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Acid Rain FAQ


 Questions and Answers

The Facts

  • What causes acid rain?

    Acid deposition is a general term that includes more than simply acid rain. Acid deposition primarily results from the transformation of sulphur dioxide (SO2) and nitrogen oxides into dry or moist secondary pollutants such as sulphuric acid (H2SO4), ammonium nitrate (NH4NO3) and nitric acid (HNO3). The transformation of SO2 and NOx to acidic particles and vapours occurs as these pollutants are transported in the atmosphere over distances of hundreds to thousands of kilometers. Acidic particles and vapours are deposited via two processes - wet and dry deposition. Wet deposition is acid rain, the process by which acids with a pH normally below 5.6 are removed from the atmosphere in rain, snow, sleet or hail. Dry deposition takes place when particles such as fly ash, sulphates, nitrates, and gases (such as SO2 and NOx), are deposited on, or absorbed onto, surfaces. The gases can then be converted into acids when they contact water.

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  • What does acid mean?

    An acid is a substance with a sour taste that is characterized chemically by the ability to react with a base to form a salt. Acids turn blue litmus paper (also called pH paper) red. Strong acids can burn your skin.

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  • What is pH?

    A pH scale is used to measure the amount of acid in a liquid-like water. Because acids release hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed as "pH." This scale is used to measure the acidity of rain samples.

    PH Scale

    • 0 = maximum acidity 
    • 7 = neutral point in the middle of the scale
    • 14 = maximum alkalinity (the opposite of acidity)

    The smaller the number on the pH scale, the more acidic the substance is. Rain measuring between 0 and 5 on the pH scale is acidic and therefore called "acid rain." Small number changes on the pH scale actually mean large changes in acidity.

    For example, a change in just one unit from pH 6.0 to pH 5.0 would indicate a tenfold increase in acidity. Clean rain usually has a pH of 5.6. It is slightly acidic because of carbon dioxide which is naturally present in the atmosphere. Vinegar, by comparison, is very acidic and has a pH of 3.

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  • Where is acid rain a problem?

    Acid rain is a problem in eastern Canada because many of the water and soil systems in this region lack natural alkalinity - such as a lime base - and therefore cannot neutralize acid naturally. Provinces that are part of the Canadian Precambrian Shield, like Ontario, Quebec, New Brunswick and Nova Scotia, are hardest hit because their water and soil systems cannot fight the damaging consequences of acid rain. In fact, more than half of Canada consists of susceptible hard rock (i.e., granite) areas that do not have the capacity to effectively neutralize acid rain. If the water and soil systems were more alkaline - as in parts of western Canada and southeastern Ontario - they could neutralize or "buffer" against acid rain naturally. 

    In western Canada, there is insufficient information at this time to know whether acid rain is affecting these ecosystems. Historically, lower levels of industrialization - relative to eastern Canada - combined with natural factors such as eastwardly moving weather patterns and resistant soils (i.e., soils better able to neutralize acidity), have preserved much of western Canada from the ravages of acid rain.

    However, not all areas in western Canada are naturally protected. Lakes and soils resting on granite bedrock, for instance, cannot neutralize precipitation. These are the conditions found in areas of the Canadian Shield in northeastern Alberta, northern Saskatchewan and Manitoba, parts of western British Columbia, Nunavut and the Northwest Territories . Lakes in these areas are as defenseless to acid rain as those in northern Ontario. If sulphur dioxide and nitrogen oxide emissions continue to increase in western Canada, the same sort of harmful impacts that have happened in eastern Canada could occur.

    Visit The NatChem Website for information on how to obtain deposition data and maps.

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  • Where do sulphur dioxide emissions come from?

    Sulphur dioxide (SO2) is generally a byproduct of industrial processes and burning of fossil fuels. Ore smelting, coal-fired power generators and natural gas processing are the main contributors. In 2000, for instance, U.S. SO2 emissions were measured at 14.8 million tonnes - more than six times greater than Canada's 2.4 million tonnes. But the sources of SO2 emissions from the two countries are different. In Canada, 68% of emissions come from industrial sources and 27% comes from electric utilities (2000). In the U.S., 67% of emissions are from electric utilities (2002).

