compounds (EDCs), TCS is not considered as a chemical pollutant with high priority concerns. Low acute toxicity and assumption of not to show chronic side effects, TCS usage is not well regulated [22,23]. This leads to widespread use of TCS in various household products, thus causing an increase in TCS concentration in the aquatic and terrestrial environment. Int. J. Environ. Res. Public Health 2015, 12 5659 Table 1. General properties of TCS. CAS No. 3380-34-5 Structure Molecular formula Trade name General classification Possible use Nature Molecular weight Dissociation constant (pKa) (20 °C) Henry constant (Hc) (atm mol−1·m−3) Octanol-water Partition coefficient (log Kow) Sorption coefficient (Koc) Solubility Vapor pressure Bioconcentration factor (BCF)Photodegradation (half-life in aqueous solution) Biodegradation (half-life in aerobic soil) Biodegradation (anaerobic condition) Degradation products of TCS C12H7Cl3O2 Irgasan DP 300, FAT 80′023, CH 3565, GP41-353, Irgacare MP (the pharmaceutical grade of TCS, >99% pure) and Ster-Zac Non-prescription compound Antimicrobial, antiseptic and disinfectant Hydrophobic 289.54 8.14 1.5 × 10−7 (25 °C) 4.76 18408 12 mg·L−1 (25 °C) 5.2 × 10−6 Pa (mm Hg at 20 °C) 2.7–90 (aquatic organisms) 41 min 18 days No degradation within 70 days Methyl TCS, dioxins, chlorophenols, chloroform Similar antimicrobial activity of TCS to antibiotics and its toxicity data demand regular monitoring of its concentration in the environment, along with its safe and regulated use in the consumer products. This article provides a comprehensive literature review on TCS, its occurrence in wastewaters, biosolids, aquatic and terrestrial environment, its removal potential, toxicity levels in humans, wildlife and other aquatic organisms, its bioaccumulation potential and intermediate products. The review also addresses the research gaps in concerns related to long term exposure to TCS. 2. Physico-Chemical Properties of TCS Affecting Removal The removal of organic substances, such as TCS after release into environment depends on various physico-chemical properties of the compound. For instance, the sorption of organic compounds on sludge during wastewater treatment processes plays an important role. Depending on their log Kow values, the hydrophobic substances may adsorb onto settled sludge during primary sedimentation step in WWTP. The different physico-chemical characteristics of TCS governing its removal efficiency in conventional activated sludge treatment plants are given in Table 2. As evident from Table 2, the Int. J. Environ. Res. Public Health 2015, 12 5660 adsorption potential of TCS is high due to a high log Kow. The high Kow value of TCS is also indicator of its bioaccumulation potential. Another important property governing the removal of organic substances is their volatility. Triclosan is also non-volatile (5.3 × 10−4 Pa at 20 °C) and is moderately soluble in water (10 mg·L−1 at 20 °C). Moreover, it does not hydrolyze easily [24]. Normally, the substances with a Henry’s constant (Hc) ≥ 10−3 atm·mol−1·m−3 will easily be removed by volatilization. Hence, the volatilization losses of specific substances during wastewater treatment can be predicted based on Henry’s constant value and Hc/Log Kow ratio [11]. Table 2. Removal potential of TCS during wastewater treatment process depending on different physico-chemical properties. Physico-Chemical Property Removal Potential of TCS Adsorption potential Log Kow ≤ 2.5 Low sorption potential 2.5 < Log Kow < 4 Medium sorption potential Log Kow ≤ 4 High sorption potentialTCS Volatilization potential 4 Hc > 1 × 10 and Hc/Log Kow >1 × 109 High volatization potential Hc < 1 × 104 and Hc/Log Kow 8.1 and it converts into its neutral phenolic form if the water pH is below 7.9. In addition to pH, co-occurrence of dissolved compounds such as metals and organic matter may possibly affect photosensitivity of TCS [24]. Hence, the complex matrix of wastewater affects the efficiency of photodegradation of TCS in WWTP [25]. 