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Home , Nitrogen dioxide, Nitrogen oxide, NOx

Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998

Nitrogen

Nitrogen oxides (NOx) in the ambient air consist 1994). The United States generates about 20 mil- primarily of nitric (NO) and nitrogen di- lion metric tons of nitrogen oxides per year, about oxide (NO2). These two forms of gaseous nitro- 40% of which is emitted from mobile sources. Of gen oxides are significant pollutants of the lower the 11 million to 12 million metric tons of nitrogen atmosphere. Another form, (N2O), oxides that originate from stationary sources, is a . At the point of discharge about 30% is the result of fuel in large from man-made sources, , a colorless, industrial furnaces and 70% is from electric utility tasteless gas, is the predominant form of nitro- furnaces (Cooper and Alley 1986). gen oxide. Nitric oxide is readily converted to the much more harmful by Occurrence in Air and Routes of Exposure chemical reaction with present in the at- mosphere. Nitrogen dioxide is a yellowish-or- Annual mean concentrations of nitrogen dioxide ange to reddish-brown gas with a pungent, in urban areas throughout the world are in the irritating odor, and it is a strong oxidant. A por- range of 20–90 micrograms per cubic meter (µg/ tion of nitrogen dioxide in the atmosphere is con- m3). Maximum half-hour values and maximum 24- verted to (HNO3) and ammonium hour values of nitrogen dioxide can approach 850 salts. aerosol (acid aerosol) is removed µg/m3 and 400 µg/m3, respectively. Hourly aver- from the atmosphere through wet or dry deposi- ages near very busy roads often exceed 1,000 µg/ tion processes similar to those that remove sul- m3. Urban outdoor levels of nitrogen dioxide vary fate aerosol. according to time of day, season, and meteorologi- cal conditions. Typically, urban concentrations Major Sources peak during the morning and afternoon rush hours. Levels are also higher in winter in cold re- Only about 10% of all NO emissions come from x gions of the world than in other seasons because of anthropogenic sources (Godish 1991). The rest is produced naturally by anaerobic biological pro- the increased use of heating fuels. Finally, since the cesses in soil and water, by and volca- conversion of nitrogen dioxide from nitric oxide nic activity, and by photochemical destruction of depends on solar intensity, concentrations are of- nitrogen compounds in the upper atmosphere. ten greater on warm, sunny days. Nitrogen oxides About 50% of emissions from anthropogenic decay rapidly as polluted air moves away from the sources comes from fossil-fuel-fired heat and elec- source. Concentrations of nitrogen oxides in rural tricity generating plants and slightly less from areas without major sources are typically close to motor vehicles. Other sources include industrial background levels. However, nitrogen oxides can boilers, incinerators, the manufacture of nitric travel long distances in the upper atmosphere, con- acid and other nitrogenous chemicals, electric arc tributing to elevated ozone levels and acidic depo- welding processes, the use of in min- sitions far from sources of emissions. ing, and farm silos. Concentrations of nitrogen dioxide in homes Worldwide annual emissions of anthropogenic may considerably exceed outdoor levels and may nitrogen oxides are estimated at approximately therefore be more important for human health. 50 million metric tons (World Resources Institute Large sources of indoor nitrogen dioxide include

