Hexaboron dizinc undecaoxide

Endpoint summary

Description of key information

While boron has been shown to adversely affect male reproduction in laboratory animals, there is no clear evidence of male reproductive effects attributable to boron in studies of highly exposed workers (Whorton et al.1994; Sayli 1998, 2001; Robbins et al.2010; Scialli et al.2010, Duydu 2011) or highly exposed individuals (Barr 1993, Cortes 2011). There is also no evidence of developmental effects in humans attributable to boron in studies of populations with high exposures to boron (Tuccar et al. 1998; Col et al.2000; Chang et al.2006). However, studies of human developmental effects are not as robust as the studies of male reproduction because of developmental ascertainment issues.
An important difference between laboratory animals and humans is the intrinsically higher zinc concentration in humans compared to laboratory animals. Normal levels of zinc in soft tissues in humans are over two times greater than in comparative tissues in laboratory animals (see Appendix D, Fig. 17 and King et al. 2000; Ranjan et al.2011; Yamaguchi et al.1996; Florianczyk 2000). The higher zinc concentrations in humans are also found in the target organs of boric acid, including fetal tissue, epididymis and testes (see section 5.10.1.3. “Specific Investigations”: Ahokas et al. 1980; Dorea et al. 1987; Suescun et al. 1981).
Several in vitro and animal studies recently completed investigated the protective effect of zinc against boric acid related developmental and fertility toxicity of boric acid. These studies provide important mechanistic data on the effects of zinc on boric acid related reproductive toxicity that raises doubt about the relevance of the effects for humans (see section 5.9. “Toxicity to Reproduction”: Durand 2013; Hofman-Huther 2013; Kirkpatrick 2013a, b; Edwards 2013).
 The protective effect of the intrinsically large zinc stores in the human body against boric acid associated toxicity explains in part the absence of effects in humans exposed to high levels of boron.

Additional information

Fertility:

The potential reproductive effects of inorganic borate exposure to a population of workers at a large mining and production facility was assessed using the Standardised Birth Ratio (SBR), a measure of the ratio of observed to expected births. The average exposure for the highest exposure group was 28.4 mg B/day (approximately 0.4 mg B/kg bw/day) for two or more years. The average duration of exposure was 16 years. The number of offspring indicated no adverse effects on reproduction in these workers (Whorton et al., 1994). Exposure data used in this study was collected using the total dust sampler. The IOM equivalent exposure would be 71 mg B/day.

In a study of a highly exposed population in Turkey, where exposure comes mainly from naturally high levels of B in drinking water (up to 29 mg B/L) as well as from mining and production, no adverse effect has been reported on fertility over three generations (Sayli, 1998a,b; 2001a,b).

A study to assess the health effects of boron exposure was performed to assess the fertility/infertility of subjects exposed to borates environmentally and/or occupationally in a country with all the worlds largest deposits were described (Sayli, 2004). The study covered all centres of borate production, an area of 350 km long and 150 km wide. Drinking water piped out from springs and wells had boron concentrations 0.2 to 29 ppm (mg B/kg or mg B/L). Dust amount at work sites was below the permissible level of 10 mg/m3. The work, questionnaire based, was realized in field as an observational one. Residents were visited at home and in coffeehouses in villages and public buildings in towns, and workers at facilities and ore pits without any selection. The inquiry was mainly concerned with marital state and child-bearing properties of probands and other members in the kindred. Infertility of the primary type among 2529 probands as a convenient sample was 3.1 % changing from 0.0 % to 6.5 % regarding subpopulations from 12 centres, differences being statistically insignificant. No differences with respect to birthplace and professional state were revealed. Pedigree data showed the rate was 3.2 % covering 14320 marriages over 3 generations. No appreciable concentration of infertiles either in subgroups or in so-called "borate families" in borate towns was observed. Approached as an independent test, marriages of male and female sibs of proband and his (her) spouse ranged from 2.4 % to 4.2 %. None of these was so far higher than found in different sets of controls and of general population over 50000 families. Childlessness was found in 1.7 % among workers vs 4.3 % among employees from all facilities, the difference attributable to socio-cultural grounds. In conclusion, it can be stated that continuous boron exposure at the present level does not affect human reproductive performance adversely primarily and secondarily over 3 generations for at least 60 - 70 years.

