Impacts of Air Pollution

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The progression of air pollution in different parts of the World has led to policy initiatives and defensive expenditures as a response to the impacts on the human and natural environment. Initial policy interventions in Europe were at urban scale to combat human health impacts (for example, the UK Clean Air Acts of 1956 and 1968) and later efforts have been directed towards reducing the emissions leading to acidification and eutrophication by long-range transboundary air pollution, causing impacts far from the sources of pollution. The impacts of pollution typical of Europe and North America are now being increasingly observed in low and middle income countries at both local and regional scales. However, the nature and spectrum of the problems differ between the different

8 D. Mage, G. Ozolins, P. Peterson, A. Webster, R. Orthofer, V. Vandeweerd and M. Gwynne, Urban air pollution in megacities of the World. Atmos. Environ., 1996, 30, 681.

Polluting agent Major sources

Table 1 Sources and effects of selected air pollutants discussed in this article

Particulates (TSP)

Particulates (PM10)

Particulates (PM2 5) aerosols

NOx (including nitrates from oxidation in the atmosphere)

SO2 (and sulfate from oxidation in the atmosphere)


Volatile organic compounds

Buta-1,3-diene Benzene

Heavy metals

Fuel use: coal, oil, peat, biomass; construction process dust emissions

Diesel vehicles; aerosols; industry and land use activities

Predominantly aerosols of sulfate, nitrate and ammonium

Energy use (combustion) of fossil fuels or biomass

Energy use (any high temperature combustion causes N to be fixed from the air) - any fuel, particularly in transport

Energy use (combustion) of coal, oil; industrial processes (e.g. smelting)

Animal husbandry; fertilizer production and volatilization

Road transport; solvent use; extraction and distillation of fossil fuels; non-combustion processes Vehicle exhausts Vehicle exhausts

Energy use (combustion) of coal; metallurgical industry emissions

Photochemical reaction between NOx and VOCs in the presence of sunlight


Human health; nuisance dust; soiling; ecosystem degradation; reduced crop quality and yield Health effects; impact on visibility

Health effects; impact on visibility

Health effects; climate modification

Health effects; ecosystem acidification and eutrophication; precursor of photochemical oxidants (e.g. O3)

Ecosystem acidification; health effects; impacts on crops, forests and natural vegetation; corrosion

Gaseous impacts on vegetation (by NH3); acidification (by NH4 + ) and eutrophication

Health impacts; precursor for photochemical oxidant pollution

Health (carcinogen) Health (carcinogen)

Human health effects; food chain effects; impacts on vegetation

Impacts on human health; impacts on crop yield, tree vitality and natural vegetation; corrosion

Table 2 Excess deaths Date Place Excess deaths associated with air -

pollution incidents December 1873 London 270-700

(modified from ref. 9) December 1892 London 1000

December 1930 Meuse Valley, Belgium 63

December 1952 London 4000

November 1953 New York, USA 250

December 1962 London 340-700

December 1962 Osaka, Japan 60

regions of the World. Table 1 shows the major air pollutants, their sources and impacts.

Local Air Pollution

Local air pollution is pollution in close proximity to urban centres and industrial installations, usually at a distance of less than fifty kilometres. Close to air pollution sources, in and around urban locations and close to industrial installations, there are impacts on human health, crop yield and quality, forest health, man-made materials and monuments, and also on visibility.

Human health. Human health impacts are caused by a wide range of gases and particles (Table 1). Sulfur dioxide, in combination with particulate matter, has caused serious health impacts in Europe and North America over the last 150 years (as indicated from the data in Table 29). For example, there were a number of very bad smog episodes during the 1950s that led to the development of the Clean Air Act in the UK. Figure 5 shows the number of excess deaths related to the most infamous smog episode in London in 1952 where 4000 excess deaths were eventually recorded, which led to a public outcry forcing politicians to act.

