Tag Archives: Adams

EPA’s New Ozone Rule: Part 12

In my last post, we quoted the document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011, where the current Administrator of the EPA, Lisa Jackson (who recently announced she is leaving the agency) discusses why she decided to lower the maximimum ozone concentration limit from 0.075 ppm to between 0.060 and 0.070 ppm. Now she will explain to us how she chose the exact limit. Note that she tacitly acknowledges that she can’t demand more than is necessary. Choosing the optimal number won’t be easy, because the evidence doesn’t point to any such number (p. 174):

The Administrator next considered what standard level within the proposed range of 0.060 to 0.070 ppm would be requisite to protect public health, including the health of susceptible populations, with an adequate margin of safety — i.e., a level that is sufficient but not more than necessary to achieve that result. She recognizes that neither the health evidence nor the human exposure and health risk assessments provide any “bright line” for selecting a specific level within the proposed range.

She explains the difficulties: no laboratory studies in the range of .060 to .070 ppm, studies of people in the street indicate no particular threshold within this range, difficulty in extrapolating what we know about healthy people to people with asthma, and risk assessments made at only two levels: 0.070 ppm and 0.064 ppm. In short, no easy method of determining the best limit. Instead, she will need to base her judgement on many factors taken together (Note: The paragraph sign in brackets [¶] indicates a paragraph break that I introduced that isn’t in the original document. P. 174):

[¶]No controlled human exposure studies were conducted at intermediate levels between 0.070 and 0.060 ppm. Associations reported in epidemiological studies generally ranged from well above to well below this range, with no suggestion of a possible threshold within this range. While there is substantial evidence that asthmatics have greater responses than healthy, non-asthmatic people, there is uncertainty about the magnitude of the differences in their responses within this range. Moreover, within this range, exposure and health risk assessments estimated the exposures of concern and health risks only for standard levels of 0.070 and 0.064 ppm. Thus, there is a combination of scientific evidence and other information that the Administrator needs to consider as a whole in making the public health policy judgment to select a standard level from within the proposed range.

The Administrator declares the limit she selected (p. 175):

After weighing the strengths and the inherent uncertainties and limitations in the evidence and assessments, and taking into account the range of views and judgments expressed by the CASAC Panel, including CASAC’s most recent advice, and in the public comments, as discussed above, the Administrator finds the evidence and other information on the public health impacts from exposure to O3 warrant an 8-hour primary standard set at 0.070 ppm [emphasis mine — MHK]…

Jackson notes that the surest source of evidence, laboratory studies, offer scant evidence below the 0.080 ppm level other than the studies of Adams. In the interest of brevity I’m omitting that section. She goes on to discuss epidemiological studies, studies of people in the street. While they may not be as robust as laboratory studies, the large number of studies do offer enough evidence of a link between levels of ozone and bad health outcomes to make a judgement (p. 177):

With regard to epidemiological studies, the Administrator observes that statistically significant associations between ambient O3 levels and a wide array of respiratory symptoms and other morbidity outcomes, including school absences, emergency department visits, and hospital admissions, have been reported in a large number of studies. These associations occur across distributions of ambient O3 concentrations that generally extend from above to well below the proposed range, although the Administrator recognizes that there are questions of biological plausibility in attributing the observed effects to O3 alone at the lower end of the concentration ranges extending down to background levels.

However, Jackson does recognize that epidemiological studies have their drawbacks, as she discusses here. Samet assures her that although these studies are less reliable at concentrations that approach the natural ambient level, they are not less reliable at the 0.060 to 0.070 ppm range (p. 177):

[¶] The Administrator also recognizes the uncertainty inherent in translating information from such studies into the basis for selecting a specific level from within the proposed range. The Administrator notes that in its most recent advice, CASAC concluded that epidemiological studies are inherently more uncertain as ambient O3 concentrations decrease and effect estimates become smaller, although CASAC’s confidence in attributing reported effects on health outcomes to O3 did not change over the range of 0.060 to 0.070 ppm (Samet, 2011. p.10-11).

Now Jackson must make a value judgement. At what level concentration is the epidemiological evidence pointing to? (p. 178)

[¶]In weighing this evidence and the related uncertainties, the Administrator concludes that while the epidemiological evidence provides support for a standard set no higher than 0.070 ppm, it does not warrant selecting a lower standard level within the proposed range.

But what about people with respiratory problems? Perhaps they need a standard below 0.070 ppm. but she concludes that there is not enough information to choose a lower limit for that reason (p. 178).