    Canada cannot win the fight against acid rain on its own. Only reducing acidic emissions in both Canada and the U.S. will stop acid rain. More than half of the acid deposition in eastern Canada originates from emissions in the United States. Areas such as southeastern Ontario (Longwoods) and Sutton, Quebec receive about three-quarters of their acid deposition from the United States. In 1995, the estimated transboundary flow of sulphur dioxide from the United States to Canada was between 3.5 and 4.2 millions of tonnes per year.

    Canada - Sulphur Dioxide Emmissions

    United States - Sulphur Dioxide Emmissions

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  • Have SO2 emission levels changed at all?

    Initiated in 1985, the Eastern Canada Acid Rain program committed Canada to cap SO2 emissions in the seven provinces from Manitoba eastward at 2.3 million tonnes by 1994, a 40% reduction from 1980 levels. By 1994, all seven provinces had achieved or exceeded their targets. In 1998, the provinces, territories and the federal government signed The Canada-Wide Acid Rain Strategy for Post-2000, committing them to further actions to deal with acid rain. Progress under both the Eastern Canada Acid Rain Program and under the Post-2000 Strategy, including data on emissions, is reported in the respective annual reports of these two programs. Between 1980 and 2001, emissions of SO2 declined by approximately 50% to 2.38 million tonnes. Ineastern Canada , emissions of SO2 declined by approximately 63% between 1980 and 2001.

    National Pollutant Release Inventory

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  • Where do NOx emissions come from?

    The main source of NOx emissions is the combustion of fuels in motor vehicles, residential and commercial furnaces, industrial and electrical-utility boilers and engines, and other equipment. In 2000, Canada's largest contributor of NOx was the transportation sector, which accounted for approximately 60% of all emissions. Overall, NOx emissions amounted to 2.5 million tonnes in 2000. By comparison, U.S. NOx emissions for 2000 amounted to 21 million tonnes - 8 times more than Canada 's emissions.

    The influence of transboundary flows of air pollutants from the United States into Canada is significant. Overall about 24% of the regional-scale ozone episodes that are experienced in the United States also affect Ontario. An analysis of ozone concentrations at four sites in extreme southwestern Ontario taking wind factors into account provides an estimate that 50 to 60% of the ozone at these locations is of U.S. origin (Multi-stakeholder NOx/VOC Science Program 1997b).

    Pie Chart - Canada Emmissions, 2000

    Pie Chart - United States Emissions - 2002

    Air Pollutant Emissions, Canada
    Air Pollutant Emission Trends, U.S.

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  • Have NOx emission levels changed at all?

    In Canada , total NOx emissions have been relatively constant since 1985. As of 2000, stationary sources of NOx emissions have been reduced by more than 100,000 tonnes below the forecasted level at power plants, major combustion sources and metal smelting operations. In 2000, as part of the Ozone Annex to the Canada-US Air Quality Agreement, Canada committed to an annual cap on NO2 emissions from fossil-fuel power plants of 39,000 tonnes in central and southern Ontario and 5,000 tonnes in southern Quebec. It also committed to new stringent emission reduction standards for vehicles and fuels and measures to reduce NOx emissions from industrial boilers. These commitments are estimated to reduce annual NOx emissions from the Canadian transboundary region (defined as central and southern Ontario and southern Quebec) by approximately 39% from 1990 by 2010.

    Canada-U.S. Air Quality Agreement and "Progress Reports"
    Air Pollutant Emissions

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  • What is the difference between a target load and a critical load?

    The critical load is a measure of how much pollution an ecosystem can tolerate; in other words, the threshold above which the pollutant load harms the environment. Different regions have different critical loads. Ecosystems that can tolerate acidic pollution have high critical loads, while sensitive ecosystems have low critical loads.

    Critical loads vary across Canada. They depend on the ability of each particular ecosystem to neutralize acids. Scientists have defined the critical load for aquatic ecosystems as the amount of wet sulphate deposition that protects 95% of lakes from acidifying to a pH level of less than 6. (A pH of 7 is neutral; less than 7 is acidic; and greater than 7 is basic.) At a pH below 6, fish and other aquatic species begin to decline.

