And Now the Rest of the Story

On September 23, 2002 the San Francisco Chronicle printed a story summarizing the report, Code Red: America's Five Most Polluted National Parks, prepared by environmental groups who described The Great Smoky Mountains as the nation's most polluted national park, with air quality rivaling that of Los Angeles. Using park service data, the groups rated the Smokies as the most polluted park in the country.

The report used a plant ozone-exposure index of 60 parts per billion for comparison purposes. Ozone exposure numbers were computed by adding the hourly average concentrations for all hours experiencing 60 parts per billion or greater for the months of April through October. According to the study, the Smokies' average annual ozone exposure of 133,200 ppb-hours exceeded that experienced in Atlanta, Knoxville, Tenn., and Charlotte, N.C. Only one city in the study's analysis, Los Angeles with more than 180,000 ppb-hours, exceeded the Smokies. Over the 11-year period (1991-2001), the authors of the report stated that the Great Smoky Mountains National Park experienced two times the ozone exposure than in Knoxville, Tennessee and Atlanta, Georgia.

Figure 6 from the report is illustrated below. The figure compares the average annual ozone exposure over the 11-year period for 4 of the national parks with major metropolitan areas. The report stated that the Great Smoky Mountains, Sequoia-Kings Canyon, Yosemite, and Shenandoah National Parks experienced greater ozone exposures than Charlotte, Atlanta, Knoxville, Washington D.C., New York City, Houston, Chicago, and Denver.

Figure 7 below, from the report, describes that the average annual ozone exposure was higher at Great Smoky Mountains National Park for 1997-2001 than at Los Angeles, California. Thus, the report, Code Red: America's Five Most Polluted National Parks, gives one the impression that the ozone exposures experienced at the Great Smoky Mountains National Park for the period 1997-2001 were higher than Los Angeles and Houston, the two cities in the United States that are competing for the highest national ozone exposures.

And now, as Paul Harvey used to say, it is time to tell the Rest of the Story, the one that deals with the science behind vegetation effects assessments. Based on scientific evidence, we shall see that the Great Smoky Mountains National Park actually does not experience ozone levels close to the values experienced by Los Angeles.

In December 2000, the Federal Land Manager's Air Quality Related Values Workgroup (FLAG) Phase I Report was published. The authors of the report were the U.S. Forest Service, National Park Service, and the U.S. Fish and Wildlife Service. FLAG was formed to develop a more consistent approach for the Federal Land Managers to evaluate air pollution effects on their resources. Of particular importance was the New Source Review program, especially in the review of Prevention of Significant Deterioration of air quality permit applications. The goals of FLAG were to provide consistent policies and processes both for identifying air quality related values (AQRVs) and for evaluating the effects of air pollution on AQRVs, primarily those in Federal Class I air quality areas, but in some instances, in Class II areas.

For protecting vegetation from ozone exposure, FLAG selected a cumulative exposure index, similar to the one described in the report, Code Red: America's Five Most Polluted National Parks. However, based on years of research published in the peer-reviewed literature, FLAG recognized the importance of the potential for the higher hourly average ozone concentrations (i.e., greater than or equal to 100 ppb) to affect vegetation more than the mid-level (i.e., 60 - 90 ppb) and lower values (below 60 ppb). FLAG recommended that both the cumulative exposure index and the number of hours greater than or equal to 100 ppb be coupled together. Accounting for peak concentrations provided FLAG with important information regarding the timing of events and helped determine if a response was due to chronic or acute ozone exposure. Also, as pointed out by FLAG, much of the quantitative exposure/response information used by FLAG for the determination of critical exposure ozone levels was generated from experimentation based on fumigation treatments containing numerous occurrences of high hourly average concentrations. Thus, FLAG concluded that the concern about vegetation effects caused by ozone was linked with both a high cumulative value and a large number of hours greater than or equal to 100 ppb (N100).

Unfortunately the authors of the report, Code Red: America's Five Most Polluted National Parks, did not couple the N100 exposure index with the cumulative index they characterized. Given the fact that the National Park Service, the U.S. Forest Service, and the Fish and Wildlife Service recommended the use of the cumulative ozone exposure index similar to the one used in the Code Red: America's Five Most Polluted National Parks report AND the number of hours equal to and greater than 100 ppb (N100), it is important to re-evaluate the ozone data for the Great Smoky Mountains National Park and Los Angeles based on the FLAG recommendations. The figure below illustrates the use of the cumulative ozone exposure index that is identical to the one used in the Code Red: America's Five Most Polluted National Park report. The actual name for the cumulative exposure index is SUM06.

