Survey of Chemical Contaminants: Organics - Organics Study Notes

Overview of Organic Chemical Contaminants

Organic contaminants represent an extremely diverse category within toxicology and environmental science. This survey covers a wide range of chemical properties and environmental fates.

Classification of Organic Contaminants

Organic contaminants are categorized based on their origin and intended (or unintended) use:

Intentional Poisons: These include substances specifically designed to eradicate pests, such as insecticides, herbicides, and rodenticides.
Unintentional Poisons: These are typically byproducts of industrial processes, degradation products, or industrial waste.
Naturally-occurring Compounds: This includes Polycyclic Aromatic Hydrocarbons (PAHs).
Synthetic Molecules: These are compounds unlike anything found in nature, with DDT (Dichlorodiphenyltrichloroethane) serving as a primary example.

Polycyclic Aromatic Hydrocarbons (PAH)

Polycyclic Aromatic Hydrocarbons are a class of organic compounds characterized by the presence of two or more fused aromatic rings.

Formation: They are produced primarily through the incomplete combustion of organic material.
Toxicity: Some compounds in this class are potent carcinogens. PAHs with low molecular weight are often noted for being very toxic.
Specific Examples and Varieties: - Naphthalene - Anthracene - Phenanthrene - Pyrene - Chrysene - Benzo[j]fluoranthene - Benzo[a]pyrene - Perylene - Coronene
Source Attribution: Unsubstituted PAHs are particularly common in samples collected from pyrogenic (fire-related) sources.

Halogenated Biphenyls and Related Compounds

Polychlorinated Biphenyls (PCBs)

PCBs consist of a biphenyl base molecule to which chlorine atoms have been added. There are $209$ different related compounds, referred to as congeners.

Trade Names: Often sold as mixtures under the trade name Arochlor (e.g., Arochlor 1260).
Molecular Example: $2,3,5,3',5'\text{-Pentachloro-biphenyl}$ .
Applications: Historically used as lubricants, heat conductors in electrical transformers, and as plasticizers.
Environmental Concerns: They are toxic and have a high potential for biomagnification. This is due to their slow degradation rates in the environment and their high solubility in lipids (lipophilicity).

Polychlorinated Terphenyls (PCT)

PCTs are chemically similar to PCBs and were utilized as heat stabilizers in cooling oils.

Example: $2,5,2',2'',5''\text{-pentachloro-[1,1',4'1'']terphenyl}$ .
Membrane Permeability: While they exhibit high lipophilicity, congeners with a high number of chlorine atoms often have difficulty passing through biological membranes, which prevents them from accumulating to extremely high concentrations.

Polybrominated Biphenyls (PBBs)

PBBs share similar physical properties and environmental fate characteristics with PCBs.

Uses: Manufactured as fire retardants added to hard plastics. They are frequently incorporated into televisions and various electronic appliances.
Example: $2,3,5,3',5'\text{-Pentabromo-biphenyl}$ .

Polybrominated Diphenol Ethers (PBDEs)

PBDEs are a group of brominated organic compounds used as additive flame retardants in a wide array of commercial materials since the 1970s.

Chemical Properties: They exhibit high thermal stability. When heated, they burn to release bromine, which actively retards fire development.
Congeners: $209$ congeners are known, though production typically focuses on three specific products.
Common Applications: Flame-resistant plastics, polyurethane foam, and textiles used in carpeting, mattresses, electronics, and children’s sleepwear.

Biomonitoring Data: Indicator B8

According to data from the Centers for Disease Control and Prevention (NHANES 2003-2004), PBDE concentrations in women ages 16 to 49 years show variations by socioeconomic and ethnic factors:

Measurement: Concentrations are measured in serum as $ng/g \, \text{lipid}$ .
Income Factors: The data distinguishes between families living above and below the poverty line.
Population Groups: Includes White non-Hispanic, Black non-Hispanic, and Mexican-American groups.
Note: Estimates are adjusted for birth probability based on age and race/ethnicity to reflect exposures to women who are or may become pregnant.

Chlorofluorocarbons (CFCs) and Ozone Depletion

CFCs were first synthesized in 1948 and found heavy use as coolants and blowing agents for producing foams such as Styrofoam.

The Montreal Protocol (1987): An international treaty that imposed restrictions to phase out CFCs, which is slowly resulting in a decrease in atmospheric concentrations.
Mechanisms of Depletion (e.g., CFC-11 / $CFCl_3$ ): 1. CFCs are emitted and reach the stratosphere. 2. Sunbeams (ultraviolet rays) break down CFCs, releasing chlorine ( $Cl$ ) atoms. 3. Each chlorine atom initiates a chain reaction with ozone ( $O_3$ ) molecules, converting them into oxygen molecules ( $O_2$ ). 4. $Cl + O_3 \rightarrow ClO + O_2$
Environmental Impact: The thinning of the ozone layer allows increased levels of harmful ultraviolet rays to reach the Earth. The "Ozone Hole" is defined as areas with ozone levels < 220 \, \text{Dobson Units (DU)}.
Historical Area Trends: The average area of the ozone hole peaked between 1995 and 2005, often exceeding the area of Antarctica ( $25 \, \text{million} \, km^2$ ) and approaching the size of North America.