    A target load is the amount of pollution that is deemed achievable and politically acceptable when other factors (such as ethics, scientific uncertainties, and social and economic effects) are balanced with environmental considerations. Under the Eastern Canada Acid Rain Program, Canada committed to cap SO2 emissions in the seven provinces from Manitoba eastward at 2.3 million tonnes by 1994. The program's objective was to reduce wet sulphate deposition to a target load of no more than 20 kilograms per hectare per year (kg/ha/yr), which our scientists defined as the acceptable deposition rate to protect moderately sensitive aquatic ecosystems from acidification.

    Under the Canada-Wide Acid Rain Strategy for Post-2000, signed in 1998, governments in Canada have adopted the primary long-term goal of meeting critical loads for acid deposition across the country. Recently, maps that combine critical load values for aquatic and forest ecosystems have been developed. These maps indicate the amount of acidity (reported as acid equivalents per hectare per year (eq/ha/yr)) that the most sensitive part of the ecosystem in a particular region can receive without being damaged.

    Map graph - Terrestrial or Aquatic Critical Loads

    The maximum amount of acid deposition that a region can receive without damage to its ecosystems is known as its critical load. It depends essentially on the acid-rain neutralizing capacity of the water, rocks, and soils and, as this map of Canada shows, can vary considerably from one area to another. Critical loads were calculated using either water chemistry models (i.e., "Expert" or "SSWC") or a forest soil model (i.e., "SMB"). The index map (lower left) indicates the model selected for each grid square: red = Expert (aquatic), yellow = SSWC (aquatic), green = SMB (upland forest soils).

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  • Would acid rain remain a problem without further controls?

    Yes. Scientists predicted in 1990 that a reduction in SO2 emissions from Canada and the U.S. of approximately 75% beyond commitments in the 1991 Canada-U.S. Air Quality Agreement (AQA) would be necessary to eliminate the acid deposition problem in Canada. This science was based on the effect of sulphur-derived acids in wet deposition on aquatic ecosystems. New science presented in the 2004 Acid Deposition Science Assessment assesses the capacity of aquatic and terrestrial ecosystems to receive acids derived from both sulphur and nitrogen in wet and dry deposition. Improved estimates of dry deposition (the sum of gaseous SO2, particle sulphate, nitric acid, particle nitrate and other nitrogen species) indicate that past estimates of critical loads for aquatic ecosystems are too high, implying that past predictions of the impact of proposed control strategies have been overly optimistic. In some regions, the critical loads for forest ecosystems are even more stringent that those for aquatic ecosystems. Canada still needs to evaluate the sustainability of forest ecosystems for various levels of acid deposition given the new critical loads for terrestrial ecosystems. It is likely that new science will continue to support the need for further SO2 emission reductions of this scale or somewhat greater.

    That is why The Canada-Wide Acid Rain Strategy for Post-2000 calls for further emission reductions in both Canada and the United States. Without further controls beyond those identified in the 1991 Canada-U.S. Air Quality Agreement, areas of southern and central Ontario, southern and central Quebec, New Brunswick and Nova Scotia would continue to receive mean annual sulphate deposition amounts that exceed their critical loads. The critical load would be exceeded by up to 10 kg/ha/yr of wet sulphate in parts of central Ontario and central and southern Quebec. As a result, about 95,000 lakes would remain damaged by acid rain. Lakes in these areas have not responded to reductions in sulphate deposition as well as, or as rapidly as, those in less sensitive regions. In fact, some sensitive lakes continue to acidify. 

    In total, without further controls, almost 800,000 km2 in southeastern Canada-an area the size of France and the United Kingdom combined-would receive harmful levels of acid rain; that is, levels well above critical load limits for aquatic systems.


    Predicted wet sulphate deposition in excess of critical loads in 2010, without further controls (in kg/ha/yr).

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  • Is rain getting more or less acidic?

    One measure of the acidity of acid rain is the pH. The pH of rain depends on two things: the presence of acid-forming substances such as sulphates, and the availability of acid-neutralizing substances such as calcium and magnesium salts. Clean rain has a pH value of about 5.6. By comparison, vinegar has a pH of 3.

