International Air Quality Advisory Board
SPECIAL REPORT ON
3. CONTINENTAL ISSUES: PERSISTENT TOXIC SUBSTANCES
3.1 Quantification, Effects, Challenges, and the Grasshopper Effect
Across the North American continent, hundreds of hazardous pollutants are emitted into the atmosphere daily from many different sources. These include motor vehicles, oil and gas combustion, metallurgical industries, chemical production and manufacturing, gasoline production and distribution, and solvent use. Many hazardous pollutants have been found as contaminants in surface water, sediment, drinking water, and food. The air pathway and atmospheric deposition have increasingly been identified as factors in these occurences. Most of the pollutants only contribute to local problems and are not transported long distances. One subset of these hazardous pollutants, however, is a group known as persistent toxic substances. PTSs tend to volatilize and move through the atmosphere, frequently crossing boundaries before they are deposited. Hence, they have become a cause for continental and global concern.
Because PTSs tend to undergo biomagnification and bioaccumulation, their impact is greatest on animals at the top of the food web: predatory birds, fish, and mammals, including humans. Exposure to PTSs seldom results in acute symptoms or death, but it can impair reproductive capabilities and reduce resistance to disease. Studies of the chronic effects of exposure, however, are very difficult to conduct. For example, epidemiological studies are often impeded by the long latency period (decades in some cases) from the time of exposure until the onset of effects.
Once PTSs have been emitted into the atmosphere, their ultimate destination depends in part on their chemical and physical characteristics and in part on prevailing meteorological conditions. Most PTSs remain in the atmosphere for several hours to a few days -- long enough to cross into neighboring states, provinces, or countries. In addition, because some of these compounds are semi-volatile, they can evaporate and become airborne early in the day, and condense and deposit onto soil, vegetation or water later in the day. Some also show a similar seasonal cycle. This process of recycling through the atmosphere is known as the "grasshopper effect." Although there are considerable uncertainties that prevent exact quantification of transport, it is generally agreed that the "atmospheric region of influence" is very large.
There is considerable evidence of the long-range transport of PTSs and of their bioaccumulation in animals and humans in different parts of North America, including remote areas of the Arctic where no local sources exist. There are, however, major uncertainties regarding the source regions and magnitude of PTS emissions, and the rate of transport through the atmosphere. Although quantities in the atmosphere are often below detection, there is evidence that they nevertheless contribute significantly to bioaccumulation in fish, birds, and mammals.
Residuals from past uses of these PTSs remain in the environment. Contaminated soils and sediments abound in many areas; leachate and volatiles from landfills often contain these compounds. These are sources that are difficult to identify and quantify with any known accuracy. As an example, for more than a decade now, Lake Ontario has been acting as a net source of polychlorinated biphenyls (PCBs) that are degassing out of the lake at a rate that currently exceeds inputs.
Emissions from sources outside the continent also contribute to the accumulation of PTSs in North America, and vice versa. The transport of PTSs from outside North America to regions such as the Arctic is well documented. Figure 3-1 (Brown Snow), shows the pathway through which particles and persistent organic pollutants from the former Soviet Union are transported to the Arctic. To define the air pollution sourcereceptor relationship between North America and other continents and regions, however, considerably more data are needed.
The major PTSs and the key issues associated with them are briefly described below.
3.2 PCBs and PICs
PCBs are a group of chemicals that were introduced in 1929 and manufactured in developed countries until the late 1970s. (Some production apparently continues in parts of the former Soviet Union.) Because PCBs are chemically stable and heat resistant, they were used worldwide as electrical transformer and capacitor oils, hydraulic and heat exchange fluids, lubricating and cutting oils, and plasticizers in joint sealants. While their use is currently banned in all developed countries, substantial amounts of PCBs still remain in large capacitors and transformers.
There is little information about current use and disposal of PCBs. Improper disposal in the past has resulted in a large number of sites that contain PCB-contaminated sediments and soils. These sites are also suspected to be significant sources of atmospheric emissions, although the magnitude and extent of these emissions are not known.
