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Diss Factsheets

Toxicological information

Epidemiological data

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Administrative data

epidemiological data
Type of information:
other: worker reproductive toxicity study
Adequacy of study:
key study
Study period:
No data
other: Not applicable
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Acceptable well documented publication which meets basic scientific principles. Read-across is justified on the following basis: The family of zinc borates that include Zinc Borate 500, Zinc Borate 2335 and Zinc Borate 415 (also known as Zinc Borate 411). Zinc borate 500 is anhydrous Zinc Borate 2335 and Zinc Borate 415 has different zinc to boron ratio. Zinc borate 2335 (in common with other zinc borates such as Zinc borate 415 and 500) breaks down to Zinc Hydroxide (via Zinc oxide) and Boric Acid, therefore the family of zinc borates shares the same toxicological properties. Zinc borates are sparingly soluble salts. Hydrolysis under high dilution conditions leads to zinc hydroxide via zinc oxide and boric acid formation. Zinc hydroxide and zinc oxide solubility is low under neutral and basic conditions. This leads to a situation where zinc borate hydrolyses to zinc hydroxide, zinc oxide and boric acid at neutral pH quicker than it solubilises. Therefore, it can be assumed that at physiological conditions and neutral and lower pH zinc borate will be hydrolysed to boric acid, zinc oxide and zinc hydroxide. Hydrolysis and the rate of hydrolysis depend on the initial loading and time. At a loading of 5% (5g/100ml) zinc borate hydrolysis equilibrium may take 1-2 months, while at 1 g/l hydrolysis is complete after 5 days. At 50 mg/l hydrolysis and solubility is complete (Schubert et al., 2003). At pH 4 hydrolysis is complete. Zinc Borate 2335 breaks down as follows: 2ZnO • 3B2O3 •3.5H2O + 3.5H2O + 4H+ ↔ 6H3BO3 + 2Zn2+ 2Zn2+ + 4OH- ↔ 2Zn(OH)2 ____________________________________________________________ Overall equation 2ZnO • 3B2O3 •3.5H2O + 7.5H2O ↔ 2Zn(OH)2 + 6H3BO3 The relative zinc oxide and boric oxide % are as follows: Zinc borate 2335:zinc oxide = 37.45% (30.09% Zn) B2O3 = 48.05% (14.94% B) Water 14.5% Zinc borate 415: zinc oxide = 78.79%; (63.31% Zn) B2O3 = 16.85% (5.23% B) Water 4.36% Zinc borate, anhydrous: Zinc oxide = 45 % B2O3= 55% (17.1 % B)

Data source

Reference Type:
An overview of male reproductive studies of boron with an emphasis on studies of highly exposed Chinese workers
Scialli AR, Bonde JP, Brüske-Hohlfeld, Culver DB, Li Y & Sullivan FM.
Bibliographic source:
Reproductive Toxicology 29: 10 - 24.

Materials and methods

Study type:
other: worker reproductive toxcity study
Endpoint addressed:
toxicity to reproduction / fertility
Test guideline
according to guideline
other: No data
not specified
Principles of method if other than guideline:
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, but outcome measures in these studies were relatively insensitive, based on family size and did not include an evaluation of semen endpoints. This review outlines the general outline of boron and its reproductive effects and includes the basic procedures used in a Chinese study. Three categories of endpoints were identified: Semen analysis, reproductive outcome and sperm X:Y ratio.
GLP compliance:
not specified

Test material

Constituent 1
Reference substance name:
Boric acid and borax
Boric acid and borax
Details on test material:
- Name of test material: Boric acid; Borax


Details on study design:
A review panel reviewed and summarized papers of studies of highly exposed Chinese workers in China. Male workers at one boron mine and four boron processing plants in northeast China were studied. The 5 workplaces were selected based on the location, number of employees and the presence and cooperation of an industrial hygienist at the site. 957 men between 18 and 40 years of age agreed to an interview to provide demographic, exposure, reproductive and general health information. Of the interviews, 945 were considered eligible. Potential subjects were 25 - 35 years of age, married without a history of contact with a number of substances and disorders. In addition to general physical examination, men were evaluated for hair distribution breast tissue size; the size, firmness and location of testes, epididymides and ductus deferens and the presence of variocele of hydrocele.
A comparison group of 251 men were recruited from an area 30 miles away with low background boron exposure levels. Later in the course of the studies, another comparison group was added, consisting of 63 workers without occupational exposure to boron but drawn from the same community as the boron workers and was termed the local community control group.

Boron content of environmental and biological samples was measured. The detection limits and relative standard deviation for boron in different media were: Airborne particulates 0.01 μg/g ± 5.01 %; food 0.0063 μg/g ± 0.63 %; drinking water and urine by ICP-AES 0.0072 ng/mL ± 0.6 %; drinking water and urine by ICP-MS 0.057 μg/mL ± 1.25 %.
Personal measurements were performed in borate processing areas using IOM inhalable dust sampler. Total airborne dust concentrations ranged from 0.3 to 33 mg/m3. The boron concentration in the dust ranged from 1.5 to 4.2 %.
Ingestion was measured from the sum of boron intake from food and drink several times using a duplicate plate method. Boron workers were calculated to ingest a weighted mean of 16.9 mg B/day, while the community comparison group's boron intake was 4.25 mg B/day.
Exposure assessment:

Results and discussion

Semen analysis:
The data do not indicate that boron exposure under the conditions described impairs testicular function with respect to sperm concentration, motility morphology or chromatin denaturability. The methods used to assess these endpoints were standard methods reliably performed.

Reproductive success:
Evaluation of sex ratio did not show a significant effect of boron exposure.

Sperm X:Y ratio
There were differences in Y:X ratio across the three groups defined by boron exposure. Y:X ratio appeared to be more related to group membership than boron exposure. The within-subject variability of Y:X ratio and possible determinants of Y:X ratio are unknown, except for possible miniscule effects of age, calendar time and race. Y:X ratio is not known to be associated with impaired semen quality, reproductive success or offspring health.

There is no clear evidence of male reproductive effects attributable to boron in studies of highly exposed workers.
Confounding factors:
No data
Strengths and weaknesses:
Semen analysis:
There are questions regarding the selection of subjects for semen analysis and whether bias may have been introduced by subject selection.
The statistical power of the analyses was considered adequate.

Reproductive success:
The methods were not adequate to address the question of whether men exposed occupationally have different reproductive experiences than men not so exposed.
The assessment of sex ratio in China is considered unlikely to be reliable.

Applicant's summary and conclusion

Reproductive outcomes in the wives of 945 boron workers were not significantly different from outcomes in the wives of 249 background control men after adjustment for potential confounders. 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.