Tag Archives: 8-hour standard

EPA’s New Ozone Rule: Part 16

Posted on January 17, 2013 | 1 comment

We are continuing our discussion in our last post about why the EPA felt it necessary to formulate a new secondary standard for ground-level ozone concentration. As we noted before, initially the EPA felt it adequate for the secondary standard to be identical to the primary standard, but then it reconsidered its position.

The EPA performed an evaluation comparing primary and secondary standards and found that high cumulative exposures were widespread. Below is a summary of what they found, taken from the document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011. Note point #4 where EPA explains why it thinks the primary standard is insufficient (p. 201):

…The following key observations were drawn from comparing predicted changes in interpolated air quality under each alternative standard form and level scenario analyzed:

  1. The results of the exposure assessment indicate that then-current air quality levels could result in significant impacts to vegetation in some areas. For example, [bulleted list is my formatting — MHK]
    • For the base year (2001), a large portion of California had 12-hr W126 O3 levels above 31 ppm-hours, which has been associated with approximately up to 14% biomass loss in 50% of tree seedling cases studies.
    • Broader multi-state regions in the East (NC, TN, KY, IN, OH, PA, NJ, NY, DE, MD, VA) and West (CA, NV, AZ, OK, TX) are predicted to have levels of air quality above the W126 level of 21 ppm-hours, which is approximately equal to the secondary standard proposed in 1996 and is associated with biomass loss levels no greater than approximately 9% in 50% of tree seedling cases studied, and biomass loss levels greater than approximately 9% in the other 50%.
    • Much of the East and Arizona and California have 12- hour W126 O3 levels above 13 ppm-hours which has been associated with biomass loss levels no greater than approximately 7% biomass loss in 75% of tree seedling cases studied and biomass loss levels greater than approximately 7% in the remaining 25% of cases studied.
  2. When 2001 air quality was rolled back to meet the then current 8-hour secondary standard, the overall 3-month 12-hour W126 O3 levels were somewhat improved, but not substantially. Under this scenario, there were still many areas in California with 12-hour W126 O3 levels above 31 ppm hours. A broad multi-state region in the East (NC, TN, KY, IN, OH, PA, MD) and West (CA, NV, AZ, OK, TX) were still predicted to have O3 levels above the W126 level of 21 ppm-hours.
  3. Exposures generated for just meeting a 0.070 ppm, 4th-highest maximum 8-hour average alternative standard (the lower end of the proposed range for the primary O3 standard) showed substantially improved O3 air quality when compared to just meeting the then-current 0.08 ppm, 8-hour standard. Most areas were predicted to have O3 levels below the W126 level of 21 ppm-hr, although some areas in the East (KY, TN, MI, AR, MO, IL) and West (CA, NV, AZ, UT, NM, CO, OK, TX) were still predicted to have O3 levels above the W126 level of 13 ppm-hours.
  4. While these results suggested that meeting a proposed 0.070 ppm, 8-hour secondary standard would provide substantially improved protection in some areas, the Staff Paper recognized that other areas could continue to have elevated seasonal exposures, including forested park lands and other natural areas, and Class I areas which are federally mandated to preserve certain air quality related values. This is especially important in the high elevation forests in the western U.S. where there are few O3 monitors and where air quality patterns can result in relatively low 8-hour averages while still experiencing relatively high cumulative exposures.

Now the EPA will explain where in particular the lack of a separate secondary standard is a problem. It seems that ozone levels in high-elevation rural areas remain fairly constant during the day, so that the ozone concentration may be below the primary standard and yet deliver a large cumulative exposure. This is where attention to a cumulative-based secondary standard might be particularly useful. Note that the 8-hour average form refers to the primary standard, which depends on the average of ozone measurements taken during an eight-hour time period (p. 202):

To further characterize O3 air quality in terms of the 8-hour and alternative secondary standard forms, an analysis was performed in the 2007 Staff Paper to evaluate the extent to which county-level O3 air quality measured in terms of various levels of the 8-hour average form overlapped with that measured in terms of various levels of the 12-hour W126 cumulative, seasonal form. This analysis was limited by the lack of monitoring in rural areas where important vegetation and ecosystems are located, especially at higher elevation sites. This is because O3 air quality distributions at high elevation sites often do not reflect the typical urban and near-urban pattern of low morning and evening O3 concentrations with a high mid-day peak, but instead maintain relatively flat patterns with many concentrations in the mid-range (e.g., 0.05-0.09 ppm) for extended periods. These conditions can lead to relatively low daily maximum 8-hour averages concurrently with high cumulative values so that there is potentially less overlap between an 8-hour average and a cumulative, seasonal form at these sites. The 2007 Staff Paper concluded that it is reasonable to anticipate that additional unmonitored rural high elevation areas important for vegetation may not be adequately protected even with a lower level of the 8-hour form.

