Potential for long-range trans-boundary air pollution

The number of chlorine atoms in each molecule can vary from one to eight. Among the possible 210 compounds, 17 congeners have chlorine atoms at least in the positions 2, 3, 7 and 8 of the parent molecule and these are the most toxic, bioaccumulative and persistent ones compared to congeners lacking this configuration. All the 2,3,7,8-substituted PCDDs and PCDFs plus coplanar PCBs (with no chlorine substitution at the ortho positions) show the same type of biological and toxic response. 

Owing to their chemical, physical and biological stability, PCDD/PCDFs are able to remain in the environment for a long time. As a consequence, dioxins from so-called primary sources (formed in industrial or combustion processes) are transferred to 

Their mobility may be increased by the simultaneous presence of organic solvents such as mineral oil. The half-life of TCDD in soil has been reported as 10-12 years, whereas photochemical degradation seems to be considerably faster but with a large variation that might be explained by experimental differences (solvents used, etc.). Highly chlorinated PCDD/PCDFs seem to be more resistant to degradation than those with just a few chlorine atoms. 

PCCD/PCDFs have been found to be present in Arctic air samples, e.g. during the winter of 2000/2001 in weekly filter samples (particulate phase) collected at Alert in Canada. PCDD/PCDFs have been monitored since 1969 in fish and fish-eating birds from the Baltic. The levels of PCDD/PCDFs in guillemot eggs, expressed as TEQ, decreased from 3.3 ng/g lipids to around 1 ng/g between 1969 and 1990. Since 1990, this reduction seems to have levelled off and today it is uncertain whether there is a decrease or not. Fish (herring) show a similar picture. 

Thus both physical characteristics and environmental findings support the long-range transport of PCCD/PCDFs and PCBs. There are differences, however, both between and within the groups regarding ability to undergo LRTAP. 

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