3. Current Scenario of TCS Use and Safety Generally, TCS comes in the form of white powder. TCS has a weak aromatic, phenolic scent as it is a chlorinated aromatic compound. Ever since its invention, TCS has been widely used in numerous consumer products as illustrated in Figure 2 [6,8,10,12,26]. It is used as an active ingredient in dental products since 1980s in Europe and the mid-1990s in the United States after approval by the Food and Drug Administration [27]. More specifically, TCS is used in numerous personal care products, such as toothpastes, antibacterial soaps (bars and liquids), dishwashing liquids, deodorant soaps (bars and liquids), cosmetic and antiseptic products, and antiperspirants/deodorants [28]. Triclosan is also used in other consumer products, such as kitchen utensils, toys, bedding, clothes, fabrics, and trash bags. Int. J. Environ. Res. Public Health 2015, 12 5661 Figure 1. Molecular structures of TCS and its environmental transformation product, methyl-TCS. Figure 2. Various applications of triclosan. Int. J. Environ. Res. Public Health 2015, 12 5662 The concentration of TCS recommended by various government agencies to be used in various consumer products is given in Table 3. In 1989, the European Community Cosmetic Directive approved TCS usage as a preservative in cosmetics and toiletries up to 0.3% [28]. According to FDA, up to 0.3% TCS is permitted in toothpaste [29]. Similarly, as per the National Library of Medicine’s Household Product Database, TCS concentrations were reported to range from 0.1% to 0.3% in liquid hand soaps [30]. Table 3. Recommended levels of TCS in various consumer products (Adapted from [25]. Type of TCS-Based Product TCS Concentration (%) Oral care products Toothpaste 0.3 Mouth wash solutions 0.03 Dermally applied products (rinse off) Skin cleansers 0.3 Liquid hand soap 0.1–0.45 Dishwashing detergent 0.1 Dermally applied products (leave on) Body lotion 0.3 Facial Moisturizer 0.3 Deodorant/antiperspirants 0.3 Reference [29] [31] [28] [32] [30] [28] [28] [28] According to the FDA monograph for health care antiseptic drug products, which covered antibacterial soap products containing TCS, the recommended limits are up to 1% TCS for use in antiseptic washes and surgical hand scrubs in health care settings [33]. According to Governmental regulations in the European Union (EU) and the United States, only specified amount of triclosan can be used in some cosmetic and PCPs. TCS possesses a broad range of antimicrobial activity that encompasses several, types of nonsporulating bacteria and a few fungi, such as Plasmodium falciparum and Toxoplasma gondii [19,34]. At low concentrations, TCS inhibits the growth of microorganisms; at higher concentrations, it kills microorganisms. Different microorganisms show varied response to TCS as provided in Table 4. Triclosan blocks the active site of enoyl-acyl carrier protein reductase enzyme (ENR) thus impairing the production of bacterial lipids [35]. In consequence, cell membranes are not properly produced and bacterial proliferation stops. Therefore, only a small TCS dose is required to inhibit bacterial growth. As humans lack ENR enzyme, TCS has been considered harmless to them. Studies carried out by FDA found that TCS-fluoride paste prevented tooth deformities, such as gingivitis, tartar and plaque in a way that was superior to fluoride-only toothpastes. Over the last 30 years, TCS has also been successfully used as an antimicrobial agent in hospitals and for other biomedical purposes. The successful control of methicillin-resistant Staphylococcus aureus (MRSA) outbreaks in several clinical settings using TCS based products [36,37]. This led to the recommendation of showering/bathing with 2% TCS for the decolonization of patients whose skin is carrying MRSA [38]. However, susceptibility of MRSA strains to TCS has changed little over the last decade [39]. Later on there has been no relation found between TCS response in MRSA and other strains of S. aureus and antibiotic susceptibility or resistance [40]. Int. J. Environ. Res. Public Health 2015, 12 5663 Table 4. Different microorganisms affected by the antimicrobial action of TCS. Target Microorganisms Effective Concentrations Most sensitive strains Staphylococci, some Streptococci, some mycobacteria, Escherichia coli, Klebsiella pneumonia, Klebsiella spp., Enterobacter spp., Acinetobacter spp., Proteus spp. 0.01 mg·L−1 to 0.1 mg·L−1 and Proteus mirabilis, Plasmodium falciparum, Toxoplasma gondii Less sensitive strains Methicillin-resistant Staphylococcus aureus (MRSA) strains 0.1–2 mg·L−1 Enterococci Highly resistant strains Pseudomonas aeruginosa, Clostridium difficile Reference [33] [19] [40,142] [49] [49,143] The American Medical Association (AMA) has raised concerns about the