The figure shows that the SUM06 values for Los Angeles were much higher in the earlier years than the values experienced in the more recent years. California officials, as well as the EPA, recognize that the Los Angeles area has decreased its ozone levels considerably compared to the 1970s and 1980s. As pointed out in the report, Great Smoky Mountains National Park experienced SUM06 values similar in magnitude to those in Los Angeles for the period 1997-2001. If one considers only the SUM06 exposure index, the figure above tends to support the conclusions in the report. However, the figure below tells the Rest of the Story.

The figure shows that the number of hourly average ozone concentrations for each year in Los Angeles greater than or equal to 100 ppb (N100) is in the range 221 - 956. The range of the number of hourly average concentrations greater than or equal to 100 ppb for the Great Smoky Mountains National Park is in the range 1 - 107. Although the SUM06 cumulative exposure values, as shown in the previous figure above, are similar for both locations in the most recent years, the Los Angeles site experiences as much as 956 times more hourly average concentrations greater than or equal to 100 ppb than the Great Smoky Mountains National Park site. In other words, there are far more hourly average peak ozone concentrations occurring in Los Angeles than in the Great Smoky Mountains National Park. Thus, FLAG's concern that one might overestimate possible vegetation effects if one uses only the cumulative exposure index instead of considering both the cumulative exposure index and N100 values, has been demonstrated in the report, Code Red: America's Five Most Polluted National Park.

Should we be concerned about air pollution in our national parks? The answer is definitely "YES". However, in order for our nation's policymakers to make the best decisions concerning our national parks that are in the interests of our citizens, the public, politicians, and government officials must be exposed to the best science available without simple sound bites. The report, Code Red: America's Five Most Polluted National Park, raises concerns about air pollution in our national parks. However, the misinterpretation of scientific data does not help those of us who work hard to protect our valuable natural resources.

Dr. Allen S. Lefohn is an active research scientist who focuses on developing exposure- and dose-response relationships for predicting the effects of air pollution on vegetation and human health. He was a co-author of the exposure-response section of the EPA's Ozone Criteria Document in 1996 and authored several sections of the EPA's 2006 Ozone Criteria Document. His cumulative ozone exposure index, the W126, was adopted by FLAG as one of its two ozone exposure indices for protecting vegetation and the W126 index was proposed by EPA Clean Air Scientific Advisory Committee (CASAC) and EPA Staff to the EPA Administrator as the form for the secondary ozone standard to protect vegetation. In 2008, based on advice from the White House (Washington Post, April 8, 2008; Page D02), the EPA Administrator decided to make the secondary ozone standard the same as the primary 8-hour average standard. However, On September 16, 2009, the EPA announced it would reconsider the 2008 national ambient air quality standards (NAAQS) for ground-level ozone for both human health and environmental effects. The Agency planned to propose any needed revisions to the ozone standards by December 2009 and issue a final decision by August 2010. On January 7, 2010, the EPA announced on its web site its proposal to strengthen the national ambient air quality standards for ground-level ozone. The EPA's proposal decreased the 8-hour “primary” ozone standard level, designed to protect public health, to a level within the range of 0.060-0.070 parts per million (ppm). EPA proposed to establish a distinct cumulative, seasonal “secondary” standard, referred to as the W126 index, which was designed to protect sensitive vegetation and ecosystems, including forests, parks, wildlife refuges, and wilderness areas. EPA proposed to set the level of the W126 secondary standard within the range of 7-15 ppm-hours. The proposed revisions resulted from a reconsideration of the identical primary and secondary ozone standards set at 0.075 ppm in March 2008. On August 20, the Agency announced that it would delay its final announcement to on or around the end of October. In early November, the EPA announced that it would reach a final decision on the ozone standards by December 31, 2010. On December 8, the EPA announced that it would delay its final decision on the ozone standards until July 2011. EPA announced on July 26 that it would not make a decision on the ozone standards by its previously announced deadline of July 29. On September 2, 2011, President Obama requested that the EPA withdraw its proposed revised ozone standards.

On November 26, 2014, the EPA Administrator announced that she was proposing an ozone human health (primary) standard in the range of 65 to 70 ppb and would take comment on a standard as low as 60 ppb. For the welfare (secondary) ozone standard, she proposed that the standard be the same as the health standard if the final health standard were set in the range of 65 to 70 ppb. The Administrator believed that a health standard in this range would protect vegetation from ozone exposures of W126 values within the range of 13-17 ppm-h. She also took comment on setting a W126 value in the range of 7-13 ppm-h, which implied that she was still considering establishing a secondary standard separate in form from the human health 8-h standard. In August 2014, the EPA Staff recommended to the Administrator that she select the ozone primary standard at a specific level between 60-to-70-parts-per-billion. For the secondary standard, the EPA Staff recommended that the Administrator establish a 3-month, 12-h W126 secondary standard, which would have a specific value within the range of 7 to 17 ppm-h. In October 2015, the Administrator concluded that protection of vegetation from adverse effects could be provided by an 8-h O3 standard of 70 ppb that restricted cumulative 3-month seasonal W126 exposures to 17 ppm-hrs or lower. Follwing a review of the 2015 ozone standards, the Administrator on December 23, 2020 made the decision that both the human health and vegetation ozone standards would remain at the current levels established in 2015.