Several "products of incomplete combustion" (PICs) -- including polycyclic aromatic hydrocarbons (PAHs) and other polycyclic compounds containing oxygen and nitrogen -- are persistent in the environment. The PICs of most concern are dioxins (PCDDs) and furans (PCDFs), a group of chlorinated chemicals that are toxic to life forms in minute quantities. Dioxins and furans can enter the environment in many ways. Waste incinerators that are not equipped with efficient flue-gas cleaning systems are known to be one of the most significant sources. Wood-burning stoves and the burning of leaded automotive fuel also add to loadings in the atmosphere. Dioxins and furans also enter the environment as trace byproducts of industrial processes, including processes used in metallurgical industries. Pulp and paper mills using elemental chlorine in their bleaching process often release water contaminated with dioxins. Traces of these contaminants have also been found in chlorophenoxy acid herbicides (e.g. Agent Orange), in chlorophenol wood preservatives, and in PCB mixtures (mainly furans). Hexachlorobenzene (HCB) is another trace byproduct in the production of chlorine gas and chlorinated compounds, including several pesticides. HCB is also emitted to the atmosphere by waste incineration and metallurgical processes.
Many substances used as flame retardants have chemical properties that are similar to PCBs. Polybrominated diphenyl ethers can leach out of electrical equipment, building materials, car interiors, and textiles treated with flame retardant to contaminate the environment. Knowledge of the toxicity of brominated flame retardants is very limited.
Many PTSs are pesticides -- organic compounds with several chlorine atoms designed to be toxic to their target organisms. Most affect the nervous system and the liver; several interfere with reproduction. Although most industrialized countries have eliminated or restricted the use of first-generation pesticides over the last two decades, much of the developing world still uses them for public health and pest control purposes. Of primary concern are DDT, toxaphene, chlordane, mirex, aldrin, dieldrin, and hexachlorocyclohexanes.
Metals occur naturally in the environment as part of the earth's crust. They are emitted into the air and water through both natural and anthropogenic processes. The heavy metals of primary concern are mercury, cadmium, and lead. These three metals can have discernable toxic effects to life forms although their levels in the atmosphere are only moderately above background concentrations.
Mercury also occurs naturally as elemental mercury and in organic and inorganic compounds. Much of the mercury in the environment is strongly bonded to sediment and organic matter, making it unavailable to organisms. Microorganisms, however, can convert inorganic mercury into methyl mercury, a fat-soluble molecule that easily passes through cell membranes, accumulates in animals, and biomagnifies in the food web. The processes controlling methylation and bioaccumulation are poorly understood. Because mercury is a nerve toxin, there are health concerns about its effect on the brain, as well as damage to the reproductive, digestive, and sensory systems. Plants can also be sensitive to mercury, particularly at high concentrations, which can lead to reduced growth.
Two primary anthropogenic sources of mercury are fossil fuel combustion (particularly coal) and waste incineration. Other sources include the chlor-alkali industry and non-ferrous metals production. Mercury is commonly used in thermometers, barometers, dental fillings, batteries, switches and fluorescent lamps, and is thus present in waste containing these items.
Cadmium is toxic to most forms of life. It can be taken up directly from water, and to some extent from air and food, and has a tendency to bioaccumulate in both plants and animals. Kidney damage, decalcification of the skeleton, and emphysema are the serious chronic effects of high cadmium exposure, but these are encountered only in some occupational settings. Cadmium is a byproduct in the pyrometallurgical production of zinc, which is the leading anthropogenic source of cadmium to the environment. Cadmium is also a byproduct in the production of lead. Other major sources include fossil fuel combustion and waste incineration, and the manufacture of alloys, pigments, metal coatings, batteries, and electronics. Cadmium is also a contaminant in chemical fertilizer, manure, compost, and sewage sludge. Cigarette smoking is another major cause of cadmium accumulation in humans.
Lead in the environment is readily absorbed by sediments and soil particles, and is therefore largely unavailable to plants and animals. Leaded gasoline, however, has been and, outside of the United States and Canada, continues to be a major source of lead emissions to the environment on the continental scale. Lead is also emitted from the mining and metallurgic industries, ammunition manufacture and use, and municipal waste incineration. It accumulates in the liver, kidney, spleen, and skeleton; it can damage the nervous and gastrointestinal systems, and interfere with the formation of red blood cells.
Over the past two years, the Board has reviewed current efforts in the United States and Canada to address the issue of PTSs in the environment, including efforts underway to implement the Great Lakes Binational Toxics Strategy signed by both countries in April 1997. The Board has determined that:
The Board recommends that the Commission closely track implementation of the Great Lakes Binational Toxics Strategy at regular intervals and review the progress in completing specific elements of the strategy as identified by the Board under the strategy.