use of TCS and some other antimicrobial agents in consumer products [41]. The AMA has encouraged the FDA to study the issue on the safety and effectiveness of antimicrobials including TCS. The progress of the current FDA evaluation will be monitored by the AMA on regular basis. The AMA also indicated that further research is required on the introduction of antimicrobials in massive consumer products. In 2009, the American Public Health Association (APHA) proposed that it would recommend the banning of TCS for household and non-medical uses. However, no further action has been taken as yet. Regardless of current efforts to review and regulate the proper use of TCS, a scientific debate lingers on its potential adverse impact on human health, environment and potential association to microbial resistance. 4. Emergence of Microbial Resistance to TCS The overuse of anti-microbial products may lead to increased resistance among bacteria. Considering the published studies, there is a dilemma whether TCS does or does not encourage the development of antibiotic resistance. Triclosan-resistant bacteria can be produced readily by their in vitro exposure to increasing TCS quantities and the consequent development of resistant colonies [42]. The mechanism of microbial resistance to TCS has been described by various researchers [43,44]. According to the authors, the resistance can be attributed to: (1) overproduction of targets/amplification or; (2) modification of target. Gomez-Escalda et al. [45] found that a combination of membrane impermeability and efflux were responsible for the increased insusceptibility of E. coli isolates to TCS. Various studies demonstrated the development of microbial resistance following exposure to TCS [44,46,47]. Reiss et al. [48] described the induction of expression of an efflux pump in P. aeruginosa following TCS exposure, resulting in high-level resistance to TCS and the antibiotic, ciprofloxacin. In E. coli, resistance can be attributed to either overexpression of the TCS target enzyme enoyl reductase or to changes in cellular permeability [49]. The most resistant bacteria have slow growth rate as compared to sensitive bacteria. On the contrary, E. coli resistant to TCS actually possess enhanced growth rates. The intrinsic resistance of P. aeruginosa to TCS can be attributed to: (1) a nonsusceptible enoyl reductase; (2) an outer membrane permeability barrier or; (3) pumping of the drug Int. J. Environ. Res. Public Health 2015, 12 5664 from the cell interior to its exterior [50]. The latter has been stated as the major reason for TCS insusceptibility [51,52] in P. aeruginosa. MRSA strains, meanwhile, may or may not show decreased sensitivity to triclosan [50,53]. Study conducted by Fan et al. [54] demonstrated that all S. aureus strains with decreased sensitivity overproduced the enzyme Fab I by 3–5 fold and moreover, mutations in Fab I were found in the most resistant strains. Major concern is that the mode of action of TCS and its target site in the microbes is similar to antibiotics. The enzymes enoyl reductase (product of Fab I among Gram-positive and Gram-negative bacteria and Inh A in Mycobacterium e.g., M. smegmatic and M. tuberculosis) involved in fatty acid biosynthesis are the targets for a number of structurally unrelated drugs, including TCS. For instance, isoniazid an antibiotic used to treat tuberculosis that targets the same enzyme system [55]. Thus, TCs belongs to the group of drugs, such as isoniazid (tuberculosis) and diazoborine (experimental antibiotic) which target the enzyme enoyl reductase. Hence, a mutation in the enzyme may lead to resistance to TCS and these drugs. The overuse of TCS may result in the development of cross-resistance to antibiotics, and thereby the emergence of bacterial strains resistant to both TCS and antibiotics [56]. The laboratory studies play an important role in evaluating mechanisms of action and resistance to biocides, including TCS. These studies are mostly related to a wide range of medical applications [49,57]. Various researchers have purported to demonstrate a correlation between the use of biocides including TCS and antibiotic resistance [55,58,59]. On the contrary, few authors advocated that TCS use should be regulated as all other biocides [8,60]. There was no relationship found between TCS application and antibiotic tolerance in methicillin-resistant Staphylococcus