Tag Archives: Clean Air Act

EPA’s New Ozone Rule: Part 17

In our previous post, the EPA explained why it found a secondary standard necessary to protect vegetation Indeed, when EPA’s Clean Air Scientific Advisory Committee (CASAC) found out, they strongly objected. I can imagine that a journalist reporting on CASAC would use words like “furious”, “enraged”, “livid.” They let the EPA know in no uncertain terms how they felt as reported in the document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011 (p. 212):

Following the 2008 decision on the O3 standards, serious questions were raised as to whether the standards met the requirements of the CAA [Clean Air Act — MHK]. In April 2008, the members of the CASAC Ozone Review Panel sent a letter to EPA stating “[i]n our most-recent letters to you on this subject – dated October 2006 and March 2007 – … the Committee recommended an alternative secondary standard of cumulative form that is substantially different from the primary Ozone NAAQS in averaging time, level and form — specifically, the W126 index within the range of 7 to 15 ppm-hours, accumulated over at least the 12 “daylight” hours and the three maximum ozone months of the summer growing season” (Henderson, 2008). The letter continued: “[t]he CASAC now wishes to convey, by means of this letter, its additional, unsolicited advice with regard to the primary and secondary Ozone NAAQS. In doing so, the participating members of the CASAC Ozone Review Panel are unanimous in strongly urging you or your successor as EPA Administrator to ensure that these recommendations be considered during the next review cycle for the Ozone NAAQS that will begin next year” (id.).

Now CASAC is going to really lay into the EPA!

The letter further stated the following views:

The CASAC was … greatly disappointed that you failed to change the form of the secondary standard to make it different from the primary standard. As stated in the preamble to the Final Rule, even in the previous 1996 ozone review, ‘there was general agreement between the EPA staff, CASAC, and the Administrator, … that a cumulative, seasonal form was more biologically relevant than the previous 1-hour and new 8-hour average forms (61 FR 65716)’ for the secondary standard. Therefore, in both the previous review and in this review, the Agency staff and its advisors agreed that a change in the form of the secondary standard was scientifically well-justified.

Unfortunately, this scientifically-sound approach of using a cumulative exposure index for welfare effects was not adopted, and the default position of using the primary standard for the secondary standard was once again instituted. Keeping the same form for the secondary Ozone NAAQS as for the primary standard is not supported by current scientific knowledge indicating that different indicator variables are needed to protect vegetation compared to public health. The CASAC was further disappointed that a secondary standard of the W126 form was not considered from within the Committee’s previously-recommended range of 7 to 15 ppm-hours. The CASAC sincerely hopes that, in the next round of Ozone NAAQS review, the Agency will be able to support and establish a reasonable and scientifically-defensible cumulative form for the secondary standard.” (Henderson, 2008)

Wow! You can almost feel the burning red-hot indignation behind this rhetoric which I suspect was toned down quite a bit. In our next post, we’ll see how the EPA reacted.

EPA’s New Ozone Rule: Part 14

In my previous post, we discussed the role of an assessment EPA had done estimating how many children from 12 metropolitan areas would be exposed to different levels of ozone. We’ll close this discussion of why the EPA chose the primary standard it did with these final comments from Jackson, taken from the document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011. In this comment, she compares the exposure assessment we were discussing in the previous post to the assessment of risk of how many people are likely to experience health problems from ozone at different maximum levels. She still comes to the conclusion that a standard of .070 ppm is warranted but not lower than that (p. 182):

In considering the estimates provided by the risk assessment, the Administrator notes that significant reductions in health risks for lung function, respiratory symptoms, hospital admissions and mortality have been estimated to occur across the standard levels analyzed, including 0.084 ppm, the level of the 1997 standard, 0.080, 0.074, 0.070, and 0.064 ppm. In looking across these alternative standards, as discussed above in section II.A.2, the patterns in risk reductions are similar to the patterns observed in the exposure assessment for exposures at and above the health benchmark levels. In considering these results, the Administrator recognizes there is increasing uncertainty about the various concentration-response relationships used in the risk assessment at lower O3 concentrations, such that as estimated risk reductions increase for lower alternative standard levels so too do the uncertainties in those estimates. In light of this and other uncertainties in the assessment, the Administrator concludes that the risk assessment reinforces the exposure assessment in supporting a standard level no higher than 0.070 ppm, but it does not warrant selecting a lower standard level.

