For this reason, it combines with organic compounds to form chlorinated organic compounds. The presence of a chlorine in an organic molecule results in changes in the properties of the molecule. Chlorine in a molecule increases stability and, for example, makes the substance harder to burn. This increase in the chemical stability of the molecule also increases the biological and environmental persistence of the substance. Because the chlorine atom has a large size and because it usually replaces the much smaller hydrogen atom when it combines with organic compounds, chlorinated organic compounds have a larger volume than their nonchlorinated counterparts.
Larger molecules boil at higher temperatures and are less soluble in water.
This effect of adding chlorine to a molecule is well understood and is incremental, that is, the more chlorine added to the molecule, the less easily it is broken down and the less water soluble it becomes. Reduced water solubility usually increases solubility in fats which makes these substances tend to move or partition from the environment into organisms and, because they are not easily broken down, they can then move through the food chain. The incremental effects of increasing the chlorine content of a molecule are fundamental to our understanding that the ecologically important properties of chlorinated substances will vary with the number of chlorine atoms in the molecule and that the resulting effects on toxicity, environmental transport, and transformation are generally understood and predictable. Only a small number of chlorinated substances possess all of these properties to the extent that they have been judged to present unacceptable human health and environmental risks. These substances are easily recognized and include the familiar dioxins, DDT, dieldrin, chlordane and PCBs and others that are currently the subject of a United Nations initiative for worldwide phase-out and banning.