Then the EPA seems to reverse its position. Since we can’t be confident that the primary standard will be adequate, especially in rural areas and remote areas where data on ozone might be sparse, we may need to establish a secondary standard. Whereas before the EPA wanted to err on the side of less regulation, now they want to err on the side of more regulation (p. 203):

It continues to remain uncertain as to the extent to which air quality improvements designed to reduce 8-hour O3 average concentrations would reduce O3 exposures measured by a seasonal, cumulative W126 index. The 2007 Staff Paper indicated this to be an important consideration because:

  1. The biological database stresses the importance of cumulative, seasonal exposures in determining plant response;
  2. Plants have not been specifically tested for the importance of daily maximum 8-hour O3 concentrations in relation to plant response;
  3. The effects of attainment of a 8-hour standard in upwind urban areas on rural air quality distributions cannot be characterized with confidence due to the lack of monitoring data in rural and remote areas.

These factors remain important considerations in the Administrator’s reconsideration of whether the current 8-hour form can appropriately provide requisite protection for vegetation.

Question on point #3: If we can’t be sure of the effects of attainment of an 8-hour standard on rural areas because we don’t have enough monitoring data, how would we be any more sure of the effects of attainment of the secondary standard?

The EPA’s own CASAC (Clean Air Scientific Advisory Committee) was also very unhappy with the decision to make the secondary standard equal to the primary standard. We will see what they have to say in the next post.

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EPA’ s New Ozone Rule: Part 15

Posted on January 15, 2013 | 1 comment

A major innovation of EPA’s 2010 revision of the ozone standard was the introduction of what is called a secondary standard that is different from the primary standard. The secondary standard has existed before, but it was always set identical to the primary standard. To summarize the two standards:

  • The primary standard is intended to protect the public health. It is currently based on the fourth-highest 8-hour average ozone concentration reading in a year.
  • The secondary standard is meant to protect property, economic interests, and other concerns. It is based on a cumulative ozone concentration over time. Ozone readings are taken hourly between 8 a.m. and 7 p.m., adjusted by what is called the W126 rule, and then summed during a three-month period. Units are in ppm-hours. See what I wrote in this blog about the secondary standard in the post “EPA’s New Ozone Rule: Part 6.” To view, click here.

Now if one standard was consistently stricter than the other, the EPA could simply adopt the stricter standard. That it felt necessary to formulate two standards can only mean that in some places one standard will be harder to meet, and in other places the other standard will be the stricter. The EPA wants to meet both standards everywhere, a condition we Orthodox Jews call being machmir for both shitos.

What I don’t understand yet is why the primary standard, which is meant to safeguard public health, is based on a highest one-time average, whereas the secondary standard, meant to protect property, is based on a cumulative measure. A cumulative standard makes sense, because research shows that the extent of damage to plants caused by ozone depends on cumulative exposure. But perhaps damage to human health also depends on cumulative exposure, just as the damage caused by radiation to human health depends on cumulative exposure. Why not make the primary standard cumulative as well? Be that as it may, currently the primary standard remains based on a highest one-time average, while the secondary standard remains identical to the primary standard.

What I want to do in this post is quote EPA in its own words why it felt a new secondary standard was necessary, discussed in the document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011.

From the outset, the EPA is clear that the secondary standard was formulated because of ozone’s effects on plants (p. 196):

…The 2006 Criteria Document concluded that O3 exposure indices that cumulate differentially weighted hourly concentrations are the best candidates for relating exposure to plant growth responses…

It is interesting that the EPA recognized the value of a secondary standard long before 2010 (p. 197):

At the conclusion of the 1997 review, the biological basis for a cumulative, seasonal form was not in dispute. There was general agreement between the EPA staff, CASAC, and the Administrator, based on their review of the air quality criteria, that a cumulative, seasonal form was more biologically relevant than the previous 1-hour and new 8-hour average forms (61 FR 65716).