Recent evidence suggests that fine particulates, less than 10 ¿um (PM10) and, in particular, less than 2.5 ¿um (PM2 5), size fractions that represent an inhalation hazard, are contributing to respiratory and cardiopulmonary disease™ resulting in mortality and hospital admissions at pollutant concentrations well below air quality standards.n Indeed, epidemiological evidence from the United States is consistent with a linear non- threshold response for the population as a whole. 10 The PM10 and PM2 5 concentrations are higher in parts of Asia than in Europe or North America and similar responses may be seen in these areas, although further epidemiological studies are required in developing countries to determine whether people in different socio-economic circumstances respond in the same or different ways to air pollutants.12 However, as there is no evidence that a

9 Modified from D. M. Elsom, Atmospheric Pollution: A Global Problem, Second Edition, Blackwell, Oxford, 1992.

10 M. Lippman, The 1997 US EPA standards for particulate matter and ozone, in Air Pollution and Health, ed. R. E. Hester and R. M. Harrison, Issues in Environmental Science and Technology, No. 10, The Royal Society of Chemistry, Cambridge, 1998, pp. 75-99.

11 F. Murray, Impacts on Health, in Regional Air Pollution in Developing Countries, Background Document for Policy Dialogue. Stockholm Environment Institute, York, 1998, pp. 5-11.

12 F. Murray, G. McGranahan and J. C. I. Kuylenstierna. The application of models to assess health effects of air pollution in Asia, Water, Air, Soil Pollut. (in press).

Figure 5 Deaths related to sulfur dioxide and smoke concentration in the London smog, December 195213

Figure 5 Deaths related to sulfur dioxide and smoke concentration in the London smog, December 195213

December 1952

threshold to PM-associated health effects exists it may be reasonable to expect that reductions in PM10 and PM25 will result in improvements in peoples health.14 Oxides of nitrogen are associated with increased incidence of lower respiratory tract infection in children and decreased airway responsiveness in asthmatics. 15 It has proved difficult to disentangle the impact of outdoor NOX impacts from indoor, and from the presence of other pollutants, but published estimates suggest respiratory effects in children at annual average NO2 concentrations in the range 50-75¿ugm~3 or higher.i®

Ozone-related health effects include changes in lung capacity, flow resistance, epithelial permeability and reactivity to bronchoactive challenges. Short term effects on lung function can be observed a few hours after the exposure and may persist for hours or days after the exposure ceases. 10 These effects are apparently reversible, but repeated exposure can exacerbate or prolong these effects, although the current knowledge of the chronic health effects are much less complete. 10 Field studies also show decreased athletic performance and increased incidence of asthma attacks and respiratory symptoms in asthmatics. 11

There has been a steady improvement in air quality in cities of Europe since the 1970s due to policies that reduce pollutants such as sulfur dioxide and particulates, and excess death rates have fallen. In Asia, some urban areas seem to be following the same pollution trajectory as cities such as London up until the

13 E. T. Wilkins, Air pollution and aspects of the London fog of December 1952, Quart J. R. Met. Soc., 1954, 80, 267.

14 M. Lippman, Air Pollution and Health - Studies in North America and Europe, in Health and Air Pollution in Developing Countries, ed. G. McGranahan and F. Murray, Stockholm Environment Institute, York, 1999, pp. 29-41.

15 L. J. Folinsbee, Human health effects of air pollution, Environ. Health Perspect., 1992,100, 45-56.

16 WHO, Updating and Revision of the Air Quality Guidelines for Europe, Reports of WHO Working Groups, WHO Regional Office for Europe, Copenhagen, 1995.

1950s. In China annual average SO2 concentrations of402 ^g m~3 and particulate matter (TSP) concentrations of 690 ¿ugm~3 have been recorded in 'urban' sites in Chongqing in 1985-89.17 Globally more than 1200 million people may be exposed to excessive levels of sulfur dioxide and more than 1400 million people to excessive levels of suspended particulate matter. i8

Specifically, some traffic-related pollution (giving rise to oxides of nitrogen, CO, particulates and VOCs) has continued to increase in many parts of the world as vehicle numbers increase, often negating improvements in engine or fuel technology. Lead in petrol has been phased out in most of Europe, and the health damaging lead concentrations have been reduced. Focus in Europe has now been placed on reducing PM10, NOX, benzene and buta-1,3-diene (both carcinogens) and plans are being implemented to reduce these pollutants. 19 The nature of the pollution in urban areas in Europe is therefore changing, even though the overall impact has reduced. Air pollution from the transport sector now dominates in some developing country cities, where particulate emissions from diesel vehicles can be very high, leaded petrol is still in use and where hot, sunny conditions readily give rise to photochemical smogs, leading to increased ozone- and NOx-related health impacts.