    Although the acidity of acid rain has declined since 1980, rain is still acidic in eastern Canada. For example, the average pH of rain in Ontario's Muskoka-Haliburton area is about 4.5 - about 40 times more acidic than normal.

    Reductions in the acidity of acid rain are due to reductions in emissions of SO2.



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  • How does acid rain affect lakes, rivers and streams?

    Lakes that have been acidified cannot support the same variety of life as healthy lakes. As a lake becomes more acidic, crayfish and clam populations are the first to disappear, then various types of fish. Many types of plankton-minute organisms that form the basis of the lake's food chain-are also affected. As fish stocks dwindle, so do populations of loons and other water birds that feed on them. The lakes, however, do not become totally dead. Some life forms actually benefit from the increased acidity. Lake-bottom plants and mosses, for instance, thrive in acid lakes. So do blackfly larvae.

    Not all lakes that are exposed to acid rain become acidified. In areas where there is plenty of limestone rock, lakes are better able to neutralize acid. In areas where rock is mostly granite, the lakes cannot neutralize acid. Unfortunately, much of eastern Canada-where most of the acid rain falls-has a lot of granite rock and therefore a very low capacity for neutralizing acids.

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  • What happens to the fish, frogs, birds and bugs that live there?

    There are many ways the acidification of lakes, rivers and streams harm fish. Mass fish mortalities occur (during the spring snow melt) when highly acidic pollutants-that have built up in the snow over the winter-begin to drain into common waterways. Such happenings have been well documented for salmon and trout in Norway.

    More often, fish gradually disappear from these waterways as their environment slowly becomes intolerable. Some kinds of fish such as smallmouth bass, walleye, brook trout and salmon, are more sensitive to acidity than others and tend to disappear first.

    Even those species that appear to be surviving may be suffering from acid stress in a number of different ways. One of the first signs of acid stress is the failure of females to spawn. Sometimes, even if the female is successful in spawning the hatchlings or fry are unable to survive in the highly acidic waters. This explains why some acidic lakes only have older fish in them. A good catch of adult fish in such a lake could mislead an angler into thinking that all is well.

    Other effects of acidified lakes on fish include: decreased growth, inability to regulate their own body chemistry, reduced egg deposition, deformities in young fish and increased susceptibility to naturally occurring diseases.

    Here are the effects of an acidified ecosystem on the natural environment:

    As water pH approachesEffects
    • crustaceans, insects, and some plankton species begin to disappear.
    • major changes in the makeup of the plankton community occur.
    • less desirable species of mosses and plankton may begin to invade.
    • the progressive loss of some fish populations is likely, with the more highly valued species being generally the least tolerant of acidity.
    Less than 5.0
    • the water is largely devoid of fish.
    • the bottom is covered with undecayed material.
    • the nearshore areas may be dominated by mosses.
    • terrestrial animals, dependent on aquatic ecosystems, are affected. Waterfowl, for example, depend on aquatic organisms for nourishment and nutrients. As these food sources are reduced or eliminated, the quality of habitat declines and the reproductive success of birds is affected.

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  • Are the lakes recovering?

    Some acidified lakes are recovering, but many more are not. Of 202 lakes that have been studied since the early 1980s, 33% have reduced levels of acidity while 56% have shown no change and 11% have actually become more acidic. The greatest improvements have been seen in the Sudbury area, where local emissions of acid-causing pollutants have declined by 90% in the last three decades. Here, fish populations have rebounded and fish-eating birds, such as loons, have increased. However, no substantial wildlife recovery has been seen beyond the Sudbury area. The least improvement has been seen in Atlantic Canada, even though lakes in this region were never as highly acidified as those in some parts of Ontario and Quebec. Since 1990, scientists have confirmed that maintaining lake pH at 6.0 or more is the most appropriate criterion for calculating critical loads. This pH level encourages healthy aquatic systems in lakes, rivers and streams.

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  • What does acid rain do to trees?

    The impact of acid rain on trees ranges from minimal to severe, depending on the region of the country and on the acidity of the rain. Acid rain, acid fog and acid vapour damage the surfaces of leaves and needles, reduce a tree's ability to withstand cold, and inhibit plant germination and reproduction. Consequently, tree vitality and regenerative capability are reduced.