aureus and P. aeruginosa during a 10 year study conducted by [32]. Marshall et al. [61] reported no differences in overall titers of bacteria or frequencies of antibiotic resistance in a snap-shot investigation among homes using or not using bactericide products. Similarly, a comprehensive study by Cole et al. [62] found no relationship between the use of biocides including TCS and antibiotic resistance in homes with use/no use of biocidal agents. There was a concern that the use of TCS in dental hygiene products results in the development of TCS-resistant bacteria that are less sensible to common antibiotics. In view of this, an expert panel review concluded that there was no evidence of resistance development in the opportunistic or pathogenic microorganisms following the exposure to TCS [63]. The interim use of TCS containing dental hygienic products does not affect the stable microflora of the mouth or changes the susceptibility of Streptococci to antibiotics. However, chronic exposure to TCS demonstrated less significant decrease in antibiotic susceptibility in dental bacteria [64]. Usually, the introduction of bacteriostatic compounds to hinder plaque growth is seen as necessary [65]. Although TCS resistance in laboratory experiments may be linked with changes in antibiotic susceptibility, but comprehensive environmental investigations have not yet clearly established any relationship between TCS usage and antibiotic resistance. It is now well known that laboratory findings do not always apply in the real world environment [42]. In general, bacterial resistance to disinfectants is not a new phenomenon. The phenomenon of decreased susceptibility to various disinfectants was being described over a century ago by various researchers as thoroughly reviewed by Russell, [66], before the introduction of TCS. The study conducted by Tan et al. [67] indicated that resistance to TCS and other biocides is increasing. This conclusion was generally based upon minimum inhibitory concentrations (MIC) in laboratory experiments rather than Int. J. Environ. Res. Public Health 2015, 12 5665 bactericidal estimations. There might not be a correlation between a poor rate of kill and sensitivity at MIC level [49]. The use of MIC investigation to study emerging bacterial resistance is important as it can indicate a trend towards some resistance properties [40,68]. As resistance develops in a step-wise manner, it is judicious to conserve use and continued surveillance of susceptibility to antimicrobials. 5. Toxicity of TCS Triclosan possesses broad-spectrum antimicrobial action and has been classified as a Class III drug (compounds with high solubility and low permeability) by FDA [69]. Due to environmental concerns, TCS was declared as Priority Existing Chemical for full assessment under the Industrial Chemicals (Notification and Assessment) Act, 1989 (the Act) in the Chemical Gazette of 6 May 2003 [70]. Some signs of it have been already reported as TCS was not only found in WWTPs, but even in urine, plasma and breast milk in humans [20,71,72]. Studies have thus yielded contradictory findings regarding links between TCS and adverse health impacts in humans and animals. 5.1. Toxicity in Humans Absorption, distribution, metabolism and excretion are rapid in the case of TCS in human body. TCS is metabolized to glucuronide and sulfate conjugates (phase II metabolism) and are primarily excreted via urine. These hydrophilic conjugates of TCS limit the bioaccumulation of TCS. Some studies indicated that TCS is comparatively non-toxic to humans and other mammals. Conversely, studies indicated that TCS exposure resulted in contact dermatitis, or skin irritation [73]. A photo-allergic dermatitis (PACD) reaction can be triggered when the skin comes in contact with TCS and is further exposed to sunlight [74]. PACD can result in symptoms, such as eczematous rash on the body parts with combined TCS and sunlight exposure. According to the claims made by various manufacturers of TCS-containing toothpaste and soaps, the active ingredient continues to work even up to 12 h after use. This prolonged exposure to TCS in turn increases the risk of PACD. Triclosan has been found in urine, plasma, and breast milk of humans [16,20,75,76], but typically without attribution to specific sources of TCS exposure. High levels of TCS were found in 60% of human milk samples indicating the absorption