Chlorinated Phenols and Pesticides

Chlorinated Phenols

Produced during industrial processes including papermill processing and the kraft bleaching of wood pulp.

Uses: Employed as wood preservatives, biocides, and fungicides.
Examples: - $2,4,5\text{-Trichlorophenol (TCP)}$ - $2,3,4,5,6\text{-Pentachlorophenol (PCP)}$

Organochlorine Pesticides

This class was designed strictly as poisons. They tend to be lipid-soluble and biomagnify in the food chain.

DDT (Dichlorodiphenyltrichloroethane): Described as the "poster child" of organochlorine pesticides. It is highly lipid-soluble and biomagnifies.
DDT Derivatives: - DDE: Used similarly to DDT. - DDD: A degradation product of DDT.
Other Examples: Chlordane, $1,2,3,4,5,6\text{-Hexachloro-cyclohexane}$ , Methoxychlor, Lindane, Toxaphene, Aldrin, Dieldrin, and Endrin.

Modern Insecticide Classes

Organophosphate Insecticides

These are powerful neurotoxicants that differ significantly from organochlorines.

Mechanism: They act as AChE (Acetylcholinesterase) inhibitors. By inhibiting this enzyme, they cause an increase of Acetylcholine ( $Ach$ ) in the synaptic cleft, leading to the overstimulation of $Ach$ receptors in the postsynaptic neuron.
Relative Characteristics: Compared to organochlorines, they are more toxic to mammals, less lipid-soluble, and breakdown significantly faster in the environment.
Examples: Methyl parathion, Chlorpyrifos, and Diazinon (derived from Orthophosphoric acid).

Carbamate Insecticides

Mechanism: Similar to organophosphates (AChE inhibition).
Relative Toxicity: More toxic to mammals than organochlorines, but generally less toxic than organophosphates.
Characteristics: Less lipid-soluble than organochlorines and faster environmental breakdown.
Root Compound: Carbamic acid.
Examples: Carbaryl, Carbofuran, and Primicarb.

Pyrethroid Insecticides

These are synthetic analogs of naturally-produced pyrethrins, which are found in chrysanthemum (pyrethrum) plants.

Environmental Fate: They break down very quickly and are readily metabolized by mammals, reducing long-term accumulation risks.
Examples: Permethrin, Allethrin, Fenvalerate, and Pyrethrin I.

Dioxins, Furans, and Environmental Health

Polychlorinated Dibenzodioxins (PCDD) and Dibenzofurans (PCDF)

Origins: These are created as byproducts in chemical production (notably "Agent Orange" from the Vietnam War era) and during combustion processes.
Properties: Extremely persistent in the environment.
Examples: $2,3,7,8\text{-Tetrachloro-dibenzo-p-dioxin}$ and $2,3,7,8\text{-Tetrachloro-dibenzofuran}$ .

Case Studies and news

Great Lake Cormorants: Studies show mechanisms of action manifesting as reproductive failure, endocrine disruption, and cancers. There is also specific concern regarding fish-eating birds and mid-Atlantic peregrine falcons.
Viktor Yushchenko (2004): During the Ukrainian elections of November 21, 2004, opposition leader (and later president) Yushchenko was poisoned. His appearance changed significantly between March 2004 and December 2004, developing severe chloracne, a clinical symptom of dioxin or pesticide poisoning.

Pharmaceuticals and Herbicides

Pharmaceuticals

Diclofenac: An anti-inflammatory used for both humans and cattle. It has been identified as a cause of massive extinction in old-world vultures, causing a population decrease of $99\%$ .

Diverse Herbicides

Bipyridine Herbicides: Diquat and Paraquat.
Triazines: A class of nitrogen heterocyclic compounds including Atrazine and Metribuzin.
Phenoxy Herbicides: Used for monocot control. - 2,4-D: $(2,4\text{-Dichloro-phenoxy)-acetic acid}$ . Part of the "Agent Orange" mixture. - 2,4,5-T: $(2,4,5\text{-Trichloro-phenoxy)-acetic acid}$ .

Discussion Framework for POPs

When reviewing literature on Persistent Organic Pollutants (POPs), students should address:

The specific question being addressed by the researchers.
The methodology used to address the question.
The specific numerical or qualitative conclusions reached.
The overall implications of those conclusions for environmental safety.
The plausibility and believability of the conclusions based on the provided evidence.