PCDFs are unwanted byproducts that appear in the environ ment and in manufactured products as contaminants because of improperly controlled combustion processes. They are also produced when unexpected heating to temperatures over 600°C occurs as in lightning strikes or electrical fires in PCB-containing transformers. Like PCBs, these chemicals are very stable and highly lipophilic. They are poorly metabolized and very resistant to environmental degradation. Several significant environmen industrial sites have occurred. Recent publications have demon strated an elevated incidence of subsequent chronic diseases (eg, diabetes, metabolic syndrome, and obesity) in exposed persons.
Laboratory studies of the blood concentrations of TCDD and its metabolites have provided insight into the persistence and metabolism of the contaminants.
In laboratory animals, TCDD has produced a variety of toxic effects. Wasting syndrome (severe weight loss accompanied by reduction of muscle mass and adipose tissue), thymic atrophy, epidermal changes, hepatotoxicity, immunotoxicity, effects on reproduction and development, teratogenicity, and carcinogenic ity have been produced. The effects observed in workers involved in the manufacture of 2,4,5-T (and therefore presumably exposed to TCDD) consisted of contact dermatitis and chloracne. In severely TCDD-intoxicated patients, discrete chloracne may be the only manifestation.
The presence of TCDD in 2,4,5-T, commercially known as
Silvex, was believed to be responsible for other human toxicities associated with the herbicide. There is epidemiologic evidence for an association between occupational exposure to the phenoxy herbicides and an excess incidence of non-Hodgkin’s lymphoma.
The TCDD contaminant in these herbicides seems to play a role in a number of cancers such as soft tissue sarcomas, lung cancer,
Hodgkin’s lymphomas, and others. TCDD is considered an IARC class 1, known human carcinogen. Other halogenated compounds of this type are not currently classifiable as to carcinogenicity; they are listed as IARC class 3.
Fluorinated hydrocarbon chemicals have been of commercial interest since the mid-20th century. Their uses have included coolant materials in air conditioning systems; artificial oxygen-carrying substances in experimental clinical studies; and heat-, stain-, and stick-resistant coatings for cookware, fabrics, and other materials. The fluorocarbons were produced in very large quantities and have become widespread in the environment. When it later became apparent that migration of lower molecular weight fluorocarbons to the troposphere had a deleterious effect on the protective ozone layer, they were banned from use. The higher molecular weight, more highly fluorinated compounds, now called perfluorinated substances (eg, Teflon), have remained in broad use. Like the heavily chlorinated and brominated hydrocarbons, their commercial usefulness has been complicated by a recognition of adverse environmental and suspected human toxic impacts that resemble some of the adverse qualities of the other halogenated hydrocarbons. A useful reference is the Centers for Disease Control and Prevention (CDC) fact sheet on PFCs. It is found at https://www.cdc.gov/biomonitoring/pdf/ PFCs_FactSheet.pdf.