potential of TCS into the body [15]. According to National Health and Nutrition Examination Survey (NHANES) data collected during 2003–2004, TCS was found in 75% of the analyzed urine samples [76,77]. The urinary data were collected for adult men and women and children between the ages of 6 and 11. NHANES is an ongoing annual survey conducted since 1999 by the US Centers for Disease Control and Prevention (CDC) aimed to collect data on selected chemicals, including TCS. This data is used to evaluate the nutrition quality and general health of the US population. Moreover, due to lipophilic nature of TCS, it may bioaccumulate in fatty tissues. Nevertheless, no study until date has established the carcinogenic, mutagenic, or teratogenic effects of TCS. Another area of concern is related to the hypothesis that TCS augments the production of chloroform. A study carried out by Fiss et al. [78] described that TCS may involve in the generation of chloroform, under certain conditions can almost double the chloroform formation in the drinking water treated with chlorine. On the contrary, studies [79] showed that there was no production of measurable quantities of chloroform within a normal tooth-brush when using toothpaste containing TCS and Int. J. Environ. Res. Public Health 2015, 12 5666 normal chlorinated drinking water. According to US EPA classification, chloroform is a possible human carcinogen. As a consequence, there was a campaign in UK underlining the potential of TCS to cause cancer, although Hao et al. studies [79] revealed that the amount of chloroform generated was lower in volume. Meanwhile, TCS in household dishwashing soaps reacts with chlorinated H2O to produce significant quantities of chloroform, a probable human carcinogen [80]. 5.2. Toxicity in Animals and Other Organisms The toxic effects of TCS were also studied in various animal models. For instance, its negative effect on the metabolism of thyroidal hormones causes hypothermia and an overall depression of the central nervous system (CNS) of mice [81]. The exposure to 0.03 mg·L−1 TCS was associated with induction of the expression of the metamorphic genes in tadpoles, which induced their premature metamorphosis [82]. Similarly, the study carried out by Kumar et al. [83] interrelated TCS exposure with decreased sperm production in male rats. The authors proposed the hypothesis that TCS blocks the metabolism of thyroid hormone as it presents a structure similar to the thyroidal hormone in regards to the binding of the specific receptors. Later, the endogenous hormones cannot bind to the occupied receptors. Its close structure resemblance to certain estrogens triggered masculinization of secondary characters in rice fishes [84]. A recent study by James et al. [85] pointed out that TCS can inhibit the estrogen sulfotransferase activity in sheep placenta which would cause negative effects in the fetus development. Although toxicity reports in humans from chronic usage of PCPs containing TCS as an active ingredient are not available, still it has been widely studied in laboratory animals. During chronic oncogenicity studies in mice, rats, and hamsters, treatment-related tumors were found only in the liver of male and female mice [23]. Application of the human relevance framework advocated that these tumors arose due to a mode of action which is not considered to be pertinent to humans [23]. However, Yueh et al. [86] found that long term exposure to TCS in mice enhances hepatocellular carcinoma. This mechanism of TCS induced liver carcinoma in mice and it should be evaluated as these findings strongly support the relevance of TCS toxicity to humans. Studies have also demonstrated that TCS accumulates in mice tissue with bioaccumulation factors of 3700–8400 [87]. This data indicates that fish contains concentrations thousands of fold higher than those found in the water column. Moreover, the bacterial transformation product of TCS in wastewater, methyl TCS is relatively lipophilic and stable in the environment, making it more likely to bioaccumulate in fatty tissue and will not photodegrade [88]. The lipid-based concentrations of methyl TCS detected in fish were considerably higher than the concentrations in lake water, indicating significant bioaccumulation of the compound. For aquatic organisms, the potential uptake mechanisms of lipophilic contaminants are direct uptake from water