The Administrator has also considered the evidence from controlled human exposure and epidemiological studies that children and adults with asthma and other lung diseases are likely to experience larger and more serious responses to O3 exposures than healthy, non-asthmatic people. … the Administrator recognizes that controlled human exposure studies conducted using healthy subjects likely underestimate effects in this susceptible population. The Administrator also recognizes, however, that there is uncertainty about the magnitude of any such differences in responses. Thus, the Administrator concludes that while this evidence supports taking into consideration the extent to which a standard would limit exposures of susceptible populations to concentrations at and above the 0.070 and 0.060 ppm benchmark levels, it does not further inform the translation of the available evidence of O3– related effects in healthy subjects into the basis for selecting any specific standard level from within the proposed range.

Perhaps some quantifiable data can shed some light on an appropriate level that will assist people with respiratory problems. That is the subject of my next post.

EPA’s New Ozone Rule: Part 11

In 2008, the EPA under Administrator Stephen Johnson revised the primary ozone standard to 75 ppb. He was succeeded the next year by Lisa Jackson, the appointee of the incoming Obama administration. Soon after, the EPA began its reconsideration of the new ozone standard, and Ms. Jackson decided to revise the standard, lowering it to 70 ppb.

Her rationale is recorded in the EPA document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011, pages 61 through 186. In this section, Jackson’s positions are summarized, then comments from interested parties appear together with EPA’s responses. A short piece summarizes the comments of the Clean Air Scientific Advisory Committee (CASAC), followed by the rationale for the final decision. A second section, pages 192 through 296, describes the rationale for the secondary standard, the standard meant to protect property and other interests.

It’s a lot to read, and I can’t say I read every word. However, the impression from what I did read was that Jackson wasn’t in possession of any evidence that Johnson didn’t have. Rather, she placed different weight on the evidence. What Johnson saw as sufficient to lower the primary standard to 75 ppb and no further, Jackson felt compelled to lower the standard down to 70 ppb. Here is the summary section “Conclusions on the Level of the Primary Standard”, page 167 ff., with my comments interspersed. The frequent references to Samet are to a 67-page letter written to Jackson in March 2011 from Dr. Jonathan M. Samet, chair of CASAC with the subject line Clean Air Scientific Advisory Committee (CASAC) Response to Charge Questions on the Reconsideration of the 2008 Ozone National Ambient Air Quality Standards. If you wish to read the letter, click here.

Note: The paragraph sign in brackets [¶] indicates a paragraph break that I introduced that isn’t in the original document.

To begin, let’s read what the Jackson set out to do in EPA’s own words:

As a result of the reconsideration, the Administrator has determined that a different level of the primary O3 standard than the 0.075 ppm level set in 2008 is requisite to protect public health with an adequate margin of safety. For the reasons discussed below, the Administrator has decided to set the level of the 8-hour primary O3 at 0.070 ppm…

What influenced her to make this decision?

In the 2010 proposal, the Administrator [Jackson — MHK] concluded it was appropriate to propose to set the primary O3 standard below 0.075 ppm. This conclusion was based on the evidence and exposure/risk-based considerations … and the Administrator’s determination that 0.075 ppm was a level at which the evidence provides a high degree of certainty about the adverse effects of O3 exposure on healthy people. The Administrator’s public health policy judgment on the proposed range for the level of the primary O3 standard was framed by the evidence and exposure/risk-based considerations discussed above in this notice and informed by the following key observations and conclusions on the controlled human exposure and epidemiological studies and the results of the human exposure and health risk assessments.

She will now state four reasons why the evidence suggests that the standard should be lowered (p. 168).

(1) There is a strong body of evidence from controlled human exposure studies evaluating healthy people at O3 exposure levels of 0.080 ppm and above that demonstrated lung function decrements, respiratory symptoms, pulmonary inflammation, and other medically significant airway responses. Newly available for the 2008 review, there is the limited but important evidence of lung function decrements and respiratory symptoms in healthy people down to O3 exposure levels of 0.060 ppm…

I believe Johnson had this same evidence. I suspect that if we sat the two administrators together, they would argue about the importance of limited evidence. When is limited evidence important evidence?

