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Home , NOx, Ozone

Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998

Ground-Level

Ozone (O3) is a colorless, reactive oxidant gas that is plants may account for as much as two thirds of a major constituent of atmospheric . Ground- ambient VOCs in some locations (USEPA 1986). level ozone is formed in the air by the photochemi- Anaerobic biological processes, , and vol- cal reaction of and (NOx), canic activity are the main natural contributors facilitated by a variety of volatile organic com- to atmospheric NOx, occasionally accounting for pounds (VOCs), which are photochemically reactive as much as 90% of all NOx emissions (Godish . The relative importance of the vari- 1991). ous VOCs in the oxidation process depends on their Motor vehicles are the main anthropogenic chemical structure and reactivity. Ozone may be sources of ground-level ozone precursors. Other formed by the reaction of NO and VOCs under the x anthropogenic sources of VOCs include emissions influence of sunlight hundreds of kilometers from the source of emissions. from the chemical and petroleum industries and Ozone are influenced by the from organic solvents in small stationary sources intensity of solar radiation, the absolute concen- such as dry cleaners. Significant amounts of NOx originate from the of fossil fuels in trations of NOx and VOCs, and the ratio of NOx and VOCs. Diurnal and seasonal variations oc- power plants, industrial processes, and home cur in response to changes in sunlight. In addi- heaters. tion, ground-level ozone accumulation occurs when sea breezes cause circulation of air over an Health Impacts of Exposure area or when temperature-induced air inversions trap the compounds that produce smog (Chilton The main health concern of exposure to ambient and Sholtz 1989). Peak ground-level ozone con- ground-level ozone is its effect on the respiratory centrations are measured in the afternoon. Mean system, especially on lung function. Several fac- concentrations are generally highest during the tors influence these health impacts, including the summer. Peak concentrations of ground-level concentrations of ground-level ozone in the at- ozone rarely last longer than two to three hours mosphere, the duration of exposure, average vol- (WHO 1979). ume of air breathed per minute (ventilation rate), Registered average natural background concen- and the length of intervals between short-term ex- trations of ground-level ozone are around 30–100 posures. micrograms per cubic meter (µg/m3). Short-term Most of the evidence on the health impacts of (one-hour) mean ambient concentrations in urban ground-level ozone comes from animal studies and areas may exceed 300–800 µg/m3 (WHO 1979). controlled clinical studies of humans focusing on Main Sources short-term acute exposure. Clinical studies have docu- mented an association between short-term exposure Both natural and anthropogenic sources contrib- to ground-level ozone at concentrations of 200–500 ute to the emission of ground-level ozone pre- µg/m3 and mild temporary eye and respiratory irrita- cursors, and the composition of emissions sources tion as indicated by symptoms such as cough, throat may show large variations across locations. VOCs dryness, eye and chest discomfort, thoracic pain, and occurring naturally due to emissions from trees and headache (WHO 1979, 1987). Temporary decrements