through exposed surfaces, mainly gills (bioconcentration), and also through the consumption of food (biomagnification) [21]. James et al. [89] demonstrated that demethylation of methyl TCS was slower than TCS conjugation in cattle fish. The bioaccumulation and slow conversion of methyl TCS in lower level consumers could serve as potential carriers of triclosan from the environment to higher level consumers in food chain. The structure and the function of algal communities in ecosystems receiving treated wastewater effluent may be affected by TCS contaminated wastewaters [90]. These alterations may result in shifts Int. J. Environ. Res. Public Health 2015, 12 5667 in nutrient processing capacity and natural food web structure of these streams. TCS was also identified as the responsible key pollutant for the observed effects on growth of the green algae, Scenedesmus valuolatus under realistic exposure conditions [91]. Various studies investigated the toxicity of TCS on higher aquatic organisms [92–95]. Acute toxicity values ranged from 1.4 to 3000 μg·L−1 with EC50 values for crustaceans (Daphnia magna mortality at 390 μg·L−1), insects (Chironomus tentans survival at 3000 μg·L−1), fish (Pimephales promelas mortality at 260 μg·L−1), higher plants (Lemna gibba growth inhibition at 62.5 μg·L−1) and microalgal species (Scenedesmus subspicatus growth inhibition at 1.4 μg·L−1, Skeletonema sp. at 66 μg·L−1). Moreover, the standard test organism, Selenastrum capricornutum (growth inhibition at 4.7 μg·L−1) was reported to be 30-fold more sensitive to TCS than the bacterium Vibrio fischeri (bioluminescence inhibition at 150 μg·L−1) [96]. The microalgae were found to be the most sensitive organism to TCS [92,94,97]. With the increasing concentrations of TCS in the environment, bacterial strains are more likely to adapt by developing resistance [59]. TCS has various important medical applications, thus the future goal must be to retain these important applications while eliminating the unnecessary ones for its safe use. All toxicity studies on TCS highlight the risks and suggest ban on TCS usage. In consequence, the FDA proposed, for comprehensive assessment of TCS toxicity on human health and animals, to regulate its further usage in consumer products until more information is available. Even though this proposal does not include environmental fate of TCS, this factor should be included in complete profiling of any chemical introduced into consumer products. In this sense, in 2010, more than 80 organizations petitioned EPA to ban TCS usage beyond pesticides. Minnesota has banned sale of any cleaning product (soaps) that contains triclosan on 16 May 2014. This ban makes the most manufacturers to phase out triclosan until early 2017. In 2013, FDA announced that final action on TCS usage in soaps will be taken by 2016 across the world. To complete the North American scenario, in Canada, approximately 1730 products including personal care products, cosmetics and health products containing triclosan were reported in 2011. Some reports indicate that triclosan would be a wide ranging contaminant in Canada. Therefore, from 2015 on, Health Canada is in the process to ban TCS. 6. Occurrence of TCS in Aquatic and Terrestrial Environment Incomplete removal of TCS from WWTPs and the applications of TCS laden biosolids into agricultural soils, leads to TCS being distributed in aquatic and terrestrial environment. Table 5 shows the prevalence of TCS in different environmental compartments worldwide. Environmental concentrations of TCS varied with surface water type (lake/river/streams with known input of raw wastewater) ranging from 1.4–40,000 ng·L−1; sea water 1 × 10 and Hc/Log Kow >1 × 109 High volatization potential Hc < 1 × 104 and Hc/Log Kow 8.1 and it converts into its neutral phenolic form if the water pH is below 7.9. In addition to pH, co-occurrence of dissolved compounds such as metals and organic matter may possibly affect photosensitivity of TCS [24]. Hence, the complex matrix of wastewater affects the efficiency of photodegradation of TCS in WWTP [25]. 