General PBTs are a unique classification of chemicals that have and will continue to impact human health and the environment worldwide. The three main attributes of a PBT (persistence, bioaccumulative and toxic) each have a huge role in the risk posed by these compounds. The ban included the use of PCBs in uncontained sources, such as adhesives,
fire retardant fabric treatments, and plasticizers in paints and cements. The persistence of these compounds is due to the high resistance to oxidation, reduction, addition, elimination and electrophilic substitution. The toxicological interactions of PCBs are affected by the number and position of the chlorine atoms, without ortho substitution are referred as coplanar and all others as non-coplanar. Ortho-PCBs may alter hormone regulation through disruption of the thyroid hormone transport by binding to
transthyretin. Coplanar PCBs are similar to dioxins and furans, both bind to the
aryl hydrocarbon receptor (AhR) in organisms and may exert dioxin-like effects, in addition to the effects shared with non-coplanar PCBs. The AhR is a
transcription factor, therefore, abnormal activation may disrupt cellular function by altering gene transcription. Humans and other organisms, which consume shellfish and/or fish contaminated with persistent bioaccumulative pollutants, have the potential to bioaccumulate these chemicals. While DDT is banned in the US, other countries such as China and Turkey still produce and use it quite regularly through
Dicofol, an insecticide that has DDT as an impurity. This continued use in other parts of the world is still a global problem due to the mobility and persistence of DDT. The initial contact from DDT is on vegetation and soil. From here, the DDT can travel many routes; for instance, when plants and vegetation are exposed to the chemical to protect from insects, the plants may absorb it. Then these plants may either be consumed by humans or other animals. These consumers ingest the chemical and begin metabolizing the toxicant, accumulating more through ingestion, and posing health risks to the organism, their offspring, and any predators. Alternatively, the ingestion of the contaminated plant by insects may lead to tolerance by the organism. Another route is the chemical travelling through the soil and ending up in ground water and in human water supply. In the case that the soil is near a moving water system, the chemical could end up in large freshwater systems or the ocean, where fish are at high risk from the toxicological effects of DDT. Lastly, the most common transport route is the evaporation of DDT into the atmosphere followed by condensation and eventually precipitation where it is released into environments anywhere on earth. Due to the long-range transport of DDT, the presence of this harmful toxicant will continue as long as it is still used anywhere and until the current contamination eventually degrades. Even after its complete discontinued use, it will still remain in the environment for many more years after because of DDT's persistent attributes. DDT causes membranes such as sense organs and nerves endings to activate repetitively by slowing down the ability for the sodium channel to close and stop releasing sodium ions. The sodium ions are what polarize the opposing synapse after it has depolarized from firing. This inhibition of closing the sodium ion channel can lead to a variety of problems including a dysfunctional nervous system, decreased motor abilities/function/control, reproductive impairment (egg-shell thinning in birds), and development deficiencies. Presently, DDT has been labeled as a possible human carcinogen based on animal liver tumor studies. DDT toxicity on humans have been associated with dizziness, tremors, irritability, and convulsions. Chronic toxicity has led to long term neurological and cognitive issues.
Mercury ;Inorganic Inorganic mercury (elemental mercury) is less bioavailable and less toxic than that of organic mercury but is still toxic, nonetheless. It is released into the environment through both natural sources as well as human activity, and it has the capability to travel long distances through the atmosphere. Around 2,700 to 6,000 tons of elemental mercury are released via natural activity such as volcanoes and erosion. Another 2,000–3,000 tons are released by human industrial activities such as coal combustion, metal smelting and cement production. Due to its high volatility and atmospheric residence time of around one year, mercury has the ability to travel across continents before being deposited. Inorganic mercury has a wide spectrum of toxicological effects that include damage to the respiratory, nervous, immune and excretory systems in humans. ;Organic Organic mercury is significantly more detrimental to the environment than its inorganic form due to its widespread distribution as well as its higher mobility, general toxicity and rates of bioaccumulation than that of the inorganic form. Environmental organic mercury is mainly created by the transformation of elemental (inorganic) mercury via anaerobic bacteria into methylated mercury (organic). The global distribution of organic mercury is the result of general mobility of the compound, activation via bacteria and transportation from animal consumption. This atmospheric retention time along with mercury's resistance to degradation factors such as electromagnetic radiation and oxidation, which are two of the main factors leading to degradation of many PBTs in the atmosphere, allows mercury from any source to be transported extensively. This characteristic of mercury transportation globally along with its high toxicity is the reasoning behind its incorporation into the BNS list of PBTs. ==Notable PBT environmental impacts==