Tuccar et al. (1998) investigated the effects of boron on reproductive and developmental effects in three generations of families living in boron rich regions of Turkey. This study was part of a larger study of the health effects of boron in residents living in boron rich territories of Turkey (Sayli 2001a,b; Sayli et al. 1998b; Sayli 1998a; Sayli 2003). The study population was divided into three subgroups based on levels of environmental boron exposure. Region I included residents living in boron rich territories, located close to borate pits and a processing plant. Drinking water in Region I come from natural springs and wells that contain as much as 29 ppm B. Region II residents lived far away from borate deposits. The concentration of boron in drinking water serving residents of Region II was between 0.30 and 0.50 ppm.   Region III residents were born and live within the study region with some residents close to and some far from deposits and pits. Daily exposures of 6.77 mg/day for males living in the boron-rich region and 1.26 mg/day for controls was later estimated for residents of these regions by Korkmaz et al. (2007). However, no exposure estimates of women during their pregnancies were available. A total of 226 families over three generations from Region I, 164 families from Region II and 177 families from Region III were included in the study. Questionnaires were administered by home visits, and workers were contacted at the borate plants and pits. The questionnaires obtained information on number of pregnancies, early infant deaths, congenital malformations, stillbirths and spontaneous abortions. The infant death rate was higher in Region II, the region with the low boron levels, compared to the other regions. No other significant differences in developmental effects were observed between high boron exposed populations compared to low boron exposed populations. The observed number of congenital malformations was not sufficient in the study groups to allow for statistical evaluation.

Boron treatment of rats, mice and dogs has been associated with testicular toxicity, characterised by inhibited spermiation at lower dose levels and a reduction in epididymal sperm count at higher dose levels. Studies in human workers and populations have not identified adverse effects of boron exposure on fertility (Robbins et al. 2010, Scialli et al. 2010).

Chinese workers were studied by a research team from the Beijing University of Science and Technology and the China National Environmental Monitoring Centre in collaboration with University of California at Los Angeles (Robbins et al. 2010). Boron exposure/dose measures in workplace inhalable dust, dietary food/fluids, blood semen and urine were collected from boron workers and two comparison worker groups (n = 192) over three months and correlations between boron and semen parameters. Parameters for total sperm count, sperm concentration, motility and morphology were not significantly different across the three boron exposure comparison groups. Continuous measures of boron in workers' post-work shift urine and blood were correlated with percent normal morphology but this did not remain statistically significant after controlling for age, abstinence interval, smoking, alcohol intake, pesticide exposure and mg blood levels. No other significant correlations between boron levels and conventional semen parameters were found. DNA strand breakage and percent apoptotic cells were similar cross the exposure groups and not correlated with boron levels in post-work shift urine or blood (p > 0.05). Sperm aneuploidy and diploidy did not differ by exposure group or boron levels (Robbins et al. 2010).

In addition to this, boron mining and processing workers in North-East China have been investigated for a relationship between boron exposure and reproductive health (Chang et al, 2006). Reproductive health outcomes were explored, including delayed pregnancy, multiple births, spontaneous miscarriages, induced abortions, stillbirths and unusual male:female offspring.  On average boron workers fathered nearly 2.0 pregnancies compared with 2.1 pregnancies in the control group (P = 0.6). Of the self-reported pregnancies fathered by boron workers, an average of 1.3 resulted in livebirths, compared to an average of 1.4 for the comparison group (P = 0.3). A significant difference existed between groups in delay in pregnancy, defined as the inability to conceive within 1 year of desiring a child, with boron workers experiencing greater delays. However in logistic regression models adjusting for age, education, race, tobacco, alcohol and soybean consumption the difference was no longer statistically significant (P = 0.11) with an odds ratio of 1.7 for boron workers compared to the control group (95 % confidence interval, 0.09 to 3.5).  

The infant death rate in Region 2 (low boron area) was higher than those of other regions (significantly different). Although it is difficult to recognize spontaneous abortions and stillbirths in a retrospective study depending on the description only the probands (mostly females) these were considered separately, but no differences were found. The observed number of congenital malformation was not sufficient within the study groups to perform statistical tests. There was no evidence that boron affects human development adversely.