3. Current Scenario of TCS Use and Safety Generally, TCS comes in the form of white powder. TCS has a weak aromatic, phenolic scent as it is a chlorinated aromatic compound. Ever since its invention, TCS has been widely used in numerous consumer products as illustrated in Figure 2 [6,8,10,12,26]. It is used as an active ingredient in dental products since 1980s in Europe and the mid-1990s in the United States after approval by the Food and Drug Administration [27]. More specifically, TCS is used in numerous personal care products, such as toothpastes, antibacterial soaps (bars and liquids), dishwashing liquids, deodorant soaps (bars and liquids), cosmetic and antiseptic products, and antiperspirants/deodorants [28]. Triclosan is also used in other consumer products, such as kitchen utensils, toys, bedding, clothes, fabrics, and trash bags. Int. J. Environ. Res. Public Health 2015, 12 5661 Figure 1. Molecular structures of TCS and its environmental transformation product, methyl-TCS. Figure 2. Various applications of triclosan. Int. J. Environ. Res. Public Health 2015, 12 5662 The concentration of TCS recommended by various government agencies to be used in various consumer products is given in Table 3. In 1989, the European Community Cosmetic Directive approved TCS usage as a preservative in cosmetics and toiletries up to 0.3% [28]. According to FDA, up to 0.3% TCS is permitted in toothpaste [29]. Similarly, as per the National Library of Medicine’s Household Product Database, TCS concentrations were reported to range from 0.1% to 0.3% in liquid hand soaps [30]. Table 3. Recommended levels of TCS in various consumer products (Adapted from [25]. Type of TCS-Based Product TCS Concentration (%) Oral care products Toothpaste 0.3 Mouth wash solutions 0.03 Dermally applied products (rinse off) Skin cleansers 0.3 Liquid hand soap 0.1–0.45 Dishwashing detergent 0.1 Dermally applied products (leave on) Body lotion 0.3 Facial Moisturizer 0.3 Deodorant/antiperspirants 0.3 Reference [29] [31] [28] [32] [30] [28] [28] [28] According to the FDA monograph for health care antiseptic drug products, which covered antibacterial soap products containing TCS, the recommended limits are up to 1% TCS for use in antiseptic washes and surgical hand scrubs in health care settings [33]. According to Governmental regulations in the European Union (EU) and the United States, only specified amount of triclosan can be used in some cosmetic and PCPs. TCS possesses a broad range of antimicrobial activity that encompasses several, types of nonsporulating bacteria and a few fungi, such as Plasmodium falciparum and Toxoplasma gondii [19,34]. At low concentrations, TCS inhibits the growth of microorganisms; at higher concentrations, it kills microorganisms. Different microorganisms show varied response to TCS as provided in Table 4. Triclosan blocks the active site of enoyl-acyl carrier protein reductase enzyme (ENR) thus impairing the production of bacterial lipids [35]. In consequence, cell membranes are not properly produced and bacterial proliferation stops. Therefore, only a small TCS dose is required to inhibit bacterial growth. As humans lack ENR enzyme, TCS has been considered harmless to them. Studies carried out by FDA found that TCS-fluoride paste prevented tooth deformities, such as gingivitis, tartar and plaque in a way that was superior to fluoride-only toothpastes. Over the last 30 years, TCS has also been successfully used as an antimicrobial agent in hospitals and for other biomedical purposes. The successful control of methicillin-resistant Staphylococcus aureus (MRSA) outbreaks in several clinical settings using TCS based products [36,37]. This led to the recommendation of showering/bathing with 2% TCS for the decolonization of patients whose skin is carrying MRSA [38]. However, susceptibility of MRSA strains to TCS has changed little over the last decade [39]. Later on there has been no relation found between TCS response in MRSA and other strains of S. aureus and antibiotic susceptibility or resistance [40]. Int. J. Environ. Res. Public Health 2015, 12 5663 Table 4. Different microorganisms affected by the antimicrobial action of TCS. Target Microorganisms Effective Concentrations Most sensitive strains Staphylococci, some Streptococci, some mycobacteria, Escherichia coli, Klebsiella pneumonia, Klebsiella spp., Enterobacter spp., Acinetobacter spp., Proteus spp. 0.01 mg·L−1 to 0.1 mg·L−1 and Proteus mirabilis, Plasmodium falciparum, Toxoplasma gondii Less sensitive strains Methicillin-resistant Staphylococcus aureus (MRSA) strains 0.1–2 mg·L−1 Enterococci Highly resistant strains Pseudomonas aeruginosa, Clostridium difficile Reference [33] [19] [40,142] [49] [49,143] The