    Acid rain also depletes supplies of important nutrients (e.g. calcium and magnesium) from soils. The loss of these nutrients is known to reduce the health and growth of trees (see below).

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  • How else does acid rain affect forests?

    Prolonged exposure to acid rain causes forest soils to lose valuable nutrients. It also increases the concentration of aluminum in the soil, which interferes with the uptake of nutrients by the trees. Lack of nutrients causes trees to grow more slowly or to stop growing altogether. More visible damage, such as defoliation, may show up later. Trees exposed to acid rain may also have more difficulty withstanding other stresses, such as drought, disease, insect pests and cold weather.

    The ability of forests to withstand acidification depends on the ability of the forest soils to neutralize the acids. This is determined by much the same geological conditions that affect the acidification of lakes. Consequently, the threat to forests is largest in those areas where lakes are also seriously threatened - in central Ontario, southern Quebec, and the Atlantic provinces. These areas receive about twice the level of acid rain that forests can tolerate without long-term damage. Forests in upland areas may also experience damage from acid fog that often forms at higher elevations.

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  • Are these effects reversible?

    Acid rain has caused severe depletion of nutrients in forest soils in parts of Ontario, Quebec and the Atlantic provinces, as well as in the northeastern United States. While this may be reversible, it would take many years - in some areas hundreds of years - for soil nutrients to be replenished to former levels through natural processes such as weathering, even if acid rain were eliminated completely. For now, forests in affected areas - where acid rain exceeds the critical loads - are using the pool of minerals accumulated during post-ice age times, although some monitoring sites are already deficient in minerals and visual damage to forests has appeared. The loss of nutrients in forest soils may threaten the long-term sustainability of forests in areas with sensitive soils. If current levels of acid rain continue into the future, the growth and productivity of ~ 50% of Canada's eastern boreal forests will be negatively affected.


    The maximum amount of acid deposition that a region can receive without damage to its ecosystems is known as its critical load. It depends essentially on the acid-rain neutralizing capacity of the water, rocks, and soils. This map, of the upland forest soil Steady-state critical load exceedances for southeastern Canada (eq/ha/yr), shows areas of eastern Canada where the levels of acid deposition exceed the capacity of the soils to neutralize the acid without harming the long-term sustainability of the environment. The Steady-state exceedance calculations assume that the forests are not harvested.

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Air Quality

  • Are there connections to other air pollution problems?

    Yes. Burning fossil fuel also creates urban smog, climate change and releases mercury into the air.


    SO2 can react with water vapour and other chemicals in the air to form very fine particles of sulphate. These airborne particles form a key element of smog and are a significant health hazard. Fine particles lodge deep within the lungs and can cause inflammation and tissue damages. Seniors and persons with heart and respiratory diseases are particularly vulnerable. Recent studies show strong links between high levels of airborne sulphate particles and increased hospital admissions and higher death rates.

    Urban smog also forms a haze in the air that reduces the visibility of distant objects. The areas that are most affected are the Windsor-Quebec corridor in eastern Canada and British Columbia's Lower Fraser Valley where scenery and buildings are often obscured.

    Climate change

    It is expected that with climate change, there will be accompanying higher temperatures and drought. Climate change can also cause harmless sulphur compounds that have built up in wetlands and soils to become acid-forming sulphates. When the wet weather returns, the sulphates are flushed into the surrounding lakes and increase their acidity.


    Higher concentrations of mercury commonly found in acid lakes can cause reproductive problems in birds.

    Ultra violet (UV) radiation

    Plankton and other organisms that live near the surface of an acid lake are more vulnerable to increased UV levels that result from a thinner ozone layer. This happens because acidity reduces the amount of dissolved organic matter in the water, making it clearer and allowing the UV to penetrate to greater depths.

    Swedish Non-Governmental Organization Secretariat on Acid Rain

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Your Health

  • What is the link between acid rain and human health?

    Sulphur dioxide can react with water vapour and other chemicals in the air to form very fine particles of sulphate. These airborne particles form a key component of urban smog and are now recognized as a significant health hazard.

