FEATURE
Science, Uncertainty, and EPA's New Ozone Standards There are still gray areas in the science about how ozone harms human health and vegetation. ALLEN S. LEFOHN n July 19, EPA is scheduled to make Its final decision on the promulgation of new National Ambient Air Quality Standards (NAAQS) for ozone (03) and particulate matter. As a result of an accelerated review schedule caused by an American Lung Association lawsuit, last December EPA formally announced revisions to the two standards. New forms of the primary (human health) and secondary (vegetation) standards for 03 were proposed. There has been considerable debate in the media, the federal government, and Congress about the uncertainty associated with the scientific database and the attainability of the proposed standards. The experiments that form the basis for the proposed 0 3 standards are not perfect. But some people now believe it is possible to extract from human health and vegetation experiments the key ingredients that can be used to construct the most appropriate form and level of standards to protect the public's health and welfare. 0 3 and related chemicals have long been recognized as pollutants that affect public health and vegetation (i). Children and other sensitive people have experienced a wide range of 03-induced health effects, including decreased lung function, increased respiratory symptoms, hospital admissions and emergency room visits for respiratory causes, and inflammations of the lung. During EPA's review of the standard, originally formulated in the 1970s, the agency found that several key experiments published in the late 1980s and early 1990s showed that individuals exposed to extended daily periods of 0 3 at levels below the current 1-hour (h) standard exhibited health effects. EPA believed it was necessary to change the form of the primary standard because additional protection, not offered by the existing 1-h standard, was needed for children and other at-risk populations (2). Thus, EPA proposed that the form of the primary standard be changed from the current 1-h daily maximum to a
daily maximum 8-h average concentration at a level of 0.08 parts per million (ppm). EPA proposed taking die third highest 8-h daily maximum concentration for each of 3 years and then taking the average of these 3 concentrations. Past experience with the current 1-h, 0.12-ppm primary and secondary standards has shown that geographic areas go in and out of compliance because of natural meteorological variations. For example, for the period 1993-95, there were 46 areas in the United States that violated the 0 3 standard (EPA, public announcement); for the period 1992-94, there were only 33. The reason is that 1995 was a hot, dry year in which meteorology apparently played an important role in the increased number of violating areas. Because 0 3 is phytotoxic to plant species and can produce acute foliar injuries, reduced crop yield and biomass production, and shifts in competitive advantages of vegetation species in mixed populations, EPA wanted to establish a secondary standard to protect vegetation (2). EPA believed that the current 1-h 0 3 standard was not sufficiently protective of crops and forests because 03 effects are cumulative and not necessarily related to the onetime maximum peak, on which the current 1-h standard focuses (2). Two alternative forms of a secondary standard have been proposed. One would be based on a level identical to the proposed primary 8-h standard and the other on a new seasonal accumulating-type standard Although EPA devoted considerable time and effort to reviewing and summarizing the relevant science concerning human health effects and vegetation in the peer-reviewed literature, there are still areas of uncertainty associated with the data that form the scientific basis of the recommendations for bodi standards.