Indoor air pollution is a severe problem in many developing countries, particularly in many parts of Africa and Asia. This is especially so for women, cooking over open fires in houses that have poor ventilation. It is a major problem in rural areas and villages, where the poor may be using dung or fuelwood for cooking, rather than cleaner fuels such as kerosene or gas. Roughly 20-35 per cent of total energy consumption in developing countries uses 'traditional fuels' of wood and other biomass fuels.20 However, the scale of the health impact of indoor air pollution is difficult to estimate as there are relatively few data, but it has been estimated2i that 2.8 million premature deaths per annum may be the result of indoor air pollution.

Biomass burning is an air pollution problem more clearly related to Asia, Africa and Latin America than with Europe or North America. The total emission of particulate matter and nitrogen oxides can be high, but generally spread over large areas. The smoke haze episode of 1997 in Indonesia and surrounding countries was related to fires used for forest clearance burning out of control. This became a regional health problem with air quality indices reaching hazardous levels and states of emergency declared in areas in several countries in South-East Asia. More than 8000 people were admitted to hospital in Malaysia with health problems related to air pollution.11

17 T. Larssen, H.-M. Seip, A. Semb, J. Mulder, I. P. Muniz, R.D. Vogt, E. Lydersen, V. Angell, T. Dagang and O. Eilertsen, Acid deposition and its effects in China: an overview, Environ. Sci. Policy, 1999, 2, 9-24.

18 UNEP, Urban Air Pollution. UNEP/GEMS Environment Library No. 4, United Nations Environment Programme, Nairobi, Kenya, 1991.

19 DETR, Air Quality and Transport. Part IV The Environment Act 1995 Local Air Quality Management, Department of the Environment, Transport and the Regions, London, 2000.

20 WRI, World Resources: 1998-99, World Resources Institute. Oxford University Press, Oxford, 1998.

21 K. R. Smith, Indoor air pollution in developing countries: growing evidence of its role in the global disease burden, in Proceedings of Indoor Air '96, 7th International Conference on Indoor Air Quality and Climate, Institute of Public Health, Tokyo, 1996.

The cost of air pollution to developing country cities can be very high. Using dose-response relationships developed for health and particulate matter, the URBAIR project calculated the costs of air pollution impacts on health in Mumbai, Metro Manilla, Jakarta and the Kathmandu Valley caused by PM10 exposure.22 The cost of excess deaths was calculated using the human capital approach and chronic impacts based upon restricted activity days, asthma attacks and respiratory symptom days. The health costs in the these cities varied from US $2.84 million in Kathmandu to US $127 million per annum in Jakarta, with Mumbai and Manila also showing health costs greater than US $100 million per annum.

Air quality guidelines to protect human health have been produced by international organizations such as the World Health Organisation (WHO), regional organizations, such as EU and national governments (see Table 3 for some examples). When comparing pollutant concentrations in cities of Asia to air quality guidelines, the concentrations of particulate matter consistently register as being of serious concern. For example, concentrations in Bangkok, Jakarta, Manila, Beijing, Delhi and Seoul exceeded WHO guidelines by more than a factor of two in 1992.23 In addition, SO2 pollution in NE Asian cities, such as Beijing and Seoul, also indicates a serious situation. Ozone causes 'moderate to heavy' pollution in Jakarta and Beijing, showing similar exceedance of WHO guidelines (up to a factor of two) to New York or Tokyo.23

Impacts of gaseous air pollution on vegetation. Ozone and sulfur dioxide have impacts on crops, forests and natural vegetation in both industrialized and developing countries. The direct impacts of elevated SO2 concentrations are largely confined to urban areas, peri-urban areas and close to industrial sources of pollution, although dry and wet deposition of resulting acidifying substances occurs on a continental scale (see Section 2). Ozone, once formed, may be transported in the atmosphere and affect much larger areas. Gaseous air pollutants may affect vegetation through visible injury and/or effects on growth and yield (invisible injury) and through subtle physiological, chemical or anatomical changes.28 Visible damage to leaves may have direct impacts on the market value of crops such as spinach and tobacco. In the 1960s it was widely believed that yield could only be affected when visible injury was present, but a large body of knowledge has now shown that significant yield losses can occur in

22 World Bank, Urban Air Quality Management Strategy in Asia: Guidebook, World Bank (ISBN: 0-8213-4032-8). Washington, DC, 1997.