 

Shifts in sex ratio at birth were investigated (Robbins et al. 2008). The paternal origin were assessed by assaying sperm X:Y ratio in men exposed to a range of environmental and workplace boron. Participants included 63 workers in the boron industry; 39 men living in an area of high environmental boron but not employed in the boron industry and 44 controls living in an area of low environmental boron. Total daily boron was calculated as the sum of boron in 24-h duplicate food and fluid intakes plus personal air sampling for workplace inhalable dust. Internal dose was measured in blood, urine and semen. Sperm were analysed by fluorescence in situ hybridisation for Y- versus X-bearing cells. Potential confounders were identified using a questionnaire. Total exposure was correlated with internal dose. Daily boron exposure in boron workers from dust, food and fluid intake was 41.2 ± 37.4 mg (mean ± SD); in the high boron community comparison in was 4.3 ± 3.1; and in the low boron control is was 2.3 ± 3.0.

Total exposure was correlated with internal dose (Pearson correlation for total exposure and boron in blood = 0.63, P < 0.0001; semen = 0.80, P < 0.001; and urine = 0.79, P < 0.0001). Linear regression of logged boron in biologic fluids on Y:X ratio was significant for blood P = 0.2, semen P = 0.0003 and urine P = 0.005. Additionally when subjects were categorised by exposure groups, decreased Y:X sperm ratio was found for boron workers compared with men in a high boron environment and controls (P,0.0001). Exogenous environmental or workplace boron exposures were associated with decreases in Y-versus X-bearing sperm. However, the Y:X ratio did not correlate with the boron concentration in blood within exposure groups.

Scialli et al (2010) reviewed and summarized the papers of the study of Chinese workers that described the reproductive effects of boron exposure, particularly in North Eastern China. This study was reported in a series of publications, some of which were in Chinese and some in English. Boron workers (n = 75) had a mean daily boron intake of 31.3 mg B/day, and a subset of 16 of these men, employed at a plant where there was heavy boron contamination of the water supply, had an estimated mean daily boron intake of 125 mg B/day. Estimates of mean daily boron intake in local community and remote background controls were 4.25 mg B/day and 1.40 mg/day, respectively. Three categories of endpoints were identified: Semen analysis, reproductive outcome and sperm X: Y ratio. There were no statistically significant differences in semen characteristics between exposure groups including in the highly exposed subset, except that sperm X: Y ratio was reduced in boron workers. Within exposure groups the X: Y ratio did not correlate with the boron concentration in blood, semen and urine. While boron has been shown to adversely affect male reproduction in laboratory animals, there was no clear evidence of male reproductive effects attributable to boron in studies of highly exposed workers (Scialli et al. 2010).  

 

The Chinese semen studies in highly exposed workers are a major source of information as to human reproductive toxicity. Not only are these the most exposed workers, but the Chinese worker study is the most sensitive study that has been carried out as semen analysis was performed, a very sensitive detection system for testicular damage. The highest exposed workers were exposed to about 5 mgB/Kg/day, about one third to one quarter of the NOAEL for testis effects in rodents. However, this shows that humans are not significantly more sensitive to this type of toxic effect than rodents.

 

Moreover, Espinoza-Navaroo and colleagues analyzed the spermogram values in a sample of healthy young males, residing in Arica, Chile (Espinoza-Navarro, 2010). One hundred and two healthy university students volunteers aged 18 to 30 years answered a questionnaire about fertility, habits and andrologic diseases and provided a semen sample. Within three hours after ejaculation, semen volume, pH, sperm concentration, motility and morphology were analyzed. Six percent of volunteers had offspring, 1% declared to be infertile, 32% smoked and 78% consumed alcohol. Semen pH was 7.6 ± 0.5, volume, 2.9 ± 1.6 ml, sperm concentration, 62.8 ± 62.3 x 106/ml, normal morphology, 15.0 ± 7.9%, overall motility, 42.2 ± 23.2 % and grade A motility, 19.2 ± 18.6%. The percentage of subjects that had normal semen values was 82% for total sperm count, 76% for sperm concentration, 72% for volume, 64% for vitality, 63% for pH, 57% for morphology, 38% for overall motility and 26% for grade A motility. This sample of healthy young males had a normal sperm count in comparison with international reports.

In an occupational exposure study in Turkey with 102 exposed workers and 102 control workers, the correlation between combined environmental and occupational exposure to boron and boron levels in blood, urine and semen was analyzed (Duydu et al. 2011). This study was conducted to investigate the reproductive effects of boron exposure in workers employed in a boric acid production plant in Turkey. A high environmental oral exposure via contaminated well water of the central cantina was found for both groups leading to a re-classification of all 204 workers into 4 groups based on their calculated total Daily Boron Exposure (DBE) and their blood boron concentrations. The new control group had blood boron levels below the Limit Of Quantification (LOQ) of 48.5 ng/g, additionally a low, medium and high exposure group were derived. Average age and average period of employment compared favourably between the groups. While blood and urine levels correlate reasonably well with the calculated exposure, the correlation between blood boron levels and semen boron levels is very weak. Boron is accumulated in semen and the concentration factor is highest at the lowest exposure.