American Medical Association (AMA) has raised concerns about the use of TCS and some other antimicrobial agents in consumer products [41]. The AMA has encouraged the FDA to study the issue on the safety and effectiveness of antimicrobials including TCS. The progress of the current FDA evaluation will be monitored by the AMA on regular basis. The AMA also indicated that further research is required on the introduction of antimicrobials in massive consumer products. In 2009, the American Public Health Association (APHA) proposed that it would recommend the banning of TCS for household and non-medical uses. However, no further action has been taken as yet. Regardless of current efforts to review and regulate the proper use of TCS, a scientific debate lingers on its potential adverse impact on human health, environment and potential association to microbial resistance. 4. Emergence of Microbial Resistance to TCS The overuse of anti-microbial products may lead to increased resistance among bacteria. Considering the published studies, there is a dilemma whether TCS does or does not encourage the development of antibiotic resistance. Triclosan-resistant bacteria can be produced readily by their in vitro exposure to increasing TCS quantities and the consequent development of resistant colonies [42]. The mechanism of microbial resistance to TCS has been described by various researchers [43,44]. According to the authors, the resistance can be attributed to: (1) overproduction of targets/amplification or; (2) modification of target. Gomez-Escalda et al. [45] found that a combination of membrane impermeability and efflux were responsible for the increased insusceptibility of E. coli isolates to TCS. Various studies demonstrated the development of microbial resistance following exposure to TCS [44,46,47]. Reiss et al. [48] described the induction of expression of an efflux pump in P. aeruginosa following TCS exposure, resulting in high-level resistance to TCS and the antibiotic, ciprofloxacin. In E. coli, resistance can be attributed to either overexpression of the TCS target enzyme enoyl reductase or to changes in cellular permeability [49]. The most resistant bacteria have slow growth rate as compared to sensitive bacteria. On the contrary, E. coli resistant to TCS actually possess enhanced growth rates. The intrinsic resistance of P. aeruginosa to TCS can be attributed to: (1) a nonsusceptible enoyl reductase; (2) an outer membrane permeability barrier or; (3) pumping of the drug Int. J. Environ. Res. Public Health 2015, 12 5664 from the cell interior to its exterior [50]. The latter has been stated as the major reason for TCS insusceptibility [51,52] in P. aeruginosa. MRSA strains, meanwhile, may or may not show decreased sensitivity to triclosan [50,53]. Study conducted by Fan et al. [54] demonstrated that all S. aureus strains with decreased sensitivity overproduced the enzyme Fab I by 3–5 fold and moreover, mutations in Fab I were found in the most resistant strains. Major concern is that the mode of action of TCS and its target site in the microbes is similar to antibiotics. The enzymes enoyl reductase (product of Fab I among Gram-positive and Gram-negative bacteria and Inh A in Mycobacterium e.g., M. smegmatic and M. tuberculosis) involved in fatty acid biosynthesis are the targets for a number of structurally unrelated drugs, including TCS. For instance, isoniazid an antibiotic used to treat tuberculosis that targets the same enzyme system [55]. Thus, TCs belongs to the group of drugs, such as isoniazid (tuberculosis) and diazoborine (experimental antibiotic) which target the enzyme enoyl reductase. Hence, a mutation in the enzyme may lead to resistance to TCS and these drugs. The overuse of TCS may result in the development of cross-resistance to antibiotics, and thereby the emergence of bacterial strains resistant to both TCS and antibiotics [56]. The laboratory studies play an important role in evaluating mechanisms of action and resistance to biocides, including TCS. These studies are mostly related to a wide range of medical applications [49,57]. Various researchers have purported to demonstrate a correlation between the use of biocides including TCS and antibiotic resistance [55,58,59]. On the contrary, few authors advocated that TCS use should be regulated as all other biocides [...
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