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Boron appears to act similarly in humans and mammals in the following respects:
a) Once ingested, borates are almost completely absorbed in the gut and appear rapidly in the blood and body tissues.
b) In mammals, boron is distributed evenly throughout the body fluids. Unlike soft tissues and blood, bone takes up boron selectively to give levels more than four times higher than in blood serum. Boron also remains longer in bone, before elimination.
c) Boric acid is not metabolised (transformed) within the body. Thus the types and relative amounts of boron-containing compounds in the body will be the same in all mammals. This facilitates comparisons between work with humans and other mammalian species.
d) Boron is eliminated by the same route and at the same speed in humans and rats, with more than 90% of boron being eliminated through the urine and with half of the boron being eliminated in 24 hours or less.
Rats are the main species used in laboratory studies to determine the no-observed-adverse-effect level (NOAEL) of a substance. These similarities in the way boron acts in humans and rats increase the reliability of predicting effects in humans from effects in rats. More...
In laboratory animals, boron mainly affects the reproductive system and the development of the fetus.
In rats, the no-observed-adverse-effect level (NOAEL) for boron intake is 9.6 mg/kg body weight per day. The first effect which becomes apparent at greater intake level is reduced fetal body weight. At a boron intake level of about 13 mg/kg maternal body weight per day, the weight of rat fetuses is slightly reduced, and rib anomalies may be present. At approximately 55 mg/kg body weight per day rats experience changes in the testicles and become sterile.
In the rabbit, malformations of the heart and the circulatory system can be seen at boron intake levels of approximately 25 mg/kg body weight per day.
In the mouse, fetal body weight can be affected at approximately 80 mg/kg body weight per day.
| Type of animal | Type of effect | Daily intake of boron |
|---|---|---|
| Rat | No effect is seen. This is the NOAEL for the Rat. | 9.6 mg/kg body weight per day |
| Rat | The weight of fetuses is slightly reduced, and rib anamolies may be present. | 13 mg/kg maternal body weight per day |
| Rat | Changes in the testicles which lead to sterility | 55 mg/kg body weight per day |
| Rabbit | Malformations of the heart and the circulatory system | 25 mg/kg maternal body weight per day |
| Mouse | Fetal body weight can be affected | 80 mg/kg maternal body weight per day |
Studies on mice and rats showed no evidence of carcinogenicity of boric acid. More...
Because of the lack of human data and the limited amount of animal data, the EPA has classified boron as "not classifiable as to human carcinogenicity" in 1994.
Only a few studies on humans have investigated health effects associated with exposure to boron compounds. These show that exposure can be associated with short-term and reversible irritant effects on the upper respiratory tract, nasopharynx, and eyes. The sole long-term study did not identify any long-term health effects.
Two studies on people exposed to boron found no effects on human fertility and no statistically significant change in the relative number of boys and girls born. No studies have yet investigated other reproductive outcomes, such as time-to-pregnancy, conception delays, spontaneous abortions, and sperm quality. In order to identify populations which might be sensitive to boron exposure, and to evaluate reproductive effects more fully, further study of the role of other lifestyle or behavioural factors in relation to health and fertility is needed. More...
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