2 8 0 A • VOL. 3 1 , NO. 6, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
0013-936X/97/0931-280A$14.00/0 © 1997 American Chemical Society
O
Real-world versus controlled exposures One of the important areas of uncertainty is how closely controlled experiments performed in the lab-
The consequences of ozone exposure to human health were determined in a range of studies, including some that involved having volunteers exercise while breathing in ozone-enriched air. (Courtesy Larry Folinsbee)
oratory mimic exposures in the real world. Controlled human exposure studies constitute the core of human health effects evidence that supports the proposal for an 8-h average standard. Folinsbee and co-workers (3), Horstman and co-workers (4), and McDonnell and co-workers (5) found that exposing healthy individuals during 6.6 h of exercise resulted in significant lung function decrements. These three key experiments indicated that effects of 0 3 can accumulate over many hours and that, during exercise, some individuals experience decreased lung function and symptoms at concentrations below the current level of the standard. The studies, which form the basis for the selection of the 8-h time frame, applied constant concentrations. Analyses of actual ambient air quality data do not show prolonged constant concentrations, but rather widely varying concentrations. Experimental findings appear to indicate that higher hourly average concentrations should be given greater weight than the lower values when assessing both human health and vegetation effects. For human health effects, M. J. Hazucha and colleagues (6, 7) reported that over an 8-h period, the exposures that contained high concentration values elicited a greater biological effect than the same 0 3 dose at constant concentrations. Hazucha and co-workers implied that two different exposures exhibiting different combinations of in-
dividual hourly values but the same 8-h average might possibly produce significantly different human health effects. The magnitude of this uncertainty has not been examined. Although the controlled human chamber studies formed the major input for the form and level of the 0 3 primary standard, research results from animal, field ambient exposure, and hospital admission studies were also used (8). The main use of the animal studies was to better understand the mechanisms by which 0 3 produces biological responses and damage to the respiratory system. The field investigations consisted of summer camp and adult exercise studies. These studies showed a small but statistically significant relationship between decreased performance on the lung function tests and increasing 0 3 at concentrations at or below the current standard. The hospital admission studies examined the relationship between hourly daily O maximum concentrations and daily hospital admissions for respiratory causes The studies report a linear association in various North American locations between 0 and hospital admissions which EPA assumes to be a cause-and-effect relationship (8) Because there was no threshold concentration for the onset of biological responses caused by 0 3 exposure above background concentrations, EPA's Clean Air Scientific Advisory Committee (CASAC) thought VOL. 3 1 , NO. 6, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 8 1 A
Vegetation studies such as the ones conducted at Twin Creeks Natural Resources Center in Great Smoky Mountains National Park, Tenn., have been important in the development of a secondary ozone standard. EPA, the National Park Service, and Appalachian State University (Boone, N.C.) conducted a six-year study on native plant species to determine the exposure-response relationships for foliar injury and growth for native plants such as yellow poplar, sassafras, sweetgum, and dwarf dandelion. (Courtesy Jim Renfro, National Park Service)
that a human health risk assessment had to play a central role in identifying an appropriate level (9). The objective of the assessment was to estimate the magnitude of risk to population groups believed to be at greatest risk because of increased exposure or increased susceptibility. However, attempting to assess risk led EPA into another area of uncertainty— natural background levels. In its risk assessment, EPA combined information derived from the effects data with exposure information. As part of this analysis, EPA had to extrapolate the chamber study data below the lowest concentration level (0.08 ppm) used in the experiments. Exposure-response relationships from 0.08 ppm to an assumed background level for 0 3 were estimated. EPA defined the assumed background 0 3 as the concentrations that would be observed in the absence of anthropogenic and biogenic emissions of volatile organic compounds and nitrogen oxides in North America (2). EPA assumed an 0 3 background level of 0.04 ppm for the 8-h daily maximum concentration and used this value in its risk assessment Variable background levels The choice of the background level, according to EPA officials, is critical to die risk assessment analysis (10). If the actual background 0 3 levels at the cleanest sites 2 8 2 A • V O L . 3 1 , NO. 6, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