The EPA also explained why, rather than summing up straight ozone concentrations, it chose to sum up modified values, referred to as the W126 form. Using W126 values gives more weight to higher concentrations and much less weight to lower concentrations that would exist either naturally without human activity, or from foreign sources beyond the control of the U.S. government (p. 198):

Regarding the first consideration, the 2007 Staff Paper noted that the W126 form, by its incorporation of a continuous sigmoidal weighting scheme, does not create an artificially imposed concentration threshold, yet also gives proportionally more weight to the higher and typically more biologically potent concentrations, as supported by the scientific evidence. Second, the index value is not significantly influenced by O3 concentrations within the range of estimated PRB [policy-relevant background, the level of ozone not caused by human activity in the U.S. — MHK], as the weights assigned to concentrations in this range are very small.

Nevertheless, the EPA retained a secondary standard identical to the primary standard until 2010. Initially, the EPA felt that if the primary standard was made more strict, it would be sufficient for the secondary standard were made identical to it. A separate secondary standard that was cumulative would provide no additional protection unless it was made very strict, which can’t be justified because our knowledge of the effects of low-level ozone on vegetation is so uncertain (the paragraph sign [¶] indicates a paragraph break that I inserted. P. 209):

In considering the appropriateness of establishing a new standard defined in terms of a cumulative, seasonal form, or revising the 1997 secondary standard by making it identical to the revised primary standard, … EPA first considered the 2007 Staff Paper analysis of the projected degree of overlap between counties with air quality expected to meet the revised 8-hour primary standard, set at a level of 0.075 ppm, and alternative levels of a W126 standard based on currently monitored air quality data. This analysis showed significant overlap between the revised 8-hour primary standard and selected levels of the W126 standard form being considered, with the degree of overlap between these alternative standards depending greatly on the W126 level selected and the distribution of hourly O3 concentrations within the annual and/or 3-year average period. On this basis, as an initial matter, EPA concluded that a secondary standard set identical to the proposed primary standard would provide a significant degree of additional protection for vegetation as compared to that provided by the then-current 0.084 ppm secondary standard.

¶ In further considering the significant uncertainties that remain in the available body of evidence of O3-related vegetation effects and in the exposure and risk analyses conducted for the 2008 rulemaking, and the difficulty in determining at what point various types of vegetation effects become adverse for sensitive vegetation and ecosystems, EPA focused its consideration on a level for an alternative W126 standard at the upper end of the proposed range (i.e., 21 ppm-hours). The 2007 Staff Paper analysis showed that at that W126 standard level, there would be essentially no counties with air quality that would be expected both to exceed such an alternative W126 standard and to meet the revised 8-hour primary standard – that is, based on this analysis of currently monitored counties, a W126 standard would be unlikely to provide additional protection in any monitored areas beyond that likely to be provided by the revised primary standard.

The EPA states again that with the lack of extensive monitoring in rural areas, it is unsure how much additional protection a separate secondary standard would provide. At this point, it decided to err on the side of less regulation. Note that the term “8 hour standard” refers to the primary standard, which averages readings over eight-hour periods (p. 210):

The EPA also recognized that the general lack of rural monitoring data made uncertain the degree to which the revised 8-hour standard or an alternative W126 standard would be protective in those areas, and that there would be the potential for not providing the appropriate degree of protection for vegetation in areas with air quality distributions that result in a high cumulative, seasonal exposure but do not result in high 8-hour average exposures. While this potential for under-protection using an 8- hour standard was clear, the number and size of areas at issue and the degree of risk was hard to determine. However, EPA concluded at that time that an 8-hour standard would also tend to avoid the potential for providing more protection than is necessary, a risk that EPA concluded would arise from moving to a new form for the secondary standard despite significant uncertainty in determining the degree of risk for any exposure level and the appropriate level of protection, as well as uncertainty in predicting exposure and risk patterns.

…EPA concluded at that time that establishing a new secondary standard with a cumulative, seasonal form would result in uncertain benefits beyond those afforded by the revised primary standard and therefore may be more than necessary to provide the requisite degree of protection.

Eventually, though, the EPA changed its mind. Why will be discussed in the next post.

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