Comprehensive Study Note on PBTs, POPs, and PFAS Chemicals

Persistent, Bioaccumulative, and Toxic Chemicals (PBTs)

• Persistence Concept: High persistence refers to chemicals that are long-lasting in the environment. This characteristic makes human exposure difficult to control and nearly impossible to reverse.

• Identification of PBT Chemicals: These substances are categorized by three distinct red flags: persistence ($P$), bioaccumulation ($B$), and toxicity ($T$). Their combined impact is particularly dangerous because once released, the effects are not easily reversible.

• Representative Examples of PBTs:

  • Pesticides.

  • Firefighting foam.

  • Inner lining of microwave popcorn bags (used to prevent sticking).

  • Cosmetics.

  • Waterproof materials.

• Characteristics of Persistent Chemicals:

  • Degradation: They degrade extremely slowly and resist environmental breakdown from sunlight or microorganisms.

  • Environmental Accumulation: They remain in the environment for extended periods, accumulating in soil and aquatic systems.

  • Transport: Chemicals that evaporate via volatilization or dissolve in water can travel vast distances from their source.

  • Exposure Risks: They pose a higher risk of exposure to both humans and wildlife due to their resistance to metabolic breakdown.

  • Legacy Contamination: Many chemicals found at Love Canal were "forever chemicals."

  • Remediation Challenges: Banning or strictly restricting these products is often insufficient to stop environmental contamination within a reasonable timeframe. Effects can linger for years or even decades. Removing them typically requires high inputs of energy.

  • Half-Life: PBTs possess a significantly longer half-life than other chemical types.

Chemical Half-Life and Persistence Thresholds

• Half-Life Definition: The time required for half of the initial amount of a chemical or substance to decay or transform. It serves as a measure of how quickly a substance changes over time.

• Persistence Criteria: While modern organic pesticides have shorter half-lives than older variants, they remain problematic. To be officially classified as persistent, a chemical must have a half-life of at least:

  • $2$ months in water.

  • $6$ months in soil or sediment.

Bioaccumulation and Biomagnification

• Bioaccumulation: This is the second red flag of PBTs. It describes the buildup of chemical levels within a single organism to concentrations higher than those found in the surrounding environment.

  • Mechanism: This occurs when a chemical is absorbed faster than it can be released or metabolized.

  • Solubility Factors: Most polychlorinated chemicals are only slightly soluble in water but highly soluble in the fat of animals and humans, as well as the cuticles of plants.

• Biomagnification: This process involves the progressive increase in chemical concentration as it moves up the food web.

  • Example Path: Small fish $→$ big fish $→$ orcas.

  • Result: Apex predators like orcas contain high levels of contaminants even if they never directly consumed the original source of the pollution.

Toxicity and Regulatory Definitions

• Toxicity Factors: The third red flag of PBTs depends on three variables:

  • The dose of the substance.

  • The susceptibility of the specific species.

  • The frequency of repeated exposure.

• Governmental Definition (Canada): A substance is legally defined as toxic if it enters or may enter the environment in a quantity, concentration, or under conditions that:

  • Have or may have immediate or long-term harmful effects on the environment or its biological diversity.

  • Constitute or may constitute a danger to the environment upon which life depends.

  • Constitute or may constitute a danger in Canada to human life or health.

Persistent Organic Pollutants (POPs)

• Relationship to PBTs: All POPs are PBTs, but not all PBTs are POPs. This is because PBTs include both organic chemicals and metals/metal compounds, whereas POPs are strictly organic chemicals.

• Physicochemical Properties:

  • POPs are more soluble in octanol (an oily substance used as a substitute for fat) than in water.

  • They bioaccumulate primarily in fatty tissues.

  • Half-life requirements match PBT standards ($2$ months in water; $6$ months in soil/sediment).

• Environmental and Biological Movement:

  • Grasshopper Effect: This refers to the process where chemicals are transported to new environments; migrating animals that die in these new areas can contaminate the local ecosystem.

  • Maternal Transfer: Some POPs can transfer from a mother to a fetus via the placenta.

  • Geographic Concentration: POPs tend to concentrate in colder environments, such as the Canadian Arctic, the St. Lawrence River, and the Great Lakes Basins.