CASAC asserted that the ozone standard should be set between .060 and .070 ppm, but it preferred that the standard be set closer to 0.060. Jackson agreed with CASAC with its assertion but not with its preference, and she explains why (p. 183):

With regard to selecting a standard level from within that range, the Administrator observes that CASAC recognized that she must make a public health policy judgment to select a specific standard that in her judgment protects public health with an adequate margin of safety. The Administrator notes that CASAC found the relative strength of the evidence to be weaker at lower concentrations, and that their recommended range of 0.060 to 0.070 ppm allowed her to judge the appropriate weight to place on any uncertainties and limitations in the science in selecting a standard level within that range (Samet, 2011, p.9). The Administrator further notes that CASAC expressed the view that selecting a level below the current standard, closer to 0.060 ppm, would be “prudent,” in spite of the uncertainties (Samet, 2011, p.7-8), and that selecting a standard level at the upper end of their recommended range would provide “little” margin of safety (Samet, 2011, p.2).

In reaching her public health policy judgment, after carefully considering the available evidence and assessments, the associated uncertainties and limitations, and the advice and views of CASAC, the Administrator judges that a standard set at 0.070 ppm appropriately balances the uncertainties in the assessments and evidence with the requirement to protect public health with an adequate margin of safety for susceptible populations, especially children and people with lung disease. In so doing, she also concludes that a standard set at a lower level would be more than is necessary to protect public health with an adequate margin of safety for these susceptible populations. This judgment by the Administrator appropriately considers the requirement for a standard that is neither more nor less stringent than necessary for this purpose and recognizes that the CAA [Clean Air Act — MHK] does not require that primary standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety. Further, this judgment is consistent with and supported by the advice and unanimous recommendation of CASAC to set a standard within a range that included but was no higher than 0.070 ppm.

So there you have it. The proposed standard of 0.070 ppm was not based on a mathematical equation or a set of rigid criteria. It was a judgement call, something with which reasonable people can disagree.

So far, we’ve been discussing the rationale of EPA’s primary ozone standard, meant to safeguard the pubiic health. Next, we’ll discuss the secondary standard, formulated to help preserve property and other economic interests.

EPA’s New Ozone Rule: Part 9

Exactly what was the EPA’s reasoning behind lowering the maximum ground-level ozone concentration from 75 ppb to 70 ppb? This is the opening paragraph of the discussion in EPA’s National Ambient Air Quality Standards for Ozone, 2011, page 35.

This section presents the rationale for the Administrator’s final decision that the O3 primary standard, which was set at a level of 0.075 ppm in the 2008 final rule, should instead be set at 0.070 ppm. In developing this rationale, the Administrator recognizes that the CAA [Clean Air Act — MHK] requires her to reach a public health policy judgment as to what standard would be requisite to protect public health with an adequate margin of safety, based on scientific evidence and technical assessments that have inherent uncertainties and limitations. This judgment requires making reasoned decisions as to what weight to place on various types of evidence and assessments, and on the related uncertainties and limitations. Thus, in selecting a final level, the Administrator is seeking not only to prevent O3 levels that have been demonstrated to be harmful but also to prevent lower O3 levels that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree.

What the EPA is saying is that it isn’t enough for the maximum concentration of ground-level ozone allowable to be set just below the minimum known to cause harm. Rather, the limit must be low enough so that even if the harm is not certain but only possible, the risk of harm is low enough to be acceptable. Question is, how low must the risk be to considered acceptable? The document itself states that risk must be taken into consideration even when it can’t be precisely identified. But does that mean that any level of risk, no matter how low, is unacceptable? That would be setting a very high standard indeed. And if that is not so, what is the maximum level of risk that is acceptable? What is the cutoff point?

