Concentrations of some atmospheric pollutants have been noticeably reduced in North America. Over the period 1970 to 1993, among the six pollutants used to monitor National Ambient Air Quality in the U.S., lead showed the most dramatic improvement, with virtual outdoor elimination since the 1970s following the changeover to unleaded fuel (Government of the United States of America, 1995). Emissions of four of the other five critical pollutants also declined: carbon monoxide (by 24 per cent), volatile organic compounds (VOCs) (by 24 per cent), fine particulate matter (PM-10) (by 78 per cent), and sulphur dioxide (by 30 per cent). Only nitrogen oxide registered an increase (of 14 per cent) (Government of the United States of America, 1995). Although nitrogen oxide emissions from vehicles have declined during the last 10 years, these gains have been offset to a large extent by increased fuel combustion for electricity generation.
In Canada, industrial emissions of sulphur dioxide were cut by nearly half while emissions of total particulate matter from industrial plants declined by nearly 36 per cent between 1970 and 1985 (Environment Canada, 1996d). As in the United States, ambient concentrations of lead have dropped dramatically and, by the early 1990s, were only a small percentage of the concentrations present 20 years earlier (Environment Canada, n.d.).
Despite these improvements, air pollution is still a concern in both countries. Twenty years after the passage of the Clean Air Act in the United States, one in three Americans still lives in an area where the air is too polluted to meet Federal health standards (Browner, 1996). The major air quality issues in the United States have been identified as health risks from air pollutants in the ambient air, especially ozone and toxic chemicals; ecosystem damage from regional air pollution, including ozone and acid deposition; transboundary air pollution; and radon and other indoor air pollutants (US-UNCED, 1992).
Some central Canadian cities continue to experience unacceptable air quality, especially in summer. The most frequent causes are ground-level ozone and airborne particulates (Environment Canada, 1996d). Canadian and U.S. emissions of ozone-forming volatile organic compounds (VOCs) and nitrogen oxides, the precursors of ground-level ozone, are still close to the levels of the mid-1980s (Government of Canada, 1996). Pulmonary disease from air pollution, especially from small particulates, has been estimated to cause 50,000-60,000 deaths each year in the United States and to cost the economy a total of US$40 billion to US$50 billion in direct health care expenditures and lost productivity (Pohl and Chivian, 1996). Some experts have suggested that 6 per cent of all respiratory admissions to hospitals in Canada are smog-related (Marchi, 1996).
In recent years, comparative risk studies performed by EPA have consistently ranked indoor air pollution among the five top environmental risks to public health in the U.S. Studies of human exposure to air pollution indicate that indoor air levels of many pollutants may be two to five times-and, on some occasions, more than 100 times-higher than outdoor levels (EPA, 1994). Over the past several decades, exposure to indoor air pollutants is believed to have increased due to a variety of factors, including the construction of more tightly sealed buildings, reduced ventilation rates to save energy, the use of synthetic building materials and furnishings, and the use of chemically formulated personal care products, pesticides, and household cleaners. EPA, in close cooperation with other Federal agencies and the private sector, has begun a concerted effort to better understand indoor air pollution and to reduce people s exposure to air pollutants in offices, homes, schools, and so on (EPA, 1996b).
Electricity generation accounts for about 70 per cent of annual sulphur dioxide emissions and 30 per cent of nitrogen oxide emissions in the United States (EPA, 1996d). The largest source of sulphur dioxide emissions in Canada is the smelting of metal ores, which accounts for 50 per cent of eastern Canadian emissions. Power generation contributes 20 per cent (Environment Canada, 1996d).
Airborne acidic pollutants are often transported by large-scale weather systems thousands of kilometres from their point of origin before being deposited. In eastern North America, weather systems generally travel from southwest to northeast. Thus, pollutants emitted from sources in the industrial heartland of the midwestern states and central Canada regularly fall on the more rural and comparatively pristine areas of northeastern U.S. and southeastern Canada (Environment Canada, 1996).
Acidification of surface waters is a further cause for concern. Many lakes and streams in Canada and the United States suffer from chronic acidity because they rest atop soil with a limited capacity to neutralize acidic compounds (called "buffering capacity"). The U.S. National Surface Water Survey found that acid rain causes acidity in 75 per cent of acidic lakes and about half the acidic streams (EPA, 1996d). The Canadian Government has estimated that 14,000 lakes in eastern Canada are acidic (Government of Canada, 1996d). About 43 per cent of Canada s land, mainly in the east, is highly sensitive to acid rain (Environment Canada, 1996d). Acidification in Canada is, to a large extent, a transboundary problem because half the acid rain falling in Canada originates in the U.S. (Environment Canada, 1996). "Episodic" acidification is an added concern, particularly since it can cause large "fish kills." In the United States, for example, approximately 70 per cent of sensitive lakes in the Adirondacks of New York state and 30 per cent of streams in the mid-Appalachians of the eastern United States are likely to become acidic during such an episode (EPA, 1996d).
Recent extreme weather events, including heat waves and heavy flooding, have helped citizens to understand the possible impacts of climate change and how they could affect North America. The region produces more greenhouse gas emissions than any other region and has among the highest per capita emission levels (WRI/UNEP/UNDP/WB, 1996). Total U.S. emissions increased by 13 per cent between 1970 and 1990 (EOP, 1993) and currently comprise 22 per cent of the world total (WRI/ UNEP/UNDP/WB, 1996). Both Canada and the United States are pursuing ways to reduce their contributions as part of concerted efforts under the United Nations Framework Convention on Climate Change.
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