Xenoestrogen
Xenoestrogens are synthetic substances that differ from those produced by living organisms and imitate or enhance the effect of estrogens. The estrogenic stimulation is an unintended side-effect of these agents or their metabolites.
Xenoestrogens are part of a heterogeneous group of chemicals that are hormone or endocrine disruptors. They differ from phytoestrogens (estrogenic substances from plants), mycoestrogens (estrogenic substances from fungi), and pharmacological estrogens (estrogenic action is intended). External estrogens from a variety of sources may have a cumulative effect upon living organisms, and xenoestrogens may be part of a larger picture of a process of estrogenization of the environment. Xenoestrogens have only been recently (less than 70 years) introduced into the environment, as produced by industrial, agricultural, and chemical companies.

# Effects
Xenoestrogens have been implicated in a variety of  medical problems. Foremost is the concern that xenoestrogens as false messengers disrupt the process of reproduction. Studies have implicated observations of disturbances in wildlife with estrogenic exposure. Reproductive issues which are of concerns in humans are fetal exposure (perhaps leading to hypospadias) and decreased reproductive ability in men (i.e. decrease in sperm numbers). Another issue is the potential effect of xenoestrogens on oncogenes, specifically in relation to breast cancer.

# Presence
The ubiquitous presence of such estrogenic substances is a significant health concern, both individually and for a population. Life relies on the transmission of biochemical information to the next generation, and the presence of xenoestrogens may interfere with this transgenerational information process through "chemical confusion" (Vidaeff, Sever). Environmental protection laws and regulations are supposed to protect the environment from hormonal disruption. Agencies such as the United States Environmental Protection Agency and the World Health Organization International Program on Chemical Safety are charged to address these issues.

# Research
The study of the problem is difficult and complex. Transgenerational effects are difficult to prove, effects may be multifactorial, and the large variety of substances in question with the absence of unexposed controls do not lend themselves to easy interpretation. Believers that environmental estrogen disruption is a major health hazard are opposed by detractors  who argue that observed effects are spurious and inconsistent, or that the quantities of the agents are too low to have any effect.
A 2005 study by Belcher and coworkers demonstrated that even very low levels of a xenoestrogen, in this case Bisphenol A, could affect fetal neural development more than higher levels (PMID 16123166), indicating that classical models where dose equals response may not be applicable in susceptible tissue.

# Synthetic chemicals shown to have unintended estrogenic effects
- 4-Methylbenzylidene camphor (4-MBC) (sunscreen lotions)
- butylated hydroxyanisole / BHA (food preservative)
- atrazine (weedkiller)
- bisphenol A (monomer for polycarbonate plastic and epoxy resin; antioxidant in plasticizers)
- dieldrin (insecticide)
- DDT (insecticide)
- endosulfan (insecticide)
- erythrosine / FD&C Red No. 3
- heptachlor (insecticide)
- lindane / hexachlorocyclohexane (insecticide)
- methoxychlor (insecticide)
- nonylphenol and derivatives (industrial surfactants; emulsifiers for emulsion polymerization; laboratory detergents; pesticides)
- polychlorinated biphenyls / PCBs (in electrical oils, lubricants, adhesives, paints)
- parabens (lotions)
- phenosulfothiazine (a red dye)
- phthalates (plasticizers)
DEHP (plasticizer for PVC)
- DEHP (plasticizer for PVC)