[Dioxins: current knowledge about health effects].

Dioxins are a family of chlorinated aromatic hydrocarbons that are produced during combustion processes in the presence of a chlorine donor and as by-products of the chlorine-processing chemical industries. Several dioxins are extremely stable compounds and persist for years in the environment. Exposure to dioxins occurs mainly via the ingestion of contaminated food. The lipophilic character of dioxins prevents their excretion in the urine and causes their accumulation in body fat. The mechanisms of dioxin action are similar to those of a hormone. Dioxins bind to a specific intracellular receptor and the complex acts as a transcription factor that induces the production of a great number of proteins. Certain dioxins, particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin, are very toxic and able to induce numerous clinical conditions. The carcinogenicity of dioxins is well documented in animal models and has been described in humans after professional and accidental exposures. Recent experimental data also indicate that dioxins can cause dysfunction of the sexual and thyroid hormone systems and that the administration of dioxins induces several conditions related to hormonal dysfunction. Chronic exposure of female Rhesus monkeys increases the incidence and severity of endometriosis. The administration of dioxins during pregnancy and nursing causes altered development of the reproductive system, decreased spermatogenesis, hypothyroidism and disturbed psychomotor development in the offspring. The particular sensibility of the fetus and newborn is of concern because the exposition to dioxins is particularly important during those periods of life. In humans a series of conditions related to hormonal dysfunction as undescended testis, decreased spermatogenesis, testicular cancer and endometriosis have increased in incidence during the last decades. The chronological parallelism with the appearance of dioxins in the environment suggests that these might exert biological effects at the prevailing level of exposure. Nevertheless this hypothesis is currently unconfirmed by epidemiological studies. The implications of this scientific incertitude for the implementation of preventive measures are briefly discussed.