Although research results vary, between 3 and 5% of all wines sealed under cork display a musty taint. This is caused by a number of potent organic compounds, the most significant of which is 2,4,6-trichloroanisole or tca. These taint compounds are metabolic products of fungi naturally present in cork, or which have grown in the cork at various processing stages. Initially, the occurrence of this taint was ascribed to the washing of cork planks by chlorine-containing bleaches; these have since been replaced by peroxide, but cork taint has remained a problem. It seems that the structure of cork, which is permeated by fine pores (lenticels) to facilitate gas exchange, will always harbour fungi with the potential to produce taint compounds. A study by Duncan, Gibson, and Obradovic has demonstrated the presence of TCA in the bark of cork trees in a Portuguese cork forest. See chloroanisoles. The effect of cork taint on the wine is often characterized as an off-putting, mouldy, wet cardboard or wet dog character. It suppresses fruit and shortens the length of finish of the wine. In its most subtle form, cork taint has a slight dulling effect on the bouquet and palate. At its extreme, high levels of cork taint render a wine quite unapproachable.