• Historical Context: Synthetic POPs were developed in the mid-$20^{th}$ century for beneficial uses in disease control, pest management, crop production, and industry. Awareness of their harmful impacts has only developed within the last $25$ years.

• Classification of POPs:

  • Intentionally Produced: Chemicals created for agriculture, manufacturing, or industrial processes. Examples include PCBs, DDT, and PFAS.

  • Unintentionally Produced: By-products of industrial processes or combustion.

    • Dioxins: Result from industrial processes and combustion.

    • Furans: Formed primarily during the thermal processing of foods (coffee, baked goods, cooked meats), backyard burning, and waste incineration.

The Stockholm Convention on POPs

• Overview: A legally binding international treaty aimed at eliminating or reducing the release of POPs into the environment.

• Origin: The convention was primarily initiated following the discovery of POPs contamination in the previously pristine Arctic regions.

• Standardized Categories and the "Dirty Dozen": Initially, the convention recognized $12$ POPs divided into three categories (though the list continues to expand):

  • Pesticides: Aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, and toxaphene.

  • Industrial Chemicals: Hexachlorobenzene and polychlorinated biphenyls (PCBs).

  • By-products: Hexachlorobenzene, polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and PCBs.

Per- and Polyfluoroalkyl Substances (PFAS)

• Definition: Often called "forever chemicals," PFAS are a group of over $4700$ man-made chemicals that are nearly indestructible. They qualify as PBTs, POPs, and PFAS simultaneously.

• Primary Variants:

  • PFOA: Perfluorooctanoic acid.

  • PFOS: Perfluoro octane sulfonic acid.

• Applications and Use Cases: PFAS are used for their unique ability to repel oil and water, their resistance to temperature and chemicals, and their surfactant properties (allowing liquids to spread easily). Common products include:

  • Non-stick cookware and fast food containers.

  • Raincoats and textiles.

  • Stain-resistant carpets and upholstery.

  • Personal care products and cosmetics.

  • Electronics and vehicles.

  • Fire retardant foam.

  • Pesticides and cleaning products.

• Chemical Structure:

  • Built on fluorocarbons featuring $C-F$ (carbon-fluorine) and $C-C$ (carbon-carbon) bonds.

  • The carbon-fluorine bond is one of the strongest in chemistry, allowing these chemicals to persist for hundreds to thousands of years.

  • Functional groups often attached include carboxylic acid, sulfonic acid, sulfonyl, amine, ether, alcohol, or chlorine.

  • Note: Carboxylic acid is associated with the creation of PFOS, while sulfonic acid is associated with PFOA. Due to high toxicity, these are being replaced by shorter-chain versions.

Environmental Impact and Exposure of PFAS

• Prevalence: PFAS are ubiquitous. They are found in air, groundwater, oceans, rivers, soil, wastewater, and sludge. They are even detected in precipitation and remote areas far from release points.

• Production Volume: Estimates suggest that between $100,000$ and $500,000$ tonnes of perfluorinated compounds have been produced and are currently circulating in the biosphere.

• Sources of Exposure:

  • Industrial production sites.

  • Department of Defense ($DOD$) sites (military/army defense).

  • Publicly Owned Treatment Works ($POTW$) or sewage treatment plants.

  • Consumer products and the reuse of contaminated sludge.

Health Concerns Associated with PFAS

• High Certainty Health Concerns:

  • Developmental Effects: Delayed mammary gland development, reduced vaccine response, and lower birth weight in infants.

  • Organ Damage: Liver damage and thyroid disease.

  • Cancer: Increased risk of kidney and testicular cancer.

  • Metabolic: Increased cholesterol levels.

• Low Certainty Health Concerns:

  • Developmental/Reproductive: Obesity, early puberty onset, increased miscarriage risk, low sperm count/mobility, and increased time to pregnancy.

  • Pregnancy: Pregnancy-induced hypertension or pre-eclampsia.

  • Chronic Disease: Breast cancer and inflammatory bowel disease (ulcerative colitis).

The Canadian Context

• Detection in Canada: PFAS are routinely found in Canadian samples of air, aquatic ecosystems, landfill leachate, wastewater, biosolids, and both aquatic and terrestrial wildlife.

• Scientific Status: Research in Canada is ongoing but moves slowly due to the complexity of the science. It is currently believed that known information about PBTs representing only the "tip of the iceberg."