23 ASEAN, First ASEAN State of the Environment Report. ASEAN Secretariat, Jakarta, 1997.

24 Council of Ministers of the European Commission, Air Quality Assessment and Management, Report 96/62/EC, Brussels, 1996.

25 US EPA, National Primary and Secondary Ambient Air Quality Standards, Air Programs 40CFR Part 50, Office of Air Quality Planning and Standards, Research Triangle Park, NC, 1997.

26 US EPA, EAP's Clean Air Standards - A Common Sense Primer, Office of Air Quality Planning and Standards, Research Triangle Park, NC, 1997.

27 H. Fujimaki, Key air pollutants in Japan, in Global Air Quality Guidelines, World Health Organisation, Geneva, 1998.

28 M.R. Ashmore, Air pollution and agriculture, Outlook on Agriculture, 1991, 20 (3), 139-144.

Table 3 International guideline values for some urban air pollutants as a time-weigh ted average concentration in air12

European Union air quality


air quality



US air

quality standards

Japanese air

quality standards

Guidelines for Europed



Concentration Averaging







Mgm 3







1 hour,


1 hour


1 hour



99.7 percentile


24 hour,


24 hour


24 hour


24 hour

99 percentile







1 hour,


1 hour

99.9 percentile






24 hour





1 hour


1 hour


1 hour


8 hour


8 hour"


8 hours





1 hour


1 hour


24 hour


24 hour


50 b

24 hour,


24 hour

98 percentile







24 hour



aProposed by the Commission.24

bTakes effect from 1 January, 2005.25-26cTakes effect from 1 January, 2010.27 dThe guideline values should be read in the context of the guideline documents. 16 "Proposed goal.

fNo guideline values were set for particulate matter because there is no evident threshold for effects on morbidity and mortality.

Figure 6 Total filled grain yield (dry weight per plant in g) of Basmati rice (cv. Basmati-385) at final harvest for plants grown in open-top chambers in Pakistan (7 km south of Lahore) in air filtered of pollutants compared to unfiltered (polluted) air showing a 42% reduction in grain yield29

the absence of visible symptoms. Most work on yield reductions has taken place in Europe and North America, but work in Asia is increasingly showing significant yield reductions in crops such as rice (Figure 6) and wheat at ambient pollutant concentrations in comparison to air filtered of pollutants. As concentrations of NOx and VOCs increase in line with increasing vehicle use in developing countries, tropospheric ozone concentrations are liable to increase and the potential for yield losses will become greater.

Crop loss assessments to date have concentrated on the direct impacts of air pollution on yield, and have not taken into account effects on crop quality or the indirect impacts on yield. Reductions in income for vegetable producers and suppliers can arise from visible damage to the edible portion of the crop. In addition, there are other potential non-visible impacts of air pollution such as reductions in nutritional quality or accumulation of heavy metals, with important implications for consumers, particularly the poor.30,31

Corrosion impacts on materials. Corrosion of materials has mainly been a topic studied in Europe and North America. However, the pollution levels in Asia have increased rapidly and, as many developing countries happen to be located in warm, humid regions with high relative humidity and high frequency of rainfall, there is a great risk of extreme corrosion rates, even higher than in temperate zones at the same pollutant concentration^ From a comparison of corrosion data from China and Europe, the sensitivity to SO2 is similar in tropical climates to that in temperate, but the sensitivity to acidic wet deposition is much higher in wet tropical conditions. For non-marine sites Chinese data (Figure 7) show that corrosion rates are 4—5 times higher for carbon steel and 2—3 times higher for zinc and copper in Chinese sites than in UN/ECE test sites in Europe. This is due

29 A. Wahid, R. Maggs, S. R. A. Shamsi, J. N. B. Bell and M. R. Ashmore, Effects of air pollution in rice yield in the Pakistan Punjab. Environ. Pollut., 1995, 90, 323.

30 F. Marshall, M. Ashmore and F. Hinchcliffe, A Hidden Threat to Food Production: Air Pollution and Agriculture in the Developing World, International Institute For Environment and Development, London, 1997.

31 M. R. Ashmore and F. M. Marshall, Ozone Impacts on Agriculture: An Issue of Global Concern. Adv. Bot. Res, 1999, 29, 32—52.

3 2 J. Tidblad, A. A. Mikhailov, and V. Kucera, Acid Deposition Effects on Materials in Subtropical and Tropical Climates. Data compilation and temperate climate comparison, Swedish Corrosion Institute KI Report 2000:8E, Stockholm.