Unspoken is the realization that it is politically unwise to try to impose tougher rules on the public than is necessary to achieve the objective. To do so is to impose unnecessary economic hardship that could provoke a backlash. And indeed, we saw that backlash in September 2011. The EPA can’t admit that fact, but it is nevertheless true.

There have been a number of controlled studies examining human exposure to ozone, but most have been at the 80 ppb level1. However, studies by William C. Adams, researcher (now retired) at the University of California at Davis did expose humans to ozone at average concentrations as low as 40 ppb2. Besides exposing his subjects to steady concentrations, Adams attempted to mimic the natural environment by slowly increasing and then decreasing the ozone concentration, much as the ambient ozone concentration grows in the morning, peaks in midday, and then declines toward evening. Adams found no statistically significant difference in lung function compared to breathing filtered (ozone-free) air at the 40 ppb and 60 ppb levels. However, a later analysis of Adam’s data by the EPA did find a small statistical difference at the 60 ppb level3. EPA finds this of concern, because a small statistical drop of lung function among healthy adults could manifest itself much more forcefully among those with lung disease4.

Still, most controlled studies on ozone exposure do not test beneath the 80 ppb level. Yet the EPA notes that there is no evidence that the harmful effects of ozone stop at the 80 ppb level (start with a very high concentration of ozone and slowly lower it. The concentration level where harmful effects would stop is known as the threshold). In fact, it can be inferred that such effects extend well below that level, because of the variability of responses of the test subjects5. I believe this means that if 80 ppb was the threshold level, then if you exposed test subjects to that concentration, you would see a number of small responses, but they would all be roughly equal to each other. If, on the other hand, some test subjects experience effects much more than others, even though the effects are still small, that indicates that the effects occur well below the 80 ppb level. And small effects for healthy people can mean big effects for those with respiratory disease.

The above concerned controlled studies of subjects of laboratory experiments. EPA also looked at epidemiological studies, studies of what is happening to populations in their day-to-day lives6. Some found thresholds between 25 and 50 ppb. Other studies never found a threshold because the damage that ozone inflicted seemed linear with the concentration. As I understand this, this means that if the concentration was reduced by a specific percentage (for example, a 20% reduction), measureable effects are reduced by the same percentage multiplied by fixed factor (say a 2% reduction in concentration results in a 3% decrease in effects, a 4% reduction results in a 6% decrease in effects, and so on). On the other hand, you might expect that at a concentration near the threshold level, a further reduction would result in a greater decrease of effects (say a 2% reduction results in a 3% in effects, but a 3% reduction results in a 25% decrease in effects, and a 4% reduction results in a 95% decrease in    effects)6. These studies never saw this sort of effect, so they could not conclude there was any threshold for ozone.

The EPA also looked at studies that did subset analysis looking only at days whose ozone concentration did not exceed certain ozone concentrations (such as 80 ppb and 61 ppb), and still found associations between those concentrations and lung function decrements)6.

Regarding the existence of a threshold for the effects of ozone, the EPA concluded:

Based on the above considerations, the 2007 Staff Paper recognized that the available evidence neither supports nor refutes the existence of effect thresholds at the population level for morbidity and mortality effects, and that if a population threshold level does exist, it would likely be well below the level of the then current standard and possibly within the range of background levels. Taken together, these considerations also support the conclusion that if a population threshold level does exist, it would likely be well below the level of the 0.075 ppm, 8-hour average, standard set in 2008.7

But if the EPA needed to pick the lowest allowable concentration, should it have chosen the lowest threshold found by the studies, 25 ppb? That would not be possible, because the background level of ground-level ozone (the concentration of ozone in the U.S. that is either naturally occurring or coming from outside the U.S. and over which the U.S. government has no control. The background level varies with location and season8.) is often above that level of 25 ppb9. This being the case, setting the standard at 25 ppb would have been an impossible demand. (In fact, the 2007 Staff Paper found that below 35 ppb, it was difficult to tell effects from ozone from effects from other air pollutants9.) Even 50 ppb would be an extremely difficult and expensive goal to meet.