(2) A large number of epidemiological studies [studies that look at people in the street, not in the laboratory — MHK] have reported statistically significant associations between ambient O3 levels and a wide array of respiratory symptoms and other morbidity outcomes including school absences, emergency department visits, and hospital admissions. More specifically, positive and robust associations were found between ambient O3 concentrations and respiratory hospital admissions and emergency department visits… across distributions of ambient O3 concentrations that extend well below the 2008 standard level of 0.075 ppm…

The above is a powerful statement, which if true, would give good cause to lower the standard. But I would want to know what the contribution to morbidity outcomes is made by ambient O3 concentrations in the 0.075 – 0.070 ppm range. This is what we need to balance against any economic cost.

The next reason concerns people with respiratory problems and diseases. Note the concern that studies that look at only healthy people may be underestimating the effects of ozone on those with respiratory problems, although by how much is unknown:

(3) There is substantial evidence … indicating that children and adults with asthma and other preexisting lung diseases are at increased risk from O3 exposure… Evidence from controlled human exposure studies indicates that asthmatics are likely to experience larger and more serious effects in response to O3 exposure than healthy people. This evidence indicates that … controlled human exposure studies of lung function decrements and respiratory symptoms that evaluate only healthy, non-asthmatic subjects likely underestimate the effects of O3 exposure on asthmatics and other people with preexisting lung diseases. However, there is uncertainty about the magnitude of the differences in their responses such that we are not able to quantify the magnitude of any such differences.

Finally, a statement of confidence that lower ozone levels will improve public health:

(4) The assessments of exposures of concern and risks for a range of health effects indicate that important improvements in public health are very likely associated with O3 levels just meeting alternative standard levels evaluated in these assessments, especially for the alternative levels of 0.070 and 0.064 ppm, relative to levels at and above 0.075 ppm…

Now the following paragraph leads me to believe that Jackson did not base her decision on evidence that Johnson did not have. Rather, she interpreted the same evidence differently and was more accepting of CASAC’s recommendations (p. 171):

These observations and conclusions led the Administrator to propose to set the primary O3 standard at a level in the range of 0.060 to 0.070 ppm. In so doing she placed significant weight on the information newly available in the 2008 review that had been reviewed by CASAC, and took into consideration public comments that had been received during the 2008 review. She also placed significant weight on CASAC’s conclusion that important public health protections can be achieved by a standard set below 0.075 ppm, within the range of 0.060 to 0.070 ppm.

Here the document acknowledges the considerations that led Johnson to establish the 0.075 ppm standard, noting the value judgements he made (p. 171):

In reaching a final decision on the level of the primary O3 standard, the Administrator again considered whether the standard level of 0.075 ppm set in the 2008 final rule is sufficiently below 0.080 ppm to be requisite to protect public health with an adequate margin of safety. In considering this standard level, the Administrator looked to the rationale for selecting this level presented in the 2008 final rule… In that rationale, EPA observed that a level of 0.075 ppm is above the range of 0.060 to 0.070 ppm recommended by CASAC, and that the CASAC Panel appeared to place greater weight on the evidence from the Adams studies and on the results of the exposure and risk assessments, whereas EPA placed greater weight on the limitations and uncertainties associated with that evidence and the quantitative exposure and risk assessments. Additionally in 2008, EPA’s rationale did not discuss and thus placed no weight on exposures of concern relative to the 0.060 ppm benchmark level. Further, EPA concluded that “[a] standard set at a lower level than 0.075 ppm would only result in significant further public health protection if, in fact, there is a continuum of health risks in areas with 8-hour average O3 concentrations that are well below the concentrations observed in the key controlled human exposure studies and if the reported associations observed in epidemiological studies are, in fact, causally related to O3 at those lower levels. Based on the available evidence, [EPA] is not prepared to make these assumptions.” (73 FR 16483).

Now Jackson is going to state where she disagrees with Johnson. This strengthens my impression that the decision to lower the limit was a judgement call about which reasonable people can differ (p. 172):

In reconsidering the entire body of evidence available in the 2008 rulemaking, including the Agency’s own assessment of the epidemiological evidence in the 2006 Criteria Document, the views of CASAC, including its most recent advice (Samet, 2011), and the public comments received on the 2010 reconsideration proposal, the Administrator finds no basis to change her conclusion that important and significant risks to public health are likely to occur at a standard level of 0.075 ppm. Thus, she judges that a standard level of 0.075 ppm is not sufficient to protect public health with an adequate margin of safety. In support of this conclusion, the Administrator finds that setting a standard that would protect public health, including the health of susceptible populations, with an adequate margin of safety should reasonably depend upon giving some weight to the results of the Adams studies and EPA’s analysis of the Adams’s data, and some weight to the results of epidemiological studies of respiratory morbidity effects that may extend down to levels below 0.060 ppm.

