It has been hypothesised (and at the moment it remains a hypothesis) that some reported cases of reduced sperm count in men may be due to exposure to chemicals in the environment which mimic the natural female hormone oestrogen. There is still no evidence that there is a general problem in humans and no evidence that chemicals in general, or any chemicals specifically, are the cause. However, this hypothesis has sparked interest in the development of screening tests which could be used to identify oestrogenic substances.
The most recent in-vivo (live experimentation) studies specifically intended to look for oestrogenic effects are a series of internationally accepted and validated tests which measure changes in the reproductive organs of female rats which occur via processes under oestrogenic control. They have shown that all the phthalates ranging from dibutyl phthalate (DBP) to diisodecyl phthalate (DIDP) produce no oestrogenic effects.
In addition, numerous multi-generation fertility studies have been carried out on many different phthalates. The most recent of these are 2-generation studies which demonstrate that exposure of rats to diisononyl phthalate (DINP) and DIDP in utero, during lactation, puberty and adulthood does not affect testicular size, sperm count, morphology or motility, or produce any reproductive or fertility effects. No outcome which might be anticipated from hormone modulation was observed. The maximum level dosed was around 600 mg/kg bw/day.
In a 1995 publication Sharpe et al hypothesised that the observed effects on rat testes after administration of a low dose of butylbenzyl phthalate (BBP) were related to an oestrogenic mechanism. In fact there are some inconsistencies in this study and therefore it is being repeated in other laboratories. One of these repeat studies [23] has been completed and shows no effects on testes at these low doses.
It is true that some laboratories using newly developed in-vitro (test tube) screening assays have shown some phthalates, such as dibutyl phthalate (DBP) and butylbenzyl phthalate (BBP), to exhibit a weak positive result indicating possible oestrogenicity. However, these findings are equivocal in that these phthalates have proved to be non-oestrogenic in some studies.
Most phthalates, including DEHP, diisononyl phthalate (DINP) and diisodecyl phthalate (DIDP), have been tested and found to produce no oestrogenic effects
Recently published data from in-vitro screening tests indicates that, in contrast to other studies DINP may be weakly oestrogenic. However, these authors recognise that when plasticisers are eaten they are broken down to other molecules and that it is these to which humans are actually exposed. They have shown that these breakdown products are not active in the screening tests. They therefore conclude that results from in-vitro tests on whole phthalates may have little significance for human health and that it is the results of the tests on live animals which are important.
The potential reproductive risk posed by some phthalate esters has recently been reviewed by the Commission of the European Communities While it is the case that some phthalates have been shown to cause reproductive effects in rats and mice, these have occurred at levels 10,000 times higher than the estimated exposure to people. It is, therefore, very unlikely that any significant risk to human reproductive health is associated with the use of phthalates.
Further information and references may be found at www.phthalates.com