227 228 PROJECT GUIDELINES: in pulmonary function have been found in children health impacts, since prolonged acute exposure at hourly average ground-level ozone concentra- to ground level ozone concentrations of 240– tions of 160–300 µg/m3. Similar impacts were ob- 360 µg/m3 resulted in progressively larger served after 2.5-hour exposure of heavily changes in respiratory function. However, a exercising adults and children to concentrations cross-sectional analysis based on large samples of 240 µg/m3 (WHO 1987). Lung function losses, from multiple locations in the United States however, have been reversible and relatively mild (Schwartz 1989) found no correlation between even at concentrations of 360 µg/m3, with a great chronic ground-level ozone pollution and re- variety of personal responses (Chilton and Sholtz duced lung function except for the highest 20% 1989). Full recovery of respiratory functions nor- of ground-level ozone exposures, suggesting the mally occurs within 24 to 48 hours after expo- possibility of a lower threshold for effects of sure (WHO 1987). chronic ground-level ozone exposure. No evi- Animal studies have also demonstrated an dence has been found of an association between inflammatory response of the respiratory tract peak oxidant concentrations and daily mortal- following exposure to ground-level ozone at ity rates of the general population (WHO 1979). 1,000 µg/m3 for four hours (WHO 1987). Although biochemical and morphological alterations in the Other Impacts red blood cells were found in several animal spe- cies after exposure to ground-level ozone concen- Elevated ground-level ozone exposures affect trations of 400 µg/m3 for four hours (WHO 1987), agricultural crops and trees, especially slow- no consistent changes have been demonstrated in growing crops and long-lived trees. Ozone dam- humans, even at concentrations as high as 1,200 ages the leaves and needles of sensitive plants, µg/m3 (USEPA 1986), and extrapolation of such causing visible alterations such as defoliation impacts to humans has not been supported. and change of leaf color. In North America, tro- Exposure to elevated concentrations of ground- pospheric ozone is blamed for about 90% of the level ozone has been shown to reduce physical per- damage to plants. Agricultural crops show re- formance, since the increased ventilation rate duced plant growth and decreased yield. during physical exercise increases the effects of According to the U.S. Office of Technology exposure to ground-level ozone. There is no evi- Assessment (OTA 1988), a 120 µg/m3 sea- dence that smokers, children, older people, asth- sonal average of seven-hour mean ground- matics, or individuals with chronic obstructive level ozone concentrations is likely to lung disease are more responsive to ground-level to reductions in crop yields in the range of ozone exposure than others. Ground-level ozone 16–35% for cotton, 0.9–51% for wheat, 5.3– may, however, make the respiratory airways more 24% for soybeans, and 0.3–5.1% for corn. In responsive to other inhaled toxic substances and addition to physiological damage, ground- bacteria. In addition, a synergistic effect of ground- level ozone may cause reduced resistance level ozone and dioxide has been found, in- to fungi, bacteria, viruses, and insects, dicating that potentiates the effects reducing growth and inhibiting yield and of ground level ozone (WHO 1979). reproduction. These impacts on sensitive Besides short-term impacts, the potential for ir- species may result in declines in agricul- reversible damage to the lungs from repeated tural crop quality and the reduction of exposure over a longer period of time has been a biodiversity in natural ecosystems. health concern. Some studies have found an as- The impact of the exposure of plants to sociation between accelerated loss of lung func- ground-level ozone depends not only on the tion over a longer period of time (five years) and duration and of exposure but high oxidant levels in the atmosphere (Detels et also on its frequency, the interval between al. 1987). WHO (1987) pointed out that the length exposures, the time of day and the season, of the recovery period between successive epi- site-specific conditions, and the develop- sodes of high ground-level ozone concentrations mental stage of plants. Furthermore, and the number of episodes in a season may be ground-level ozone is part of a complex re- important factors in the nature and magnitude of lationship among several air pollutants Ground-Level Ozone 229 and other factors such as climatic and meteoro- formation of ground-level ozone, differences in the logical conditions and nutrient balances. Accord- sensitivity and response of affected receptors, and ing to some studies, for example, the presence of variations in the costs and benefits of achieving sulfur dioxide may increase the sensitivity of certain air quality requirements for ground-level plants to leaf injury by ground-level ozone (WHO ozone may call for area-specific guide values. 1987). Reinert and Heck (1982) point out that the Since ground-level ozone is formed by the pho- presence of ground-level ozone may increase the tochemical reaction of nitrogen oxides and certain growth-suppressing effects of . hydrocarbons, abatement strategies should focus Ambient Standards and Guidelines not only on reduction of emissions of these sub- stances but also on their ratio and balance. In ar-

Ambient standards and guidelines for ground- eas where NOx concentrations are high relative to level ozone are aimed at protecting human health, VOCs, the abatement of VOC emissions can reduce sensitive ecosystems, and agricultural plants the formation of ground-level ozone, while reduc- from the harmful effects of ground-level ozone. tion in nitrogen oxides may actually increase it. In Table 1 presents USEPA, , and WHO areas where the relative concentration of VOCs is reference standards and guidelines for ambient high compared with nitrogen oxides, ground-level ground-level ozone concentrations. Due to un- ozone formation is “NO -limited,” and NO reduc- certainty about chronic effects and the lack of x x tions work better than VOC abatement (OTA 1989). established dose-response function data, these standards and guidelines focus on short-term ground-level ozone concentrations. Recommendations

