More recently, additive estrogenic effects were observed by Kang et al. (2002) from the combinations of some concentrations of 17b -estradiol (a strong estrogen) with bisphenol A (a weak estrogen) or with butylparaben (also a weak estrogen). Also shown as additive in their study were the estrogenic effects from binary mixtures of butylparaben with bisphenol A and of nonylphenol (a weak estrogen) with butylparaben. The natural estrogen 17b -estradiol is produced by the ovary, whereas butylparaben is often found as an ingredient in many skin care products. Both bisphenol A and nonylphenol are common ingredients of plastics.

Another piece of convincing recent evidence was the study by Rajapakse et al. (2002). That study involved a series of classic yeast estrogenicity assays showing that xenoestrogens (i.e., those estrogens foreign to the body) in binary mixtures at low levels could double the estrogenic effects in the presence of a moderately high level of estrogen. Some of the xenoestrogens tested were phenyl salicylate, bisphenol A, resorcinol monobenzoate, and genistein. Similar findings were observed earlier by Rajapakse et al. (2001), when they tested the combined effects of bisphenol A with o,p’-DDT in the presence of 17b -estradiol. It is important to note that the natural estrogen 17b -estradiol is many thousand times more estrogenic than any of the weak xenoestrogens used in these two studies.

For endocrine disruptors that are antiandrogenic, the effects of procymidone and vinclozolin were also found additive at low levels in male castrate rats (Price et al., 2000). Procymidone and vinclozolin are pesticides having the ability to block dihydrotestosterone and testosterone actions by competing with these steroid hormones for the androgen receptor.