Clean/filtered air Polluted/unfiltered air

Figure 7 Maximum of observed corrosion rates for zinc, copper and C-steel for temperate and tropical/sub-tropical climates32





^ c




S5 ¡5


c a.

■2 S-

§8 oS












o in










Temperate (non-marine)

Tropical/subtropical (non-marine)

partly to the higher SO2 concentrations that now exist in China and partly due to climatic differences increasing the sensitivity of materials in tropical and sub-tropical conditions. However, the influence of the climate on dose-response relationships is not fully known and the data for those dose-response functions that exist (for China) are limited and require verification. The Chinese dose-response relationships indicate that although the response to dry deposition is similar to the European data, the corrosion rates for wet deposition are much higher for copper and zinc using the Chinese relationships. Several climatic differences in wet sub-tropical and tropical regions, compared to temperate climates, need to be taken into account in order to quantify corrosion effects^

(i) the higher temperature and solar radiation;

(ii) high relative humidity throughout the year;

(iii) the potential for increased dew formation (due to large differences between day- and night-time temperatures);

(iv) the different character of rain in these regions.

Impacts on visibility. There exists a close association between the concentration of particles in the atmosphere, their light scattering coefficient (properties) and the range of visibility. Although more complex relationships exist,33 a robust relationship can be defined^4

V = visibility in miles at noon in dry conditions M = mass of particulate matter (^gm-3)

3 3 G. Landrieu, Visibility Impairment by Secondary Ammonium, Sulphates, Nitrates, and Organic Particles, Draft Note Prepared for the UN/ECE Convention on LRTAP, Copenhagen 9-10 June, 1997.

34 K. Noll et al., Visibility and aerosol concentration in urban air, Atmos. Environ., 1968,2,465-475.

This stresses the direct relationship between particulate matter concentrations and visibility. According to Maddison,35 visibility range in Europe is a 'good' that has significant economic value, although little attention to such valuation has occurred in Europe. Visibility can never really be separated from the aesthetic qualities of the landscape, which casts doubt on the transferability of valuation exercises. However, many developing countries rely on visibility for tourism. In Nepal, atmospheric visibility data indicate that there has been a substantial decrease in visibility in the Kathmandu Valley since 1970. The number of days with good visibility in Winter at Kathmandu Airport has decreased from 25 days per month in 1972 to 5 days per month in 1992.3® The bowl-like topography and low wind speeds during the winter season create poor dispersion conditions, predisposing the Kathmandu Valley to serious air pollution problems22 (World Bank). A pessimistic, but accurate image of the air pollution situation in the Kathmandu Valley37 gave negative publicity to the area that could have had an adverse impact on tourism. In the early 1990s, foreign currency revenues amounted to approximately US$60 million a year. Although no 'dose-effect' relationships of air pollution and tourism are available, if it were assumed that there could be approximately 10% decrease in tourism, then this could lead to a loss of to US$6 million for Nepal. This is a very significant amount of foreign exchange for a country that has a negative balance of trade.22

Regional Air Pollution

Gaseous emissions such as sulfur dioxide and nitrogen oxides are rapidly oxidized to sulfate and nitrate, and ammonia is transformed to ammonium in the atmosphere. These pollutants can travel over very long distances, transported by the winds over hundreds of kilometres and then deposited by wet deposition in rain, occult (in cloud) deposition and dry deposition to surfaces and may cause impacts far from the source of pollution. Sulfur and nitrogen deposition may acidify ecosystems and nitrogen may be responsible for eutrophication (over-fertilization) of terrestrial ecosystems. Ozone is another gas which, once produced, may be transported over long distances and have effects on vegetation. Acidification, to a lesser extent eutrophication, and more recently ozone impacts on crops and forests are impacts that have led to the development of negotiations between countries in Europe and elsewhere that are receiving each others' pollution. The need for such negotiations becomes clear when one considers that more than 90% of the acidic deposition in Norway (where acidification of lakes has been a large problem) came from sources outside Norway during the early 1980s (EMEP model calculations38) when the negotiations were gaining momentum.

35 D. Maddison, The Economic Value of Visibility: A Survey, Centre for Social and Economic Research on the Global Environment (CSERGE), University College, London and University of East Anglia, 1997.

