| Dioxins in Clays
by Edouard Bastarache
 olychlorinated
dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are tricyclic
aromatic compounds with similar chemical and physical properties.
They are ubiquitous in the environment and usually do not occur
naturally.There are 75 positional isomers of PCDDs and 135 isomers
of PCDFs. 2,3,7,8-TCDD (TCDD) is the most toxic isomer, and the
estimated toxic risk in humans is calculated in terms of «TCDD equivalents».
For exemple, the toxic effect of different isomers is calculated
in terms of the amount that would cause the same degree of toxicity
as TCDD. Octachloro-dibenzo-p-dioxine, produced during the synthesis
of pentachlorophenol (Norback et al. 1975), is quite less toxic
than TCDD produced during the synthesis of the herbicide 2,4,5-T.
The most important sources include contaminated products such as
chlorinated phenols and their derivatives. Polychlorinated biphenyls
(PCBs); incineration of municipal, hazardous, and hospital wastes;
sewage sludge; automobile operations; fossil fuel combustion; and
emissions from fire involving PCBs are other sources. Production
of chlorophenols and their derivatives; chlorophenol wood treatment;
chlorine bleaching in the pulp industry; and production and handling
of iron, steel, and other metals, such as aluminium, also yields
dioxins.
Exposure has also occurred from herbicide use and from industrial
and transportation incidents. In occupational settings, exposure
has occurred in chemical manufacturing processes and from handling
the wastes from these processes. Ingestion, inhalation, and
dermal absorption are all thought to be routes of exposure to PCDDs
and PCDFs. However, exposure for most individuals will be small
and will come through a variety of sources. It is generally
accepted that about 98% of human exposure comes from food.
Dioxins are insoluble in water but lipophilic (soluble in fats)
and other hydrophobic materials, and bind to solid material such
as soil and fly ash. They have a low rate of metabolic breakdown,
they preferentially accumulate in adipose tissue, skin, liver, and
breast milk in mammals. The amount of dioxins expressed as TCDD
equivalents in breast milk of lactating women often exceeds the
tolerated Swedish daily intake of 5 pg/kg body weight by a factor
of 20 to 30. In soil TCDD has an extremely long half-life time,
greater than 10 years.The biologic half-life in humans has been
measured to be in the range of 5 to 8 years.TCDD represents the
most toxic synthetic compound known. It acts as a complete carcinogen
in several species.By now, an increased risk for all cancer sites
combined has been shown in cohort studies on TCDD-exposed subjects,
with an especially high risk for soft-tissue sarcoma (STS).In Seveso,
Italy, area with TCDD soil contamination, excess numbers of tumors
have been found, including lymphomas and STS. Among Vietnam
veterans, increased mortality in STS was reported.
As has been concluded, TCDD is among the most potent identified
chemical carcinogens.It is transspecies, transstrain, transsex,
multisite, and complete carcinogen.For humans, the immunotoxic effects
by dioxins are probably at least as serious as the carcinogenic
properties. It also causes chloracne in humans, which is the
most precocious clinical sign of excessive exposure to TCDD. It
may also cause allergic dermatitis and is an eye irritant.When heated
to decomposition it emits toxic fumes of CL-. NIOSH REL (Dioxin) :
Reduce to lowest feasible level. Little is still understood
about the potential effects on fertility and the developing nervous
system in children by dioxins and related chlorinated compounds.Also
of concern is the high content of dioxins in mother’s milk and in
the food chain (e.g. some fatty fish)
Conclusion :
Not knowing the isomers (different levels of toxicity) identified
in kaolins and ball clays and their concentrations, it is very difficult
to discuss this problem properly. For obvious reasons, we should
be more worried about employees working in mining and processing
these dioxin-containing clays, and by pottery factory employees
than by studio potters, hobbyists, teachers and their students.
The use of clays made without these could be recommended for those
more worried while awaiting more information on this subject from
the mining companies, even from those located on other continents.
It gives a good example why good housekeeping of your studio, avoidance
of unnecessary dusty operations and the use of a decent dust mask
are important factors. Also stay away from some fatty fish!!!
Many thanks to Edouard Bastarache M .D.
(Occupational & Environmental Medicine), author of Substitutions
for Raw Ceramic Materials. Edouard may be contacted via
email: edouardb@sorel-tracy.qc.ca
References :
- Occupational Medicine, Carl Zenz, last edition.
- Sax’s Dangerous Properties of Industrial Materials,
last edition
- Hazardous Materials Toxicology, Sullivan &
Krieger, last edition.
- Toxicologie Industrielle et Intoxications Professionnelles,
Lawerys, last edition.
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