Conclusions In the long term, countries should seek to ensure that ambient exposure to ground-level ozone does Evidence suggests that exposure to short-term not exceed the guidelines recommended by WHO peak concentrations of ground-level ozone dam- (see Table 1). In the interim, countries should set ages human health but that these impacts are rela- ambient standards for ground-level ozone that tively mild and reversible at ground-level ozone take into account the benefits to human health and levels exceeding current U.S. and WHO stan- to sensitive ecosystems of reducing exposure to dards and guidelines. Although repeated expo- ground-level ozone; the concentration levels sure to peak concentrations may result in achievable by and control cumulative impacts on lung function, inhibit- measures; and the costs involved in meeting the ing recovery, no clear evidence for such chronic effects of ground-level ozone exists. The large standards. In adopting new ambient air quality variety of sources and factors contributing to the standards, countries should set appropriate - in periods during which districts or municipalities Table 1. Reference Standards and Guidelines for that do not meet the new standards are expected Ambient Atmospheric Ozone Concentrations and will be assisted to attain these standards. (micrograms per cubic meter) Where there are large differences in either the costs Short-term Medium-term or the benefits of meeting air quality standards, it (1 hour) (8 hour) may be appropriate to establish area-specific am- Standard or guideline average average bient standards case by case.

USEPA 2,35a Prior to carrying out an environmental assess- State of California 1,80a ment (EA), a trigger value for annual average con- WHO (1979) 100–200 centrations of ground-level ozone should be agreed WHO guidelines for on by the country and the World Bank. Countries Europe (1987) 150–200 100–120 may wish to adopt EU, USEPA, or WHO a. Value not to be exceeded more than once a year. guidelines or standards as their trigger values. Sources: USEPA 1986; WHO 1979, 1987. The trigger value should be equal to or 230 PROJECT GUIDELINES: POLLUTANTS

lower than the country’s ambient standard. The Godish, Thad. 1991. Air Quality. Chelsea, Mich.: Lewis trigger value is not an ambient air quality stan- Publishers. dard but simply a threshold. If, as a result of the Krupnik, A. J., et al. 1989. Ambient Ozone and Acute project, the trigger value is predicted to be exceeded Health Effects: Evidence from Daily Data. Washington, in the area affected by the project, the EA should D.C.: Resources for the Future. seek mitigation alternatives on a regional or OTA (U.S. Congress, Office of Technology Assess- sectoral basis. ment). 1988. Urban Ozone and the Clean Air Act: Prob- In addition, good practice in airshed manage- lems and Proposals for Change. Washington, D.C.: ment should encompass the establishment of Government Printing Office. an emergency response plan during industrial ————. 1989. Catching Our Breath: Next Steps for Re- plant operation. It is recommended that this ducing Urban Ozone. OTA-0-412. Washington, D.C.: plan be put into effect when levels of air pollu- Government Printing Office. tion exceed one or more of the emergency Reinert, R. A., and W. W. Heck. 1982. “Effects of Nitro- trigger values determined for short-term concen- gen Dioxide in Combination with Sulfur Dioxide trations of sulfur dioxide, nitrogen oxides, par- and Ozone on Selected Crops.” In T. Schneider and ticulates, and ground-level ozone. The L. Grant, eds., by Nitrogen Oxides. recommended emergency trigger value for Amsterdam: Elsevier Scientific. ground-level ozone is 150 µg/m3 for one-hour av- Schwartz, Joel. 1989. “Lung Function and Chronic Ex- erage concentrations. posure to Air Pollution: A Cross-Sectional Analysis of NHANES II.” Environmental Research 50: 309–21. References and Sources USEPA (United States Environmental Protection Agency). 1986. Air Quality Criteria for Ozone and Chilton, K., and A. Sholtz. 1989. Battling Smog: A Plan Other Photochemical Oxidants. Washington, D.C. for Action. St. Louis, Mo.: Washington University, WHO (World Health Organization). 1979. “Photo- Center for the Study of American Business. chemical Oxidants.” Environmental Health Criteria 7. Detels, R., et al. 1987. “The UCLA Population Studies Geneva. of Chronic Obstructive .” Chest ————. 1987. Air Quality Guidelines for Europe. 92 (October): 594–603. Copenhagen: WHO Regional Office for Europe.