3® UNEP, State of the Environment: Nepal, United Nations Environment Programme, Regional

Resource Centre for the Asia-Pacific. Pathumthani, Thailand, 2001. 37 Article in Newsweek, October 1993.

3 8 J. Lehmhaus, J. Saltbones and A. Eliassen, Deposition Patterns and Transport Sector Analyses for a Four- Year Period, EMEP/MSC-W Report 1/85, The Norwegian Meteorological Institute, Oslo, 1985.

Figure 8 The distribution of relative ecosystem sensitivity to acidic deposition based upon the soil buffering characteristics 4i

Figure 8 The distribution of relative ecosystem sensitivity to acidic deposition based upon the soil buffering characteristics 4i

Acidification. Soil acidification occurs when base cations on the soil exchange complex are replaced by hydrogen and/or aluminium. Base cations are supplied by weathering and atmospheric deposition and removed by plant uptake and leaching. Leaching of cations depends on leaching of mobile anions such as SO42" and NO3~. In many areas input and output of SO42" are quite similar. However, adsorption and/or plant uptake may occur. The nitrogen processes are more complex. Atmospheric deposition includes both NO and NH4+; the latter may give NO3" and hydrogen ions through nitrification. Nitrogen-saturated ecosystems (where nitrogen inputs are in excess of ecosystem nutrient needs,39 such as seen in highly polluted parts of Europe) will leach a greater proportion of nitrogen deposited than ecosystems where nitrogen is limiting vegetation growth. In some areas, very little nitrate leaches at all. As bases leach from the soil, the base saturation and pH may decrease (the soil acidifies) if the soil is not sufficiently buffered by soil mineral weathering. The rate of acidification will depend on the capacity of the base cation storage on the cation exchange complex. Lake and stream water acidification occurs as the soils in the catchment acidify and the decrease in pH, increases in aluminium concentrations and the loss of fish have been the clearest and most severe impacts of acidic deposition, particularly in the sensitive regions of Scandinavia, the UK and NE North America. The soil acidification can itself have impacts on the vegetation at a site, leading to losses in biodiversity and plant vigour. The widespread tree damage in central Europe and parts of Canada and the USA have been attributed to 'acid rain'. The indications are that pollution is one of the main causes, but that the situation relating to tree damage is the result of a complex interaction of different pollutants with biotic and climatic stresses.40

Kuylenstierna et al4 have used soil buffering characteristics to map the

39 J. D. Aber, W. McDowell, K. Nadelhoffer, A. Magill, G. Berntson, M. Kamakea, S. McNulty, W. Currie, L. Rustad and I. Fernandez, Nitrogen saturation in temperate forest ecosystems, hypotheses revisited. Bioscience, 1998, 48, 921-934.

40 A. Wellburn, Air Pollution and Acid Rain: The Biological Impact, Longman, Harlow, 1990.

41 J. C. I. Kuylenstierna, H. Rodhe, S. Cinderby and K. Hicks, Acidification in developing countries:

relative sensitivity of different regions to acidification-related impacts (Figure 8). Although concentration on acidification in Asia has only recently begun, several studies have indicated evidence for soil acidification in China that may be associated with acidic deposition.^ Figure 8 shows that many ecosystems in NE and SE Asia may be very sensitive and so the risks of acidification damage may be rather high in areas with high deposition rates.

Eutrophication. Nitrogen deposition, either as ammonium or nitrate, can eutrophy (nutrify to excess) terrestrial ecosystems, causing changes to both structure and function. Nitrogen is an important plant nutrient, often limiting growth in terrestrial ecosystems. Therefore, nitrogen additions can cause increased growth which promotes the proliferation of species with a high nitrogen demand at the expense of those species requiring less nitrogen. As Tamm43 notes, most of the threatened (rare) species in central Europe have a low nitrogen demand and therefore are most at risk from widespread increases in nitrogen deposition. In developing countries, nitrogen has not been as clearly studied, but many florally diverse ecosystems could be at risk if nitrogen deposition increases.

Although freshwater ecosystems are often phosphorus limited, coastal marine ecosystems can be very sensitive to increasing concentrations of nitrate. This applies also in tropical regions where coral reefs may be significantly affected. The majority of nitrate in rivers comes from the agricultural sector, but a significant proportion comes from atmospheric deposition in some areas, such as in the Baltic Sea where 20% of inputs derived from the atmosphere.44

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