A limit of outdoor ozone concentration set at level X actually protects people from effects below X, since people spend much of their time indoors where ozone levels are naturally lower. Since they are likely to be indoors when the ozone level reaches X, their maximum exposure to ozone will probably be to levels much below X. Jackson’s argument here is that if setting the limit at 0.070 ppm will limit people’s exposure to ozone levels above 0.060 ppm:

[¶]Moreover, the Administrator concludes that, in setting such a standard, consideration should be given to how effectively alternative standard levels would serve to limit exposures of concern relative to the 0.060 ppm benchmark level as well as the 0.070 ppm benchmark level, based on EPA’s exposure and risk assessments…

So far, Jackson has explained why she feels that the limit of 0.075 ppm is inadequate. She wants to take CASAC’s recommendation of a limit between 0.060 and 0.070 ppm. But she needs to select an exact number. In my next post, she’ll explain how she did that.

EPA’s New Ozone Rule: Part 10

Before discussing how the EPA established its ground-level ozone standards in 2010, let’s look at for the standards it established in 2008 under the second Bush administration. I found the following excerpt very informative: I took it from the EPA document National Ambient Air Quality Standards for Ozone (Final Preamble, 2011), from the section “2008 Decision on the Level of the Primary Standard”, and it starts on page 57. I copied and pasted the entire section, edited down the length, then interspersed the text with my comments. Paragraph breaks not in the original text are marked with a paragraph sign in brackets [¶].

First, the EPA explains why it couldn’t leave the standard as it was. There was too much evidence that ozone causes harm at then present concentration of 84 ppb. Notice how much seems to depend on personal judgement rather than on objective criteria. In other words, you can’t program a computer to set ozone standards.

This section presents the rationale for the 2008 final decision on the primary O3 standard as presented in the 2008 final rule (73 FR 16475). EPA’s conclusions on the level of the standard began by noting that, having carefully considered the public comments on the appropriate level of the O3 standard, EPA concluded that the fundamental scientific conclusions on the effects of O3 reached in the 2006 Criteria Document and 2007 Staff Paper remained valid. … In considering the available scientific evidence, EPA concluded that a focus on the proposed range of 0.070 to 0.075 ppm was appropriate in light of the large body of controlled human exposure and epidemiological and other scientific evidence. The 2008 final rule stated that this body of evidence did not support retaining the then current 0.084 ppm 8-hour O3 standard, as suggested by some commenters, nor did it support setting a level just below 0.080 ppm, because, based on the entire body of evidence, such a level would not provide a significant increase in protection compared to the 0.084 ppm standard. Further, such a level would not be appreciably below the level in controlled human exposure studies at which adverse effects have been demonstrated (i.e., 0.080 ppm).

On one hand, the EPA couldn’t be satified with the current standard: there was too much scientific research proving that 84 ppb harmed people’s health. Lowering the standard a little bit wasn’t worth it; that would help too little. On the other hand, as we will see below, the EPA did not want to go overboard. Setting the level at 60 ppb was going too far; it had no evidence that going that far would increase protection for human health. This left the EPA with a range between 70 and 75 ppb, but the evidence in itself didn’t point to a specific level within this range (p. 58):

[¶] The 2008 final rule also stated that the body of evidence did not support setting a level of 0.060 ppm or below, as suggested by other commenters. In evaluating the information from the exposure assessment and the risk assessment, EPA judged that this information did not provide a clear enough basis for choosing a specific level within the range of 0.075 to 0.070 ppm.

But now EPA must explain why it is going against the recommendations of its own advisory committee, CASAC (Clean Air Scientific Advisory Committee). What EPA seems to saying here is that CASAC wasn’t influenced by scientific considerations alone but also by their opinions about policy. The EPA Administrator Stephen Johnson, however, asserted his policy perogative, used his own judgement, and overruled CASAC (p. 58).

In making a final judgment about the level of the primary O3 standard, EPA noted that the level of 0.075 ppm is above the range unanimously recommended by the CASAC (i.e., 0.070 to 0.060 ppm). The 2008 final rule stated that in placing great weight on the views of CASAC, careful consideration had been given to CASAC’s stated views and the scientific basis and policy views for the range it recommended. In so doing, EPA fully agreed that the scientific evidence supports the conclusion that the current standard was not adequate and must be revised.