223 224 PROJECT GUIDELINES: POLLUTANTS cigarette smoke, gas-fired appliances, and space sue disruption, obstruction of the respiratory bron- heaters. Nitrogen dioxide concentrations in kitch- chioles, and increased susceptibility to bacterial ens with unvented gas appliances can exceed 200 infection of the lungs (WHO 1987). Rats and rab- µg/m3 over a period of several days. Maximum 1- bits exposed to higher levels experience more se- hour concentrations during cooking may reach vere tissue damage, resembling emphysema. 500–1,900 µg/m3, and 1,000–2,000 µg/m3 where a The available data suggest that the physiologi- gas-fired water heater is also in use. Smoke from cal effects of nitrogen dioxide on humans and ani- one cigarette may contain 150,000–225,000 µg/m3 mals are due more to peak concentrations than to of nitric oxide and somewhat less nitrogen duration or to total dose. dioxide. Materials Health and Environmental Impacts Nitrogen dioxide in reaction to textile dyes can Health cause fading or yellowing of fabrics. Exposure to nitrogen dioxide can also weaken fabrics or reduce Epidemiologic studies have rarely detected effects their affinity for certain dyes. Industry has devoted on children or adults from exposure to outdoor ni- considerable resources to developing textiles and trogen dioxide. One study of nurses in Los Angeles dyes resistant to exposure (Canada found an association between exposure to nitro- 1987). gen dioxide and increased phlegm production (Schwartz and Zegler 1990). Studies have indi- Effects on Ecosystems cated that the use of gas appliances for cooking may have a very small effect on the human respiratory Nitrogen oxides are precursors of both acid pre- system, especially for small children, but that the cipitation and ozone, each of which is blamed for effect (if it exists) disappears as the children grow injury to plants. While nitric acid is responsible for older (WHO 1987). only a smaller part of ion (H+) concen- Available data from animal toxicological experi- tration in wet and dry acid depositions, the contri- ments rarely indicate effects of acute exposure to bution of nitrogen oxide emissions to acid nitrogen dioxide concentrations of less than 1,880 µg/m3 (WHO 1987). Asthmatics are likely to be the deposition could be more significant. It is nitrogen group most sensitive to exposure to nitrogen ox- oxide that absorbs , initiating the photo- ides. Two laboratories have reported reversible ef- chemical processes that produce nitric acid. Ap- fects on pulmonary function of asthmatics proximately 90–95% of the nitrogen oxides emitted exercising intermittently after 30 minutes of expo- from power plants is nitric oxide; this slowly con- sure to nitrogen dioxide concentrations as low as verts to nitrogen dioxide in the presence of ozone. 560 µg/m3 (WHO 1987). However, the health im- The extent and severity of the damage attribut- pact of the change in pulmonary function is un- able to acid depositions is difficult to estimate, clear; the change of about 10% is within the range since impacts vary according to soil type, plant of physiological variation and is not necessarily species, atmospheric conditions, insect popula- adverse. At levels above 3,760 µg/m3, normal sub- tions, and other factors that are not well under- jects have demonstrated substantial changes in pul- stood. in precipitation may actually monary function (WHO 1987). increase forest growth in areas with nitrogen-de- Studies with animals have found that several ficient soils. However, the fertilizing effect of ni- weeks to months of exposure to nitrogen dioxide trates (and sulfates) may be counterbalanced by concentrations less than 1,880 µg/m3 causes both the leaching of , , , reversible and irreversible lung effects and bio- and other nutrients from forest soils. There is little chemical changes. Animals exposed to nitrogen evidence that agricultural crops are being injured dioxide levels as low as 940 µg/m3 for six months by exposures to nitrates in precipitation. The may experience destruction of cilia, alveolar tis- amount of nitrates in rainwater is almost always Nitrogen Oxides 225 well below the levels applied as (NAPAP wavelengths of light are visible to the eye, nitro- 1990). gen dioxide appears yellowish to reddish-brown The most evident damage from acid depositions in color. Nitrogen oxides can also combine with is to freshwater lake and stream ecosystems. Acid photochemical oxidants to form . depositions can lower the pH of the water, with potentially serious consequences for fish, other Ambient Standards and Guidelines animal, and plant life. Lakes in areas with soils containing only small amounts of calcium or mag- The main goal of almost all the major national and nesium carbonates that could help neutralize acidi- international air quality standards and guidelines fied rain are especially at risk. Few fish species can produced over the last two decades has been to survive the sudden shifts in pH (and the effects of protect human health. Some countries have also soluble substances) resulting from atmospheric produced guidelines and standards for nitrogen depositions and runoff of contaminated waters; oxides to protect vegetation and sensitive ecosys- affected lakes may become completely devoid of tems, such as wetlands. Table 1 presents EU, USEPA, fish life. Acidification also decreases the species and WHO reference standards and guidelines for variety and abundance of other animal and plant ambient levels of nitrogen dioxide. life. “Acid pulses” have been associated with the fish kills observed in sensitive watersheds during the spring meltdown of the snowpack. The atmo- Conclusions spheric deposition of nitrogen oxides is a substan- The evidence suggests that exposure to short-term tial source of nutrients that damage estuaries by causing algal blooms and anoxic conditions. peak concentrations of nitrogen dioxide may dam- Emissions of nitrogen oxides are a precursor of age health, especially of sensitive individuals such as asthmatics. For many individuals, the most sig- ground-level ozone (O3), which is potentially a more serious problem. Plant scientists blame tro- nificant sources of repetitive exposure to peak lev- pospheric ozone for 90% of the injury to vegeta- els of nitrogen oxides come from residing in homes tion in North America. Ozone can travel long with gas cooking or heating appliances or from distances from the source and can contribute to cigarette smoking. Long-term exposures to high elevated ozone concentrations even in rural levels of nitrogen dioxide (well above the high- areas. Since the meteorological and climatic con- est ambient levels reported in urban areas in the ditions that favor the production of ozone—abun- United States) has been shown to to devel- dant sunshine—are also good for , opment of chronic lung injury and disease in ozone has the potential to cause large economic animals. However, there is still considerable un- losses from reductions in crop yields. certainty regarding chronic health effects for Nitrogen dioxide affects visibility by absorbing humans from exposure to ambient nitrogen ox- short-wavelength blue light. Since only the longer ides (NAPAP 1991).

