**3. Why are POPs resistant to most of the environmental degradation processes?**

In the initial list of the Stockholm Convention, POPs are typically polyhalogenated organic compounds which exhibit high lipid solubility. For this reason, they bioaccumulate in fatty tissues of animals. Halogenated compounds also exhibit great stability reflecting the nonreactivity of C-Cl bonds toward hydrolysis and photolytic degradation in the environment. The high electronegativity of halogens compared to carbon gives the carbon-halogen bond a significant polarity/dipole moment. The electron density is concentrated around the halogen, leaving the carbon relatively electron poor. This introduces ionic character to the bond through partial charges (Cδ<sup>+</sup> Clδ−). The partial charges on the chlorine and carbon exert electrostatic attraction between them, contributing to the unusual bond strength of the carbon-halogen bonds. The carbon-halogen bonds are known to be "the strongest in organic chemistry," because halogen forms the strongest single bond to carbon [3, 4]. The bond also strengthens and shortens as more halogens are added to the same carbon on a chemical compound.

The stability and lipophilicity of persistent organic compounds often correlate with their halogen content; thus, polyhalogenated organic compounds are of particular concern. They exert their negative effects on the environment through two processes, long-range transport, which allows them to travel far from their source, and bioaccumulation, which reconcentrates these chemical compounds to potentially dangerous levels in animal tissues [5].
