PCBs Polychlorinated biphenyls

2. What happens to PCBs in the environment?

• 2.1 How are PCB transported in the environment?
• 2.2 How do PCBs build up in living organisms?
• 2.3 To what extent do PCBs break down or persist in the environment?
• 2.4 How much PCBs are found in the environment?

2.1 How are PCB transported in the environment?

In surface waters, PCBs are present in substantial quantities in sediments. Indeed, PCBs tend to attach to the surface of organic matter, clay, and micro-particles that are still suspended in the water or have already settled down. They can remain buried in sediments for a long time and be slowly released into the water and evaporate into air. Releases through water into air occur especially when it is warm and when PCB concentrations in sediment are high as this keeps the PCB levels in water elevated.

PCBs present in air can deposit on land when it rains or snows (wet deposition), or when they attach to particulate matter, such as dust or soot, that settles on the ground (dry deposition). It is also possible that PCBs could be transported by insects which have come into contact with them (see section 2.2).

This text is a summary of: IPCS - WHO Polychlorinated biphenyls : Human health aspects. Concise international chemical assessment document 55
Section 5.1, Transport and partitioning 

2.2 How do PCBs build up in living organisms?

PCBs tend to build up in living organisms both by uptake from the environment over time (bioaccumulation) and along the food chain (biomagnification). PCBs remain stored in fatty tissues much more than in muscles or other body parts.

The degree of PCB bioaccumulation over time depends on how quickly they are taken up and eliminated by the organism, and on the ability of the organism to break down PCBs. Various animal species can transform some PCB congeners, such as insects, crabs and vertebrates, including some birds, fish, and mammals. The rate and physiological mechanism of PCB metabolism depend both on the animal species and on the specific PCB congener.

In aquatic organisms bioaccumulation of PCBs depends on the species, its habitat, and on the specific type of PCB. As the concentration of PCBs in sediments is several times higher than in water, levels are often high in bottom-feeding species.

PCBs generally biomagnify along the food-chain, which leads to greater PCB concentrations in organisms that are higher up in the food chain.

• In the aquatic environment, concentrations will thus be greater in shellfish than in the plankton on which they feed, and even greater in animals at the top of the food chain such as large predatory fish or mammals (seals, dolphins, and whales).
• On land, the biomagnification occurs, for instance, through the accumulation of PCBs from soil or plant leaves to worms or insects and finally to birds and mammals. PCBs can also be found in the eggs of contaminated birds.
• Humans can also accumulate PCBs from food. On average, in humans, the concentration of PCBs in fatty tissues is over a hundred times greater than in the food they eat.

This text is a summary of: IPCS - WHO Polychlorinated biphenyls : Human health aspects. Concise international chemical assessment document 55
Section 5.1, Transport and partitioning 

2.3 To what extent do PCBs break down or persist in the environment?

Some PCBs tend to be more persistent in the environment than others. The extent to which a PCB molecule can be broken down or transformed depends on the number of chlorine atoms it contains and where they are located. PCBs are mainly broken down either by the effect (direct or indirect) of sunlight or by microorganisms. In general, the persistence of PCB congeners increases with the number of chlorine atoms.

Sunlight can play a role in the breakdown of PCBs present in air, water and surface soil.

In the atmosphere, PCBs that are present in the atmosphere tend to react with ozone and water under the effect of sunlight. The reactions results amongst other things in chlorine atoms being removed. The more chlorine atoms there are the longer this reaction takes. The time it takes for half of the amount of PCBs (initially) present to be broken down ranges from 3.5 to 83 days for molecules with 1 to 5 chlorine atoms.

In water, PCBs are essentially broken down by the effect of sunlight (photolysis). In shallow water, in summer sunlight, it takes 17 to 210 days for half of the amount (initially) present to be broken down for molecules with 1 to 4 chlorine atoms. Breakdown by sunlight is slower during winter. PCB congeners with a greater number of chlorine atoms (seven or more) absorb more sunlight and are thus broken down more easily.

In soil and sediment, PCBs are mostly broken down by microorganisms. How quickly PCBs are broken down depends on several factors, including the number and location of chlorine atoms, PCB concentration, the type of microorganisms present, available nutrients, and temperature. Breakdown of PCBs by microorganisms, though slow, can occur whether or not oxygen is present in soil and sediments, and can also occur to some extent in water.

This text is a summary of: IPCS - WHO Polychlorinated biphenyls : Human health aspects. Concise international chemical assessment document 55
Section 5.2, Transformation and degradation 

2.4 How much PCBs are found in the environment?

In outdoor air, average PCB concentration varies between urban and rural locations, and other locations distant from PCB sources. For example, the average PCB concentration measured in a series of North-American cities (5 ng/m³) was 5 to 20 times higher than at two rural locations. In the Arctic and the Antarctic, which are relatively far from any PCB source, the average PCB concentration was 0.2 ng/m³. Since the early 1980s, there has been a slight but continuous decrease in the levels of PCBs in air observed in urban, rural, and coastal areas. By the early 90s, PCB concentrations in rainwater from continental areas had dropped to one quarter or less of their levels in the late 70s, with values decreasing from 20 to 5 ng/litre [0.02-0.005 ng/m³] in rural areas and from 50 to 10 ng/litre [0.05-0.01 ng/m³] in cities.

In indoor air PCB concentrations in the early 1980s were typically at least ten times higher than in the surrounding outdoor air. This may be due to the fact that PCBs are emitted by certain electrical appliances and devices (such as fluorescent lighting ballasts) and building materials (such as elastic sealants).

In seawater, in industrial areas, PCB levels were observed to be at least 100 times higher than further off-shore, based on samples of water taken from the upper few millimeters of the surface. Particularly high concentrations were reported in the North Sea (0.3–3 ng/litre) and in Galveston Bay, a highly industrialized area in Texas, USA (3.1 ng/litre between 1978 and 1979).

In river sediments, PCBs levels have been measured at different depths. Sediment samples showed the highest PCB concentrations in sediment layers buried during the time of maximum PCB manufacture. PCB concentrations are much lower in sediment layers that have formed following the ban on production and use of PCBs.

In fish, reported PCB levels have dropped significantly. For example, PCB concentrations in trout from Lake Ontario decreased by 80% between 1976 and 1994. For several fish species from the Great Lakes, the PCB concentrations in samples collected in the 1990s were generally below 1 µg/kg wet weight.

This text is a summary of: IPCS - WHO Polychlorinated biphenyls : Human health aspects. Concise international chemical assessment document 55
Section 6, Environmental levels and human exposure