water (related to industrial activities, spills, or landfills), and may cause diverse acute
and chronic effects to biota and wildlife. They act mostly through non-specific
modes of action like membrane disruption or reactive toxicity with biological
molecules.
27.6.5.2 Halogenated (Monocyclic) Aromatic Hydrocarbons
There are many different structural variants and applications of halo-derivatives of
benzene, toluene, phenol, and similar monocyclic aromatic compounds. For
example, m- and p-dichlorobenzenes or nitrated trichlorobenzenes are used as
chemical intermediates and solvents, p-dichlorobenzene is an insecticide fumigant
and disinfectant, mixtures of trichlorobenzenes are used for controlling termites,
hexachlorobenzene (HCB) is a fungicide and intermediate in chemical industry, etc.
Toxicity within this group of molecules is highly variable ranging from acute and
chronic toxicity through nonspecific modes of action (membrane and reactive
toxicity) to specific effects on the particular biota for insecticides and fungicides.
27.6.5.3 Polychlorinated Dibenzo-P-Dioxins and -Furans (PCDD/Fs)
PCDD/Fs are planar molecules with varying substituents. They are extremely stable
and are among the most toxic compounds ever identified, both through acute and
chronic exposures. They are also the major problematic compounds regulated by
POPs Stockholm Convention. These are not commercial products but exist as
by-products formed during chlorophenol synthesis and during incineration. Their
environmental concentrations are in general (also in comparison to other HHCs)
very low, which—fortunately—minimizes their acute toxic effects. Nevertheless,
PCDD/Fs due to high lipophilicity bioaccumulate and biomagnify in food chain
affecting thus mostly terminal predators (birds, mammals) after long-term chronic
exposures. The key mechanism of toxic action of dioxins and furans and also other
structurally related molecules (so-called “dioxin-like” toxicity—including, e.g.,
PCBs discussed below as well as PAHs and others) involves activation of the
intracellular aryl hydrocarbon receptor (AhR), which leads to synthesis of detoxification CYP450 enzymes and deregulation of various biological functions. The main
types of toxicities ascribed to dioxins and dioxin-like compounds include immune
suppression (e.g., documented mass mortalities of seals and dolphins due to sensitivity to ubiquitous viruses); porphyria (disruption of synthesis of components of
blood pigment hemoglobin associated with sensory disorders, paralysis, or neurotoxicity); cancer promotion; endocrine disruption (effects on synthesis, action, and
degradation of sex hormones estrogens and androgens); or disruption of vitamin A
metabolism (teratogenicity, growth disruption, sterility). The above-mentioned
effects were widely documented in fish, amphibians, reptiles, birds as well as
mammals, and also humans. Nevertheless, PCDD/Fs are highly toxic also, e.g., to
invertebrates through chronic mechanisms like endocrine disruption. Because of
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and chronic effects to biota and wildlife. They act mostly through non-specific
modes of action like membrane disruption or reactive toxicity with biological
molecules.
27.6.5.2 Halogenated (Monocyclic) Aromatic Hydrocarbons
There are many different structural variants and applications of halo-derivatives of
benzene, toluene, phenol, and similar monocyclic aromatic compounds. For
example, m- and p-dichlorobenzenes or nitrated trichlorobenzenes are used as
chemical intermediates and solvents, p-dichlorobenzene is an insecticide fumigant
and disinfectant, mixtures of trichlorobenzenes are used for controlling termites,
hexachlorobenzene (HCB) is a fungicide and intermediate in chemical industry, etc.
Toxicity within this group of molecules is highly variable ranging from acute and
chronic toxicity through nonspecific modes of action (membrane and reactive
toxicity) to specific effects on the particular biota for insecticides and fungicides.
27.6.5.3 Polychlorinated Dibenzo-P-Dioxins and -Furans (PCDD/Fs)
PCDD/Fs are planar molecules with varying substituents. They are extremely stable
and are among the most toxic compounds ever identified, both through acute and
chronic exposures. They are also the major problematic compounds regulated by
POPs Stockholm Convention. These are not commercial products but exist as
by-products formed during chlorophenol synthesis and during incineration. Their
environmental concentrations are in general (also in comparison to other HHCs)
very low, which—fortunately—minimizes their acute toxic effects. Nevertheless,
PCDD/Fs due to high lipophilicity bioaccumulate and biomagnify in food chain
affecting thus mostly terminal predators (birds, mammals) after long-term chronic
exposures. The key mechanism of toxic action of dioxins and furans and also other
structurally related molecules (so-called “dioxin-like” toxicity—including, e.g.,
PCBs discussed below as well as PAHs and others) involves activation of the
intracellular aryl hydrocarbon receptor (AhR), which leads to synthesis of detoxification CYP450 enzymes and deregulation of various biological functions. The main
types of toxicities ascribed to dioxins and dioxin-like compounds include immune
suppression (e.g., documented mass mortalities of seals and dolphins due to sensitivity to ubiquitous viruses); porphyria (disruption of synthesis of components of
blood pigment hemoglobin associated with sensory disorders, paralysis, or neurotoxicity); cancer promotion; endocrine disruption (effects on synthesis, action, and
degradation of sex hormones estrogens and androgens); or disruption of vitamin A
metabolism (teratogenicity, growth disruption, sterility). The above-mentioned
effects were widely documented in fish, amphibians, reptiles, birds as well as
mammals, and also humans. Nevertheless, PCDD/Fs are highly toxic also, e.g., to
invertebrates through chronic mechanisms like endocrine disruption. Because of
562
L. Bláha and J. Hofman