in the United States are above 0.04 ppm, men health effects are overestimated in EPA's risk assessment. During CASAC's deliberations, it was pointed out that the cleanest sites in the United States identified by EPA {11) experrenced a wide range of 8-h daiiy maximum concentrations. For example, the coastal sites in California showed 8-h daily maximum values in me 0.045 ppm range. However, at die inland sites, for a specific year, the third highest 8-h daily maximum concentration at Theodore Roosevelt National Park, N.D., ranged from 0.055 to 0.062 ppm. At Yellowstone National Park, Wyo., it ranged from 0.054 to 0.068 ppm. A discussion held in November 1996 at an Ozone/ Particulate Matter/Regional Haze Implementation Subcommittee meeting, operated under the Federal Advisory Committee Act process, focused on the third highest 8-h daily maximum concentration at die cleanest sites in the United States. An analysis showed that for six clean sites in the United States, die 8-h daily maximum concentration, averaged over 3 years, ranged from 0.045 to 0.061 ppm. In estimating the magnitude of the possible overestimate in EPA's risk assessment, Whitfield and Richmond (10) presented data for lung function decrements > 15%. Their estimates suggest that if the background level were closer to 0.06 ppm which is indicated for several of the cleanest sites in the United States the human health risk assessments performed bv EPA for specific cities could be overestimated bv 10-37% Also uncertain is the efficacy of die control strategies in reducing 0 3 levels. EPA's proposed 8-h average concentration standard cannot distinguish among different combinations of hourly average values within a period of time. A violation could occur with a combination of high and low hourly average concentrations or a range of only mid-level values. Whereas control strategies for the current 1-h standard focus on reducing peak hourly values, controls for the 8-h standard may have to focus on reducing mid-level concentrations. Industry sources point out that die key monitoring sites in almost 52% of the areas tiiat would violate die 8-h proposed standard have 4 or more of the 8 h in the mid-range (i.e., < 0.090 ppm) for the third highest 8-h daily maximum concentration for a specific year. At an Air & Waste Management Association meeting in San Francisco this past January, the potential difficulty for die reduction of mid-range concentrations was discussed. It appears that, at some sites, higher hourly average concentrations be reduced faster than mid-range values. Standard needed to protect vegetation Complementing its desire to tighten up its current human health standard, EPA also recognized that growth of vegetation was hindered at concentrations below the present standard of 0.12 ppm (i). EPA agreed that a secondary, more stringent standard was necessary to protect vegetation. EPA proposed a cumulative type of standard, witii a threshold of 0.06 ppm, to protect vegetation. The summary statistic, referred to as SUM06, is the maximum 3-month, 12-h value for each year, expressed in units of parts-per-million multiplied by hours of exposure (ppm-h). This measurement is a result of
the accumulation of hourly average concentrations. The SUM06 statistic is calculated by summing all hourly average concentrations > 0.06 ppm over the daily 8:00 a.m. to 8:00 p.m. period for repeated 3-month periods (e.g., April-June, May-July, etc.) over 3 years. The secondary 0 3 standard is achieved when the maximum concentration 3-month SUM06 value at an 0 3 air quality monitoring site is < 25 ppm-h. The maximum 3-month value will vary from site to site; EPA will focus on the maximum consecutive 3-month SUM06 value that is experienced at each site and judge the attainability of the secondary standard on the basis of the maximum value. The SUM06 index is different in form from the proposed 8-h primary standard. The index uses a threshold of 0.06 ppm and accumulates over time; it does not average over the period of interest. The scientific basis for considering this form for the vegetation standard is derived from work performed in the early 1980s. In 1983, Musselman {12) )eporred on the importance of the higher hourly average concentrations in comparison to the mid- and lower level values. These results were similar to the findings reported 7 years later for human health effects by Hazucha and co-workers {6, 7). .n the mid-1980s, additional studies were performed to verify these findings, and O effects were reported to be cumulative Qygj. a growth season (e g April October) {11) To develop its proposal for the form and level of the secondary standard, EPA depended heavily on peer-reviewed published data from a series of National Crop Loss Assessment Network (NCLAN) vegetation experiments. In these studies, crops were grown under farm conditions and exposed in opentop chambers to ambient and above-ambient 0 3 concentrations {13). In most cases, the number of high hourly average 0 3 concentrations in the experiments was higher than that occurring under realworld conditions. In addition, the crops were grown under ideal fertilization and water conditions {11). In its deliberations to select the level of the secondary standard, EPA depended almost solely on mathematical models derived from NCLAN data (11). Using these data, EPA developed regression models, which were used to predict exposure levels that would result in an estimated 10% yield loss. EPA predicted that 50% of the species and cultivars tested would exhibit a yield loss of 10-20% across the range of the SUM06 index of 25-38 ppm-h (2). The precise relationship between agricultural yield loss and SUM06 exposure levels is not, however, unequivocally established by the NCLAN data. The exposure regimes used in these experiments exhibited numerous occurrences of high hourly average concentrations (i.e., hourly values >0.10 ppm) in many of the experiments that produced 10% or greater growth loss at SUM06 values >25 ppm-h (11)) The magnitude of the SUM06 does not necessarily reflect the presence or absence of peak concentrations. This observation has raised concerns mat it may be possible for vegetation grown at two sites experiencing the same SUM06 value under ambient conditions but different hourly average values >0.10 ppm to exhibit different effects [1, 11,14).