Adverse effects in hormone levels were absent when exposure groups are compared to the new control group. For any of the semen parameters a statistically significant differences was seen between new control group and exposure groups. These facts indicates that boron does not have an adverse effect on the male reproductive system at typical exposure conditions.

 

In addition to the published data additional regrouping of the exposed workers was done based on the urine boron level and the semen boron level into 4 groups respectively. This data is confidential and legal property of ETI MINE SA and therefore only shortly summarised here. It has to be stated that for several parameters the scattering of values within the respective groups are large resulting often in standard deviations that have almost the same magnitude as the average value. In these cases the relative low number of volunteers per group complicates the determination of correlations.

Correlation between semen levels and adverse effects:

For Neck/mid-piece defects (%) a statistically significant difference in the percentage was seen in the pairwise comparison of the low dose with the high dose and the mid dose with the high dose but not the control with the high dose. No clear dose response is seen, and the correlation coefficient of 0.228 is very weak.

Correlation between urine levels and adverse effects:

For FSH (follicle stimulating hormone) the global null hypothesis that all group means are equal was rejected. The significant pair wise differences are between Control-Medium and Medium-High. Neither a clear dose response nor a significant correlation was found. A statistically significant correlation was seen between urine boron concentrations and LH (lutenising hormone) levels. Nevertheless this correlation is very weak.

Besides the effects stated in the two preceding paragraphs no other statistically significant effects or correlations between boron levels in body fluids and semen parameters or hormone levels were found.

The weak effects, that were seen, are all not indicative for a reproductive toxicity potential of boric acid but are rather most probably incidental. The absence of clear correlations between urine or semen boron levels and adverse effects in semen parameters strengthens the position made in the publication that boron does not have an adverse effect on the male reproductive system at typical exposure conditions.

Exposure to boron via the drinking water

The genetic and environmental regulation of lithium and boron were studied in the blood of Chilean healthy subjects (Barr, 1993). Samples of blood (n = 40) and water (n = 47) were collected at seven locations in the province of Tarapaca. Most of the healthy subjects were Aymara who had been resident in the respective communities for at least 3 years. Estimated intake of boron was 27 ± 8 mg B/day in the village of Camarones and 21 ± 7 mg B/day in Molinos. These appeared associated with drinking local river waters containing 15.2 and 11.7 mg B/L, respectively.Lithium concentrations in water and blood exhibited a linear relationship, as did the boron concentrations in these fluids. Because some of the individual subjects (n = 15) were first-degree relatives, a genetic component to the regulation of blood levels was explored. The variance in blood levels of lithium and boron was significantly greater between than within families [p < 0.0001]. There are environmental and apparent genetic contributions to the regulation of blood levels of lithium and boron in healthy human subjects. Recognition of the probable genetic component raises the possibility of Identifying transport proteins that may have relevance to the roles of these elements In human health and disease.

Additionally, a study was conducted between 2006 and 2010, which measured exposure levels of boron in drinking water and urine of volunteers in Arica, an area in the North of Chile with high levels of naturally occurring boron (Cortes, 2011). Samples were taken of tap and bottled water (173 and 22, respectively), as well as urine from 22 volunteers, and subsequently analyzed by inductively coupled plasma spectroscopy (ICP-OES). Boron varied in public tap water from 0.22 to 11.3 mg/L, with a median value of 2.9 mg/L, while concentrations of boron in bottled water varied from 0.01 to 12.2 mg/L. Neither tap nor bottled water samples had concentrations of boron within WHO recommended limits. The concentration of boron in urine varied between 0.45 and 17.4 mg/L, with a median of 4.28 mg/L and was found to be correlated with tap water sampled from the homes of the volunteers (r=0.64). Authors highly recommend that in northern Chile – where levels of boron are naturally high – that the tap and bottled water supplies be monitored in order to protect public health and that regulatory standards also be established for boron in drinking water in order to limit exposure.

