EPA response to OMB SO 2 NAAQS RIA recommendations

Lydia Wegman to: James_A.._Laity 05/27/2010 05:58 PM

Cc: Charlotte Bertrand, Bridgid Curry, Ron Evans, Amy Lamson, Neal 

Fann, Lydia Wegman, Al McGartland, Tricia Crabtree

Jim,

Please find below our responses to your comments on the RIA. We believe we have addressed your first 

comment in the RIA as it currently stands and that your second comment has been resolved in prior RIAs. 

As we discussed, you and I both plan to be out of the office tomorrow and will plan to talk on Tuesday. 

But if you need to reach me tomorrow, please call my cell phone on 919-621-3125. I should be available 

except during our flight to Boston (11 AM-1 PM) and a meeting that I have between 1:30 and 2:30 PM. I 

will shortly be sending you our response to the implementation comments. Thanks and have a good 

weekend. 

Lydia

Interagency Comments on SO2 NAAQS RIA

Q1) We understand that EPA continues to use the assumption that all species of PM 2.5 have the same

toxicity.  However, it seems appropriate in the context of an RIA that focuses primarily on reducing

sulfates to recognize the epidemiological studies suggesting different toxicities from different types

of PM2.5, as well as hypotheses regarding why  V, Ni, and EC/BC might be more harmful to human

health than sulfates (e.g., atmospheric transformations due to oxidation of SO2 to acid and then to

chemically neutral ammonium sulfates might cause harm because metals will become more toxic, or

products of acid catalysis will be harmful).  This sensitivity to the uncertainty associated with the

toxicity of sulfates in particular could be reflected in the RIA either through a sensitivity analysis

showing reduced PM co‐benefits due to targeting of sulfates by SO2 control strategies, or through a

qualitative discussion of this issue and its potential implications for estimated PM co‐benefits.

A1) Within three places in the RIA (i.e., the benefits chapter, the executive summary, and the 

cost-benefit comparison chapter), we acknowledge that all species of PM2.5 may not have the same 

toxicity and this would have implications for the benefits. However, as the Integrated Science 

Assessment (ISA) for PM concluded, there is not sufficient evidence to provide a quantitative estimate of 

the potential implications of such differential toxicity. Our current write-up describes this issue 

qualitatively, which we believe meets the request for a qualitative discussion. We would need to know 

specifically what additional language would be needed beyond what is currently in the RIA. 

Q2) We note that the sensitivity analysis that explores the implications of using a threshold model,

thereby acknowledging the greater uncertainty associated with the health effects below ambient

concentrations of 8‐10 ppb, is only presented in the Appendix of the benefits chapter.   For the

purposes of transparency we encourage EPA to present a broader range of estimates in which the

threshold model of the Pope/ACS study (that is currently only presented in the appendix) is used as

the lower anchor and the no‐threshold model of the Laden study (currently presented in the body of

the chapter) is used as the upper anchor in the primary analysis.

A2) This request is inconsistent with the state of the science and the past agreed to characterization in 

the RIA. In the final PM ISA, EPA concluded that the no-threshold assumption is appropriate and 

CASAC did not reject this. We have been using the current presentation of the no threshold results in the 

last 10 RIAs since April 2009. We are not aware of any change in the science to warrant a change in the 

presentation. Also, there is no appendix to the RIA; the sensitivity analysis with the threshold assumption 
is in the the benefits chapter of the RIA.

Ron Evans

Leader, Air Benefit & Cost Group

HEID/OAQPS/OAR/EPA

Mail Drop C-439-02

919-541-5488

919-541-0839 fax