1. Chemistry and application
Bisphenol S (BPS, CAS 80-09-1) consists of two hydroxyphenyl groups connected by a sulfonyl group and has the chemical formula C12H10O4S (Figure 1). BPS has a listed production volume between 1 000 and 10 000 tons per year according to the European Chemicals Agency (ECHA 2014). It is used as monomer in synthetic polymers such as polyethersulfone (PES) and polysulfone (PSU) as well as in epoxy resins also used for food contact (Lotti et al 2013, Lotti et al 2011, Liaw 1998 and Gao and Li 2000).
PES has replaced bisphenol A (BPA) based polycarbonates in plastic baby bottles (Simoneau et al 2011). BPA based epoxy resins in cans have been substituted with BPS (Viñas et al 2010). Further, BPS is used in thermal papers (Liao et al 2012) and it has been detected in recycled food carton and food packaging paper (Liao et al 2012).
Investigations of acute toxicity have shown LD50s of BPS ranging from 1.6 to > 5 grams/kg body weight (bw) (ECHA 2014). According to the European Chemicals Agency (ECHA) database, all but 1 of 10 in vitro genotoxicity studies returned negative results. One recent peer-reviewed study showed evidence of genotoxicity in immunofluorescence and chromosomal aberration tests (Lee et al 2009). Two repeated dose studies with limited documentation reported NOAELs ranging from 40 to 97 mg/kg bw/day on endpoints including nephrotoxicity and body weight gain (ECHA 2014). Based on two further repeated dose studies, the U.S. Environmental Protection Agency (EPA) concluded that BPS has a high hazard potential when used as a printing ink alternative. According to ECHA, one study with limited documentation has been carried out on reproductive and developmental toxicity; it reports No Observed Adverse Effect Levels (NOAELs) of 60 mg/kg bw/day for reproductive toxicity (ECHA 2014). Regarding endocrine disrupting properties, BPA and BPS are considered to have comparable estrogenic potencies (Grignard et al 2012, Kuruto-Niwa et al 2005). Both have a much lower estrogen receptor binding affinity than estradiol. In vitro analyses showed BPS to be an activator of both human estrogen receptors (ER) and a weak antagonist of the androgen receptor (AR), whereas interference with the pregnane X-receptor (PXR) could not be confirmed (Molina-Molina et al 2013). In rat pituitary cells BPS at femto- to picomolar concentrations interrupted cell signaling affecting cell proliferation, cell death and prolactin release indicates carcinogenic properties of BPS (Viñas and Watson 2013). BPS endocrine responses were different and stronger in combination with BPA and nonylphenol (Viñas and Watson 2013).
A 2011 study on polyethersulfone baby bottles with a limit of detection (LOD) of 100 ng/kg did not detect any migration of BPS (Simoneau et al 2011). In 2011, Spanish researchers only detected BPA but not BPS migration in soft drinks (LOD 5-25 ng/L) (Gallart-Ayala et al 2011). Another Spanish study detected BPS at concentrations up to 170 and 35 ng/ml in supernatants and solid foods packaged in epoxycoated cans, partially exceeding the SML specified in EC 10/2011 (Viñas et al 2010). BPS was also detected in foods packaged in cartons at levels up to 143 ng/g (Liao et al 2012), but much less frequently and at lower levels in food contact papers (detection rate: 8.3%, highest concentration: 12 ng/g). In an occurrence survey of bisphenols in China (2012) and the U.S. (2008-2012), BPS was found in around 20 % of samples in both countries (Liao and Kannan 2013a, Liao and Kannan 2013b). BPS contributed less than 10% to overall bisphenols exposure in both China and the U.S.. The highest concentrations were measured in meat and meat products. Canned food had the highest mean concentrations (Liao and Kannan 2013a, Liao and Kannan 2013b). According to a biomonitoring study carried out in the U.S. and several Asian countries, BPS was ubiquitous in samples from Japan and Vietnam; highest concentrations were found in urine from Japan, followed by the U.S. and China. Based on a simple pharmacokinetic approach, estimated daily intakes (EDIs) of BPS were calculated to be 3.47, 1.48, 0.707 µg/person/day for Japan, the U.S. and China, respectively (Liao et al 2012). BPS was further found in sediments from industrialized areas in the U.S., Japan and Korea, reaching almost 2 mg/g dry weight in Korea (Liao etb al 2012). Together with BPA, BPS represented 90% of all bisphenol concentrations in sediments. BPS is less environmentally degradable than other bisphenols; degradation has only been accomplished under specific conditions (Danzl et al 2009, Sakai et al. 2007, Ike et al 2006, Ogata et al 2012, Toyama et al 2013, Cao et al 2013). BPS levels in biomonitoring studies and sediments may further increase in the coming years, due to the replacement of BPA with BPS.
Under the European Regulation EC 10/2011, BPS is authorized for use as a monomer in plastic food contact articles with a specific migration limit of 0.05 mg/kg food. The SML is not based on toxicological assessment but rather on BPS’ low migration rates. According to the 2011 report of the European Food Safety Authority’s (EFSA) Scientific cooperation Working Group (ESCO WG), no national legislation on BPS exists in the European Member States. In the U.S., BPS is authorized as an indirect food additive to be used as a monomer in the production of polyethersulfone resins intended for repeated use. No specific regulation of BPS is set in the Code of Federal Regulation (CFR, Title 21, Sec.177.2440).
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