Statistical Power

Study size and statistical power are often issues raised with epidemiological studies. The statistical power of the worker studies in China and Turkey are actually better than the animal studies. Robbins et al. (2010) reported a 90% power to detect a 20% difference between the exposure groups for the majority of motility parameters. Scialli et al. (2010) calculated the detection of a 25% difference in sperm concentration between groups at the 5% significance level with about 80% statistical power. Additionally, the study power to detect a doubling of risk of failure to meet WHO criteria for normal semen analysis at the 5% significance level is also about 80%. The German Federal Institute for Occupational Safety and Health calculated the statistical power to detect fertility effects in rodent bioassays. The authors reported a 90% power to detect approximately 32% difference in sperm count in rats, approximately 20% difference in sperm motility (%), and 86% detectable difference in abnormal sperm (%). In B6C3F1 mice, detectable differences for sperm count, sperm motility and abnormal sperm were 50%, 31% and 47% respectively. In CD-1 mice, detectable differences of 30%, 21-30%, and 66-106% for sperm concentration, motility and abnormal sperm. (Mangelsdorf and Buschmann, 2002, refer to Section: Specific investigations: other studies).

Developmental: 

While developmental effects of boron have been observed in rodent bioassays that include fetal body weight reduction and minor skeletal variations, there is no evidence of developmental effects in humans attributable to boron in studies of populations with high exposures to boron (Tuccar et al 1998; Col et al. 2000; Chang et al 2006). Three epidemiological studies evaluating high environmental exposures to boron and developmental effects in humans have been conducted. Epidemiological studies of human developmental effects have shown an absence of effects in exposed borate workers and populations living in areas with high environmental levels of boron. 

Percutaneous absorption

The in vivo results in human volunteers show that percutaneous absorption of boron, as boric acid, borax, and disodium octaborate tetrahydrate, through intact human skin, is low and is significantly less than the average daily dietary intake (Wester et al., 1998; Hui et al., 1996). in vivo percutaneous absorption was 0.226 (SD = 0.125), 0.210 (SD = 0.194) and 0.122 (SD = 0.108) mean percentage dose for boric acid, borax and disodium octaborate tetrahydrate, respectively. In an in vitro study with human skin, where the amount of borate available to the skin surface was not limited to the amount that could be kept in contact with the surface, larger amounts of boron could be absorbed (Wester et al., 1998; Hartway et al., 1997).

Data-derived assessment factors for boron

Inorganic borates are encountered in many settings worldwide, spurring international efforts to develop exposure guidance (US EPA, 2004; WHO, 2009; ATSDR, 2010, cited in Maier et al. 2014) and occupational exposure limits (OEL) (ACGIH, 2005; MAK, 2011, cited in Maier et al. 2014). An updated OEL to reflect new data and current international risk assessment frameworks is derived (Maier et al. 2014). The toxicity and epidemiology data on inorganic borates to identify relevant adverse effects were assessed. International risk assessment frameworks (IPCS, 2005, 2007, cited in Maier et al. 2014) were used to evaluate endpoint candidates: reproductive toxicity , developmental toxicity , and sensory irritation. For each endpoint a preliminary OEL was derived and adjusted based on consideration of toxicokinetics, toxicodynamics and other uncertainties. Selection of the endpoint point of departures (PODs) is supported by dose-response modeling. Developmental toxicity was the most sensitive systemic effect. An OEL of 1.6 mg B / m³ was estimated for this effect based on a POD of 63 mg B / m3 with an uncertainty factor (UF) of 40. Sensory irritation was considered to be the most sensitive effect for the portal of entry. An OEL of 1.4 mg B / m³ was estimated for this effect based on the identified POD and an UF of 1. An OEL of 1.4 mg B / m3 as an 8 -h time - weighted average (TWA) is recommended.

The US EPA (2004, cited in Maier et al. 2014) divided the POD (10.3 mg B/kg-day) by a composite UF of 66 for deriving their Reference Dose (RfD). This composite factor was calculated by multiplying the subfactors of 3.3 for interspecies differences in toxicokinetics (based on data for boron clearance rates in rats versus humans), a default value of 3.2 for interspecies differences in toxicodynamics, a value of 2.0 for variability in human toxicokinetics (based on data on human variability in glomerular filtration rate), and the default factor of 3.2 to account for variability in human toxicodynamics.

Based on these results, the assessment factors of 1.5 and 2.0 for variability in human toxicokinetics (based on data on human variability in glomerular filtration rate) for sensitive population and workers, respectively, are appropriate for DNEL derivation in this chemical safety assessment. To account for variability in human toxicodynamics, the assessment factor of 3.2 will be used.