With respect to CASAC’s recommended range of standard levels, EPA observed that the basis for CASAC’s recommendation appeared to be a mixture of scientific and policy considerations. While in general agreement with CASAC’s views concerning the interpretation of the scientific evidence, EPA noted that there was no bright line clearly directing the choice of level, and the choice of what was appropriate was clearly a public health policy judgment entrusted to the EPA Administrator. This judgment must include consideration of the strengths and limitations of the evidence and the appropriate inferences to be drawn from the evidence and the exposure and risk assessments.

The EPA Administrator will now explain that his judgement differed from CASAC’s because he put different weight on the available evidence. The Adams studies which indicated health effects on healthy sujuects at 60 ppb in the laboratory were too limited. The exposure and risk assessments done by CASAC were too uncertain (p. 59).

[¶] In reviewing the basis for the CASAC Panel’s recommendation for the range of the O3 standard, EPA observed that it reached a different policy judgment than the CASAC Panel based on apparently placing different weight in two areas: the role of the evidence from the Adams studies and the relative weight placed on the results from the exposure and risk assessments. While EPA found the evidence reporting effects at the 0.060 ppm level from the Adams studies to be too limited to support a primary focus at this level, EPA observed that the CASAC Panel appeared to place greater weight on this evidence, as indicated by its recommendation of a range down to 0.060 ppm. … However, EPA more heavily weighed the implications of the uncertainties associated with the Agency’s quantitative human exposure and health risk assessments. Given these uncertainties, EPA did not agree that these assessment results appropriately served as a primary basis for concluding that levels at or below 0.070 ppm were required for the 8-hour O3 standard.

Now comes EPA’s final explanation for setting the ozone standard at 75 ppb. Note the interesting argument that if the standard is set at 75 ppb, most people will not be exposed to more than 70 ppb, probably because ozone levels are always lower indoors, and people are not always outdoors when ozone levels are at their highest. EPA also explains what would have convinced it that a standard lower than 75 ppb carried enough additional public health protection to justify itself (p. 60).

The 2008 final rule stated that … EPA decided to revise the level of the primary 8-hour O3 standard to 0.075 ppm. EPA judged … that a standard set at this level would be requisite to protect public health with an adequate margin of safety, including the health of sensitive subpopulations, from serious health effects including respiratory morbidity, that were judged to be causally associated with short-term and prolonged exposures to O3, and premature mortality. EPA also judged that a standard set at this level provides a significant increase in protection compared to the 0.084 ppm standard, and is appreciably below 0.080 ppm, the level in controlled human exposure studies at which adverse effects have been demonstrated.

[¶] At a level of 0.075 ppm, exposures at and above the benchmark of 0.080 ppm are essentially eliminated, and exposures at and above the benchmark of 0.070 are substantially reduced or eliminated for the vast majority of people in susceptible populations. A standard set at a level lower than 0.075 would only result in significant further public health protection if, in fact, there is a continuum of health risks in areas with 8-hour average O3 concentrations that are well below the concentrations observed in the key controlled human exposure studies and if the reported associations observed in epidemiological studies are, in fact, causally related to O3 at those lower levels. Based on the available evidence, EPA was not prepared to make these assumptions.

[¶] Taking into account the uncertainties that remained in interpreting the evidence from available controlled human exposure and epidemiological studies at very low levels, EPA noted that the likelihood of obtaining benefits to public health decreased with a standard set below 0.075 ppm O3, while the likelihood of requiring reductions in ambient concentrations that go beyond those that are needed to protect public health increased.

[¶] EPA judged that the appropriate balance to be drawn … was to set the 8-hour primary standard at 0.075 ppm. EPA expressed the view that a standard set at 0.075 ppm would be sufficient to protect public health with an adequate margin of safety, and did not believe that a lower standard was needed to provide this degree of protection. EPA further asserted that this judgment appropriately considered the requirement for a standard that was neither more nor less stringent than necessary for this purpose and recognized 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.

So we see that the 2008 standard of 75 ppb was clearly a judgement call. There is no sure way of determining where exactly the costs of reducing ozone outweigh the health benefits. From reading the section, it seems almost like a gut decision what risks are acceptable and how much evidence is necessary to prove harm. I think it is no more than an educated guess where the balance lies, and in 2008, the EPA thought it lay at 75 ppb. Why did its opinion change in 2010? That is the subject of the next post.

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.

Footnotes:

  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