Dear Editor,

Recently, Bolt and colleagues published a review article on possible effects of boron on human reproduction (Bolt et al., 2020[3]). The metalloid boron is required as an additive of fiberglass, structural material and ceramics. Human exposure typically occurs in mining. Since more than a decade, a discussion is ongoing concerning categorization of inorganic boron compounds as reproductive toxins (Moore, 1997[8]; Hasegawa et al., 2013[5]; ECHA, 2010[4]; SCCS, 2010[11]; ATSDR, 2010[1]). The authors summarized key findings relevant for this discussion (Bolt et al., 2020[3]):

Boron causes testicular toxicity in rats, mice and dogs.

Evaluation of reproductive and developmental toxicity remains a major challenge (Sachinidis et al., 2019[10]; Leist et al., 2017[7]; Shinde et al., 2015[14], 2016[13], 2017[12]). Currently, several research projects aim at the development of in vitro tests of developmental toxicity (Waldmann et al., 2017[15]; Rempel et al., 2015[9]; Balmer et al., 2014[2]). For this purpose, human stem cells have been exposed to test compounds to study if their differentiation is compromised (Weng et al., 2014[17]; Waldmann et al., 2014[16]; Krug et al., 2013[6]). However, although mechanisms can be evaluated in these in vitro systems, it still remains challenging to extrapolate to the in vivo situation and derive e.g. NOAELs. Therefore, the evaluation of possible human developmental effects of boron still depends on animal experiments and as far as available on epidemiological studies. The systematic review of Bolt and colleagues suggests that occupational exposure of humans to boron results in boron blood concentrations that are clearly below levels that cause adverse effects in animal experiments.

The author declares no conflict of interest.