Concerns about peak exposures In an attempt to correct for this uncertainty, Musselman and colleagues {14) proposed an allernative form of the secondary standard to protect agricultural crops from damage. The authors proposed to combine the SUM06 index with a specified number of hourly peak values (e.g., >0.10 ppm). This proposal was discussed at a workshop for vegetation experts in Raleigh, N.C. {15). Participants thought there could be "injury" to vegetation even if no hourly 0 3 values exceeded 0.10 ppm. However, injury has been defined differently than "damage" by EPA. 0 3 injures plants by spotting leaves and causing premature needle or leaf senescence, reduced photosynthesis, reduced carbohydrate production and allocation, and reduced plant vigor. Damage is defined as loss of economic value, usually associated with growth reduction in crops or trees. In some cases, injury and damage can be synonymous, such as when injured foliage alone results in an economic or Recent experiments aesthetic loss. After deliberation, the have shown that workshop participants decided {16) to forward the individuals exposed Musselman proposal to EPA for further consider- to ozone levels below ation. EPA acknowledged the current standard that any exposure index based on the NCLAN ex- exhibit health periments should consider the presence of these effects. peak concentrations {11). However, EPA ultimately did not take precautions to guarantee that areas that would violate the proposed secondary standard would also experience similar numbers of peak hourly values that occurred in the NCLAN experiments In selecting a secondary standard to protect vegetation, EPA focused on two exposure indices. Neither could adequately guarantee the presence of the peak hourly concentrations described above. Both are cumulative exposure indices: the SUM06 and the sigmoidally weighted W126 exposure index {11). The SUM06 is a threshold-weighting index, which uses a threshold concentration of 0.06 ppm. On the other hand, the sigmoidally weighted W126 is a cumulativeweighting index that assigns increasing weight to values from 0 to 0.10 ppm (although it gives little weight to values below 0.040 ppm) and a unit weight to all values >0 10 ppm With both the SUM06 and W126 it is possible to experience high values at both lowand high-elevation sites without experiencing peak hourly concentrations After considering the peer-review literature, EPA concluded that both indices performed similarly as exposure measures to predict the exposure-response relationships observed in the NCLAN crop studies. In the absence of research results that differentiate the predictive power of these two forms, EPA took into account policy considerations, as well as recommendations from the scientific community, and ultimately selected the SUM06 exposure index as a secondary standard. However, EPA did not document its reasons for proposing the use of the 3-month, 12-h SUM06 index that excluded consideration of the peaks. VOL.31, NO. 6, 1997 /ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 8 3 A
One standard or two? An important question addressed by EPA was whether the attainment of the proposed primary 8-h standard would also guarantee adequate protection for vegetation; if so, there would be no reason to propose a different secondary standard. Using air quality data from its Aerometric Information Retrieval System (AIRS), EPA examined the 8-h daily maximum and 3-month, 12-h SUM06 values for the period 1991-93 for 581 counties. The analysis revealed that almost all areas that fell within a SUM06 range of 25-38 ppm-h would also violate the 8-h standard (2). EPA noted that although these analyses indicate that the adoption of an 8-h, 0.08-ppm primary standard would provide increased protection, it was not known whether air quality improvements, as indicated by reduced 8-h 0 3 concentrations, would result in similar reductions in the SUM06 index. After extensive deliberations, EPA decided to propose a secondary standard that was different in form from the primary standard. The implementation of the SUM06 secondary standard may present challenges similar to the ones inherent in the primary standard for people responsible for recommending and applying control strategies. Many of the monitoring sites that violate the proposed SUM06 standard will experience no peak concentrations. As is the case for the primary 8-h standard, the sites that experience the peaks may have an easier time reaching attainment than those that experience most of their hourly average concentrations in the mid-range. If the proposal for 3. modified SUM06-type standard were adopted by EPA, then designers and implementers of control strategies for the secondary standard would need to focus only on
those sites mat experience the peak concentrations similar to those in the NCLAN experiments. Improving the science Research has played an important role in discussions of the primary and secondary standards, but uncertainty does exist. Future experiments should attempt to better define the magnitude and the ramifications of these uncertainties. Clearly, human health and vegetation experiments should include 0 3 exposures that mimic the actual occurrences of hourly average concentrations experienced under realworld conditions. Human health researchers might want to do fewer experiments that apply constant concentration over extended periods of time and do more experiments with varying exposure regimes, perhaps using patterns of ozone exposure observed in major metropolitan areas. If human health research results continue to show that peak concentrations are more important than the middle and lower values, more research might be directed at understanding the effect of varying hourly concentrations on human health. In addition, researchers might consider coupling these human health effects results with the development of more accurate mathematical descriptions of these changes so that the dose-response relationships can be better linked to the form and levels or the primary standard.