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  • What are the health effects of particulate matter (PM)?

    Fine particles, or particulate matter (PM), can lodge deep within the lungs, where they cause inflammation and damage to tissues. These particles are particularly dangerous to the elderly and to people with heart and respiratory diseases. Recent studies have identified strong links between high levels of airborne sulphate particles and increased hospital admissions for heart and respiratory problems, increased asthma-symptom days, as well as higher death rates from these ailments.


    The air pollution health effects pyramid is a diagrammatic presentation of the relationship between the severity and frequency of health effects, with the mildest and most common effects at the bottom of the pyramid, e.g., symptoms, and the least common but more severe at the top of the pyramid, e.g., premature mortality. The pyramid demonstrates that as severity decreases, the number of people affected increases.

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  • What are the costs to Canadians of these health effects?

    By using computer models, scientists and economists can estimate the costs of these health effects to Canadians. They do this by computer simulations, where they eliminate SO2 emissions in increasing amounts to predict how the cases of heart and respiratory problems and premature mortality would decline. This decline in health effects represents a significant potential benefit to Canadians; however, it also represents the cost to Canadians of living with current SO2 emission levels.

    For example, the expected health benefits to Canada of a 50% SO2 reduction in both eastern Canada and the U.S. (i.e., reductions above and beyond the current commitments in the Eastern Canada Acid Rain Program and U.S. Acid Rain Program) are:

    • 550 premature deaths per year would be avoided;
    • 1,520 emergency room visits per year would be unnecessary; and
    • 210,070 asthma symptom days per year would be avoided.

    Economists estimate that society values these health benefits in a range from just under $500 million per year up to $5 billion per year.

    The U.S. has also estimated the health benefits of their current Acid Rain Program, both to their citizens as well as to Canadians. The average total annual estimated health benefit (in 1994 dollars) for 1997 in the United States is US$10.6 billion, and rises to US$40.0 billion by the year 2010, when the U.S. Acid Rain Program is fully implemented.

    The estimated benefits for Canada occur primarily in the Windsor-Quebec corridor, where the greatest share of the Canadian population likely to be affected by transboundary transport of SO2 emissions from the eastern U.S. is located. The average total annual estimated health benefit for Canada is US$955 million, or well over a billion Canadian dollars by the year 2010.

    U.S. report on health benefits

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Case Studies

  • Sudbury

    The Sudbury region has a well-known history for very high local SO2 emissions and associated acid deposition. Furthermore, it has a broad sensitivity to acid rain. The degree of historical damage to the landscape, combined with efforts of the Ontario government and industry to improve conditions, makes the Sudbury area an unintentional but important "experiment" on a whole ecosystem acidification and recovery process.

    Of the 7000 lakes estimated to have been damaged by smelter emissions, most are located in the hilly forested areas, underlain by granite bedrock, northeast and southwest of Sudbury. As a result, sport fish losses from acidification in this area have also been heavy. In fact, most of Canada's well-documented cases of fisheries losses from acid rain are in the Sudbury area (not forgetting, of course, the losses of Atlantic salmon from Nova Scotia rivers and some sports fish losses in areas of Quebec).

    Over 35 years ago, scientists began studying the lakes and ponds near Sudbury. Since then, a vast amount of information has been collected that has clearly established the damaging effect of smelter emissions on the chemistry and biology of water bodies. This information has since been widely used throughout Canada and the rest of the world in the debate for cleaner air. Dramatic chemical improvements in Sudbury area lakes have been observed following substantial reductions in local smelter emissions. Between 1980 and 1997, Inco and Falconbridge, the two major producers of smelter emissions in the Sudbury area, reduced their SO2 emissions by 75% and 56% respectively.

    Overall, the widespread chemical and biological improvements seen in lakes of the Sudbury area demonstrate the resiliency of aquatic systems and provide strong support for the use of emission controls to combat aquatic acidification. However, many area lakes are still acidic and contaminated with metals.

    Major Sulphur Dioxide (SO2) sources in Sudbury, Ontario (kilotonnes)

     198019901997SO2 cap
    INCO (Copper Cliff)812617200265
    FALCONBRIDGE (Sudbury)1237054100

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