Table 1. Reference Standards and Guidelines for Ambient Levels of Nitrogen Dioxide (micrograms per cubic meter) Standard or guideline Annual average 24-hour average 1-hour average

EU limit values (1985) 200a USEPA standards (1992) 100b WHO guidelines (1977) 190–320c WHO guidelines for Europe (1987) 150 400 a. 98th percentile calculated from the mean values per hour or per period of less than an hour taken throughout the year. b. Arithmetic mean. c. Not to be exceeded more than once a month. Only a short-term exposure limit has been suggested. Sources: European Community 1985 (EU); United States 1992, 40 CFR, Part 60; WHO 1977, 1987. 226 PROJECT GUIDELINES: POLLUTANTS

Recommendations more of the emergency trigger values (determined for short-term concentrations of dioxide, In the long term, countries should seek to ensure nitrogen oxides, particulates, and ozone). The rec- that ambient exposure to nitrogen dioxide does ommended emergency trigger value for nitrogen not exceed the WHO recommended guidelines. oxides is 150 µg/m3 for the 24-hour average con- In the interim, countries should set ambient stan- centrations. dards for nitrogen dioxide that take into account the benefits to human health and to sensitive eco- References and Sources systems of reducing exposure to nitrogen diox- ide; the concentration levels achievable by Canada, Federal-Provincial Advisory Committee on pollution prevention and control measures; and Air Quality. 1987. Review of National Ambient Air the costs involved in meeting the standards. In Quality Objectives for Nitrogen Dioxide. Ottawa: En- adopting new ambient air quality standards, vironment Canada. countries should set appropriate phase-in peri- Cooper, C. David, and F. C. Alley. 1986. ods during which districts or municipalities that Control: A Design Approach. Prospect Heights, Ill.: do not meet the new standards are expected and Waveland Press. will be assisted to attain these standards. Where European Community. 1985. Directive 85/203, March there are large differences between the costs and 7. Brussels. benefits of meeting air quality standards, it may be appropriate to establish area-specific ambient Godish, Thad. 1991. Air Quality. Chelsea, Mich.: Lewis standards case by case. Publishers. Prior to carrying out an environmental assess- NAPAP (National Acid Precipitation Assessment Pro- ment (EA), a trigger value for the annual aver- gram). Various years, 1987–91. Washington, D.C.: age concentrations of nitrogen oxides should be Government Printing Office. agreed on by the country and the World Bank Ostro, Bart. 1994. “Estimating the Health Effects of Air Group. Countries may wish to adopt EU, USEPA, Pollutants: A Method with an Application to or WHO guidelines or standards as their trigger Jakarta.” Policy Research Working Paper 1301. values. The trigger value should be equal to or World Bank, Policy Research Department, Washing- lower than the country’s ambient standard. The ton, D.C. trigger value is not an ambient air quality stan- Schwartz, Joel, and S. Zegler. 1990. “Passive Smoking, dard but simply a threshold. If, as a result of the Air Pollution and Acute Respiratory Symptoms in project, the trigger value is predicted to be ex- a Diary Study of Student Nurses.” American Review ceeded in the area affected by the project, the EA of Respiratory Disease 141: 62–67. should seek mitigation alternatives on a regional or sectoral basis. The World Bank Group will clas- United States. CFR (Code of Federal Regulations). Wash- sify airsheds as moderately degraded if concen- ington, D.C.: Government Printing Office. tration levels are above 100 µg/m3 annual average USEPA (United States Environmental Protection or if the 98th percentile of 24-hour mean values Agency). 1990. National Air Quality and Emission over a period of one year is estimated to exceed Trends Report, 1990. EPA-450/4-91-023. Research Triangle Park, N.C. 150 µg/m3 of nitrogen oxides. Airsheds will be clas- sified as having poor air quality with respect to WHO (World Health Organization). 1977. “Oxides nitrogen dioxide if the 95th percentile of 24-hour of Nitrogen.” Environmental Health Criteria 4. mean values of nitrogen dioxide for the airshed Geneva. over a period of one year is estimated to exceed ————. 1987. Air Quality Guidelines for Europe. 150 µg/m3. WHO Regional Publications, European Series Good practice in airshed management should 23. Copenhagen: WHO Regional Office for Eu- encompass the establishment of an emergency rope. response plan during industrial plant operation. World Resources Institute. 1994. World Resources It is recommended that this plan be put into ef- 1994–95: A Guide to the Global Environment. New fect when levels of air pollution exceed one or York: Oxford University Press.