Organic Pollutants: How They Spread And Persist

Persistent organic pollutants (POPs) are toxic chemicals that adversely affect human health and the environment. POPs are resistant to degradation through chemical, biological, and photolytic processes and persist in the environment for long periods. They can be transported by wind, water, and migratory species across international borders, affecting people and wildlife far from their source. POPs accumulate in living organisms through the food chain, a process known as biomagnification, resulting in higher concentrations in top predators. Human exposure to POPs occurs through food, air, water, and various products, leading to health issues such as endocrine disruption, reproductive disorders, and increased cancer risk. To address the global concern of POPs, international treaties like the Stockholm Convention and the Aarhus Protocol aim to reduce or eliminate the production and use of these hazardous chemicals.

Transport by wind and water

Persistent organic pollutants (POPs) are toxic chemicals that adversely affect human health and the environment. POPs are resistant to degradation through chemical, biological, and photolytic processes. They persist for long periods in the environment and can accumulate and pass from one species to another through the food chain. POPs are transported by wind and water, and most POPs generated in one country can and do affect people and wildlife far from where they are released.

POPs can enter and affect regions like Alaska and the Arctic through wind and water. Global wind patterns allow POPs from East Asia and Northern Europe to reach Alaska. POPs can also travel through rivers from Southeast and Central Asia into the Pacific Ocean, which eventually flow into the Arctic Ocean. Climate change has also affected POP levels by causing changes in patterns of Arctic land use and emissions, increasing activities in the Arctic such as mining and shipping, and changing wildlife migration routes.

POPs can enter the gas phase under certain environmental temperatures and volatilize from soils, vegetation, and bodies of water into the atmosphere, resisting breakdown reactions in the air, and travelling long distances before being re-deposited. They can be transported over long distances via wind and water currents, even to places where they have never been used, such as Antarctica and the Arctic.

Indoor environments are a major source of human exposure to POPs through inhalation and ingestion. Studies have shown that indoor POP levels in the air and dust can exceed outdoor POP concentrations in the air and soil. In 2022, levels of at least four perfluoroalkyl acids (PFAAs) in rainwater worldwide greatly exceeded the EPA's lifetime drinking water health advisories and safety standards in other countries, indicating the global spread of these chemicals in the atmosphere.

Bioaccumulation in organisms

Bioaccumulation is the process by which toxins enter the food web by building up in individual organisms. It is essential for an organism to grow and develop, but the accumulation of harmful substances can also occur. Organisms can take up chemicals by breathing, absorbing them through the skin, or swallowing. When the concentration of a chemical is higher within an organism compared to its surroundings (air or water), it is referred to as bioconcentration.

Bioaccumulation occurs at the base of a food web, usually within primary producers like phytoplankton. These microscopic photosynthetic organisms absorb Persistent Organic Pollutants (POPs) directly from seawater and accumulate them in their bodies over time. The toxins build up in their tissues because they are absorbed from the water at a rate faster than they can be metabolized.

Some animal species use bioaccumulation as a mode of defence: by consuming toxic plants or animal prey, an animal may accumulate the toxin, which then acts as a deterrent to a potential predator. An example is the tobacco hornworm, which concentrates nicotine to a toxic level in its body as it consumes tobacco plants.

Bioaccumulation can also occur in turtles when synthetic organic contaminants (e.g. PFAS), heavy metals, or high levels of trace elements enter a singular organism, potentially affecting their health. Coastal fish, such as the smooth toadfish, and seabirds, such as the Atlantic puffin, are often monitored for heavy metal bioaccumulation.

Bioaccumulation and long-range transport are the reasons why POPs can accumulate in organisms like whales, even in remote areas like Antarctica.

Long-range transport

Persistent Organic Pollutants (POPs) are toxic chemicals that adversely affect human health and the environment. POPs are resistant to degradation through chemical, biological, and photolytic processes. They persist for long periods in the environment and can accumulate and pass from one species to another through the food chain.