In its December 2020 decision, the Administrator noted that it was important to address the exposure regime patterns used in the crop and forest seedling experiments. The Administrator noted that the vegetation experimental exposure protocols used to introduce enhanced ozone concentrations into the chambers resulted in numerous hourly average concentrations greater than or equal to 100 ppb for some of the crops and tree seedling species (but not all) in the experiments. While frequent occurrences under ambient conditions of hourly average concentrations greater than or equal to 100 ppb were prevelant in the 1980s and 1990s, this is not the case today. Thus, some of the exposure regimes used in the NCLAN and forest seedling experiments did not match the ambient exposure regimes currently experienced in the United States. Lefohn and Foley (1992) and Lefohn et al. (1997) noted the frequent occurrences of hourly average concentrations greater than or equal to 100 ppb in the NCLAN and forest seedling experiments and suggested that an additional exposure metric that described the number of hourly average concentrations greater than or equal to 100 ppb (i.e., N100 index) be coupled with the W126 metric for assessing vegetation effects if the exposure-response relationships were based on some of the NCLAN and forest seedling experiments performed in the 1980s and 1990s. Lee et al. (2022) reported in their analysis of tree seedling data that the most sensitive species in their analysis experienced a biomass loss of 5% at a W126 of 2.5–9.2 ppm-hrs and that the N100 values ranged from 0 to 7 at those exposures. The requirement for an N100 index has been discussed in Musselman et al. (2006) and Davis and Orendovici (2006). Additional discussion of the N100 metric coupled with the W126 index is provided by clicking here.

Following the December 2020 decision, on October 29, 2021, the Agency announced it would reconsider the 2020 O3 NAAQS final action. During the reconsideration process, CASAC recommended to the Administrator that the form of the secondary standard should be changed to the cumulative W126 exposure metric, an index recommended by several previous CASAC ozone panels, as well as at times by the EPA, to protect vegetation. CASAC recommended in 2023 that the Administrator consider that the level of the W126 metric be in the range of 7 to 9 ppm-hrs. Upon considering the CASAC recommendations for the human health and vegetation ozone standards as part of the reconsideration process, in August 2023 the EPA decided to initiate a new review of the ozone NAAQS, which meant that the entire ozone rulemaking process would begin once again. The current 70 ppb 8-h O3 standard promulgated in the US EPA's 2015 decision (Federal Register, 2015) serves as a surrogate to achieve O3 levels at or below a W126 value of 17 ppm-hrs, which is above the range of W126 values of 7 to 9 ppm-hrs recommended by CASAC. Dr. Lefohn is a world authority on the subjects of exposure- and dose-response of air pollution effects and the setting of air pollution standards. Please see a list of his publications for further information.

 

References

Davis, D. D.; Orendovici, T. (2006). Incidence of ozone symptoms on vegetation within a National Wildlife Refuge in New Jersey, USA. Environmental Pollution. 143:555-564.

Federal Register, National Ambient Air Quality Standards for Ozone (2015). 40 CFR Part 50, 51, 52, 53, and 58, pp 65292-65468.

Lee, E.H., Andersen, C.P., Beedlow, P.A., Tingey, D.T., Koike, S., Dubois, J.-J., Kaylor, S.D., Novak, K., Rice, R.B., Neufeld, H.S., Herrick, J.D. (2022). Ozone exposure-response relationships parametrized for sixteen tree species with varying sensitivity in the United States, Atmospheric Environment (2022), doi: https://doi.org/10.1016/j.atmosenv.2022.119191.

Lefohn, A. S.; Foley, J. K. (1992) NCLAN results and their application to the standard-setting process: protecting vegetation from surface ozone exposures. J. Air Waste Manage. Assoc. 42: 1046-1052.

Lefohn, A.S.; Jackson, W.; Shadwick, D.S.; Knudsen, H.P. (1997) Effect of surface ozone exposures on vegetation grown in the southern Appalachian Mountains: Identification of possible areas of concern. Atmospheric Environment 31(11): 1695-1708.

Musselman R. C.; Lefohn A. S.; Massman W. J.; Heath, R. L. (2006) A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.

Copyright © 1995-2025 A.S.L. & Associates. All rights reserved.