  1. U.S. Environmental Protection Agency, National Ambient Air Quality Standards for Ozone, Final Preamble, 2011, p.38
  2. Adams,W.C., Comparison of chamber 6.6-h exposures to 0.04-0.08 ppm ozone via square-wave and triangular profiles on pulmonary responses Inhalation Toxicology vol. 18: pp. 127-136. For the abstract, click here.
  3. U.S. Environmental Protection Agency, National Ambient Air Quality Standards for Ozone, Final Preamble, 2011, p.38
  4. ibid.p.39
  5. ibid.p.40
  6. ibid.p.42
  7. ibid.p.43
  8. For a detailed discussion of background ozone levels, see U.S. Environmental Protection Agency, Integrated Science Assessment for Ozone and Related Photochemical Oxidants, Third External Review Draft, June 2012, Section 3.4, “Background Ozone Concentrations”, p.3-32ff.
  9. U.S. Environmental Protection Agency, National Ambient Air Quality Standards for Ozone, Final Preamble, 2011, p.42, p.107

EPA’s New Ozone Rule: Part 8

In 2008, the EPA under George W. Bush reduced the maximum allowable concentration of ground-level ozone from 80 ppb to 75 ppb1. Two years later, the EPA decided to reduce the limit still further to 70 ppb.2. What made the EPA decide to do so in only two years? This was unusual because the Clean Air Act only requires the EPA to review its policy on ozone once every five years, the next review required in 20133. What was the rush?

In April 2008, soon after the EPA lowered the standard, the Clean Air Scientific Advisory Committee (CASAC, EPA’s scientific advisory board on clean air4) sent the EPA a letter strongly disagreeing with the new standard, claiming that the new ozone standard was not low enough to provide a margin of safety. It wanted a primary standard between 60 and 70 ppb. In addition, CASAC felt that a different secondary standard should be established to protect property and the environment. This standard should be cumulative rather than be based on highest average readings5.

A month later, a number of groups challenged EPA’s standards in court. Some of them felt the standard went too far: business interests and some states. Other petitioners felt the standard did not go far enough: environmental organizations, public health organizations, and other states. These lawsuits were consolidated into one: State of Mississippi et al v. U.S. Environmental Protection Agency. In March 2009, the EPA filed an unopposed motion to hold the lawsuit in abeyance while it reviewed the new standard. 6 The revised standard, which lowered the maximum allowable concentration from 75 ppb to 70 pbb, was published in July 20117. In September 2011, the Obama administration requested that the EPA rescind its new standard8.

The document which lays out this new standard, National Ambient Air Quality Standards for Ozone, Final Preamble published July 7, 2011, lays out a detailed explanation of EPA thinking: why it didn’t think 75 ppb was a good enough standard, why 60 ppb was too low and 70 ppb was about right, and why it felt a new secondary standard to protect property and the environment was necessary9. I am going to try to summarize that thinking here.


  1. U.S. Environmental Protection Agency, Integrated Science Assessment for Ozone and Related Photochemical Oxidants, Third External Review Draft, June 2012, p.lxxiii.
  2. U.S. Environmental Protection Agency, National Ambient Air Quality Standards for Ozone, Final Preamble, 2011, p.6.
  3. United States Code, Title 42, Chapter 85, §7409 (d)(1). To view, click here.
  4. The Clean Air Act requires that an independent scientific body review the NAAQS at five-year intervals and make recommendations. CASAC currently fulfils this role. See United States Code, Title 42, Chapter 85, §7409 (d)(2). To view, click here.
  5. U.S. Environmental Protection Agency, National Ambient Air Quality Standards for Ozone, Final Preamble, 2011, p.18.
  6. ibid.pp.29-30
  7. This is the National Ambient Air Quality Standards for Ozone, Final Preamble, 2011 that has been referred to above.
  8. Statement by the President on the Ozone National Ambient Air Qualities Standards. White House website. To view, click here.
  9. U.S. Environmental Protection Agency, National Ambient Air Quality Standards for Ozone, Final Preamble, 2011. The rationale for the primary standard (section II) starts on p. 34 and the rationale for the secondary standard (section III) starts on p. 192.