POPs are transported over long distances through the atmosphere via air and water, even to places where they have never been used, such as Antarctica and the Arctic. This long-range transportability is a significant concern as it exposes humans and animals worldwide to these pollutants for extended periods. The stability and lipophilicity of POPs allow them to travel far from their source and accumulate in fatty tissues, leading to potentially dangerous levels of concentration.

The long-range transport of POPs is facilitated by their ability to enter the gas phase under certain environmental temperatures. They volatilize from soils, vegetation, and bodies of water into the atmosphere, resisting breakdown reactions in the air before being re-deposited. This process enables them to spread across vast distances and affect areas far removed from the original source of pollution.

Additionally, oceanic currents and sea spray play a role in the long-range transport of POPs. Ionic PFCs, for example, can be transported directly by oceanic currents or sea spray. The strong affinity of PFCs for proteinous tissues contributes to their accumulation and persistence in the environment.

The United States has actively contributed to global efforts to address POPs by signing agreements such as the regional protocol of the United Nations Economic Commission for Europe on POPs under the Convention on Long-Range Transboundary Air Pollution. These international collaborations aim to protect human health and the environment from the adverse effects of POPs through coordinated actions and scientific research.

Food chain transmission

Persistent organic pollutants (POPs) are toxic chemicals that adversely affect human health and the environment. They are resistant to degradation through chemical, biological, and photolytic processes. POPs can be transported by wind and water, and affect people and wildlife far from where they are released.

POPs can enter the food chain through a process called bioaccumulation. This is where POPs accumulate in the body fat of living organisms and become more concentrated as they move from one creature to another. This process is known as biomagnification. Bioaccumulation and long-range transport are the reasons why POPs can accumulate in organisms like whales, even in remote areas like Antarctica.

For example, a 1997 study by the Arctic Monitoring and Assessment Programme found that caribou in Canada's Northwest Territories had high levels of POPs. This is because the caribou accumulate POPs from the plants they eat, which have absorbed POPs from the soil and water. The POPs become more concentrated as they move up the food chain, from the plants to the caribou.

Freshwater organisms are particularly at risk from bioaccumulation and biomagnification of POPs. This is because POPs have a low water solubility, so they bond strongly to particulate matter in aquatic sediments. These sediments can serve as reservoirs or "sinks" for POPs. When disturbed, they can be reintroduced into the ecosystem and food chain, potentially becoming a source of local or global contamination.

The transfer of POPs through river food webs has been studied, with researchers investigating the transfer pathways of polychlorinated biphenyls, polybrominated diphenyl ethers, and organochlorine. These chemicals are widespread across freshwater ecosystems and can have significant impacts on individual organisms and populations.

International risk management

To address the global concern of POPs, international treaties such as the Stockholm Convention and the Aarhus Protocol have been implemented. The Stockholm Convention, ratified by 185 countries and the European Union as of 2024, aims to protect human health and the environment from POPs. It calls for the reduction or elimination of the production, use, and release of specific POPs. The convention also establishes a scientific review process to identify additional POPs of global concern.

The Global Monitoring Plan, under the Stockholm Convention, collects comparable data on POPs concentrations in various regions to track changes over time and understand their environmental transport. This data is measured in core matrices such as air, human milk, and water, as well as matrices of national interest like fish, butter, and sediment. The convention also requires parties to develop National Implementation Plans (NIPs) to outline their strategies for fulfilling their obligations under the convention.

The Arctic Council's Arctic Monitoring and Assessment Programme (AMAP) is another initiative focused on reducing POPs within the Arctic nations. AMAP provides member nations with information on environmental threats and scientific advice on remedial and preventive actions to protect the Arctic environment from contaminants, including POPs.

Additionally, the World Health Organization (WHO) and the United Nations Environment Programme (UNEP) collaborate on a monitoring program for POPs in breast milk to assess human exposure and track progress in reducing POPs. This monitoring program has shown a steady decline in POPs levels since 1980 in most countries.

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