2 8 4 A • VOL.31, NO. 6, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY/NEWS
Vegetation research in the late 1970s and early 1980s applied ambient-type 0 3 exposures. Unfortunately, the fumigation protocol used in the NCLAN experiments resulted in numerous peak hourly average concentrations in many of the experimental treatments near the 10% growth reduction level. Considering the large uncertainty associated with a standard that does not recognize the presence of the frequent peak hourly values in the NCLAN experiments, future vegetation research efforts should focus on applying real-world 0 3 hourly data that mimic actual geographic locations. Additional research efforts might continue to explore the effects of meteorological factors, such as temperature, humidity, and soil moisture. The magnitude of the SUM06 index is a measure of the plant's exposure to 0 3 . Future vegetation research might be directed at better understanding the mechanisms that are responsible for 0 3 uptake (i.e., dose) by the plant and under what conditions uptake is most optimal. It may be possible to integrate these results so that the SUM06 exposure index can be replaced with a more doserelated statistic. References (1)
(2) (3) (4) (5) (6) (7) (8) (9)
(10)
(11)
(12) (13)
(14) (15) (16)
Review of National Ambient Air Quality Standards for Ozone-Assessment of Scientific and Technical Information; Office of Air Quality Planning and Standards. U.S. Environmental Protection Agency: Research Triangle Park, NC, 1996; EPA/452/R-96/007. U.S. Environmental Protection Agency. Fed. Regist. Dec. 13, 1996, 61 (241), 65716-50. Folinsbee, L. J.; McDonnell, W. E; Horstman, D. H. /. Air Pollut. Control Assoc. 1988, 38, 28-35. Horstman, D. H. et al. Am. Rev. Respir. Dis. .990,142,115863. McDonnell, W. F. et al. Arch. Env. Health. 1991,46(3), 14550. Hazucha, M. I.; Seal, E., Jr.; Folinsbee, L. Am. Rev. Respir. Dis. 1990,141(4), A71. Hazucha, M. J.; Folinsbee, L. J.; Seal, E., Jr. Am. Rev. Respir. Dis. 1992, 146, 1487-93. Wolff, G. T. Environmental Manager 1996, 27-32. Letter, CASAC closure on the primary standard portion of the staff paper for ozone to Carol M. Browner from George Wolff; Nov. 30, 1995; EPA-SAB-CASAC-LTR-96002. Whitfield, R. G.; Richmond, H. M. Presented at the 89th Annual Meeting of the Air & Waste Management Association, Nashville, TN; Air & Waste Management Association: Pittsburgh, PA, 1996. Air Quality Criteria for Ozone and Related Photochemical Oxidants; Office of Research and Development. U.S. Environmental Protection Agency: Washington, DC, 1996; EPA/600/P-93/004a-cE Musselman, R. C; Oshima, R. J.; Gallavan, R. E.J.Am. Soc. Hort. Sci. 1983, 108, 347-51. Preston, E. M.; Tingey, D. T. In Assessment of Crop Loss from Air Pollutants; Heck, W.; Taylor, O. C; Tingey, D. X, Eds.; Elsevier Applied Science: London, 1988; pp. 4562. Musselman, R. G; McCool, P M.; Lefohn, A. S.J. Air Waste Manage. Assoc. 1994, 44, 1383-90. Heck, W. W.; Cowling, E. B. Environmental Manager 1997, 23-33. Heck, W. W; Cowling, E. B. Final Overview of SOS Standards Workshop; North Carolina State University: Raleigh, NC, 1996.
Allen S. Lefohn, president ofA.S.L & Associates, Helena, Mont., has performdd research on biological exposureresponse and air quality characterization.
