Department of Pesticide Regulation 

Mary-Ann Warmerdam 

Director M E M O R A N D U M Arnold Schwarzenegger 

Governor 

1001 I Street • P.O. Box 4015 • Sacramento, California 95812-4015 • www.cdpr.ca.gov

A Department of the California Environmental Protection Agency 

 Printed on recycled paper, 100% post-consumer--processed chlorine-free. 

TO: Randy Segawa 

 Environmental Program Manager I 

 Environmental Monitoring Branch 

 (916) 324-4137 

FROM: Terri Barry Original signed by

 Research Scientist III 

 Environmental Monitoring Branch 

 (916) 324-4140 

DATE: January 29, 2010 

SUBJECT: Response to public comments on the Methyl Iodide Risk Characterization Document 

Background 

The Department of Pesticide Regulation (DPR) received comments on the Methyl Iodide (MeI) 

Risk Characterization Document (RCD) from several outside parties. This memorandum 

provides responses to comments on the air dispersion modeling and the environmental fate 

sections. 

Comments were received from the following parties: Arysta, Worksafe, National Resource 

Defense Council (NRDC), Pesticide Action Network North America (PANNA), and K. Collins. 

Each set of comments will be discussed separately below. 

Responses to Comments 

Arysta – 

Arysta’s comments are centered on the idea that DPR has used “outdated methods” for the 

exposure assessment. DPR is using screening methods for the RCD. Screening methods are 

acceptable and appropriate to produce air concentration estimates. DPR has used this approach 

since 1992 when mitigation measures in the form of buffer zones were developed for methyl 

bromide. Because of the long history of use, screening methods used at DPR are well understood 

and characterized. In fact, the document “Screening Procedures for Estimating the Air Quality 

Impact of Stationary Sources, Revised (EPA-454/R-92-019)” is currently on the U.S. EPA 

Support Center for Regulatory Atmospheric Modeling internet website 

(http://www.epa.gov/scram001/guidance_permit.htm). This document includes methods for 

estimating screening level air concentrations for area sources and has an update that specifically 

applied to screening estimates for area sources which can be found at the following website: 

http://www.epa.gov/scram001/guidance/guide/scrupd.pdf. Thus, it is still a U.S. EPA accepted 

method. Screening methods are reasonable worst-case estimates. 
Randy Segawa 

January 29, 2010 

Page 2 

The U.S. EPA Screening Procedures document states: “The simple screening procedure (Phase 

1) is applied to determine if the source poses a potential threat to air quality. The purpose of first 

applying a simple screening procedure is to conserve resources by eliminating from further 

analysis those sources that clearly will not cause or contribute to ambient concentrations in 

excess of short-term air quality standards or allowable concentration increments. A relatively 

large degree of "conservatism" is incorporated in that screening procedure to provide reasonable 

assurance that maximum concentrations will not be underestimated.” 

Arysta states: “The assumption of a constant wind direction over 24 hours, or for that matter 

8 hours, is particularly unrealistic. Furthermore, the wind speed assumption is at the extreme 

low-end of the distribution of wind speeds in the environment. In fact, the dispersion model 

used by DPR will not even accept a smaller wind speed than 1 m/sec.”

Air concentration produced using screening methods are not “incorrect.” As discussed above, the 

document “Screening Procedures for Estimating the Air Quality Impact of Stationary Sources, 

Revised (EPA-454/R-92-019)” is currently on the U.S. EPA Support Center for Regulatory 

Atmospheric Modeling website. The meteorological conditions used to product the air 

concentrations are not “artificial.” They are screening meteorological scenarios that can, and do, 

occur. 

Arysta states: “The illogical nature of DPR’s calculations can be observed by comparing the 

concentration estimates averaged over 8-hour and 24-hours in Table 13.” 

There are several interacting and confounding factors that make the Arysta comparisons 

inappropriate: 1) the maximum 8-hr flux was chosen using a rolling average. This highest 

average flux was then paired with the reasonable worst-case meteorological conditions for the 8-

hr scenario. This estimate is totally independent of the 24-hr scenario and should be interpreted 

as such. 2) The meteorological scenarios are different for the 8-hr versus the 24-hr averaging 

times. The 8-hr case uses D stability, the 24-hr case uses C stability. Even if all other factors 

were held constant (which they are not since the flux estimates are specific to the averaging 

time), the difference in stability class produces significant nonlinear changes in air 

concentrations between averaging times. Thus, the 8-hr air concentration is expected to be 

substantially higher than the 24-hr air concentration because it was derived independently using 

maximum 8-hr flux and more stable atmospheric conditions. The example calculation Arysta 

presents, “Even if the concentration were zero for the other 16 hours of the day, given a 

concentration of 9.5 μg/L for the first 8 hours, the “maximum” 24-hour concentration would be 

3.2 μg/L,” is not valid because of this difference in atmospheric stability. 
Randy Segawa 

January 29, 2010 

Page 3 

 3) The effect of scaling on averaging times is not proportional. For example, the maximum 

continuous 8-hr concentration during a continuous 24-hr monitoring period will be in the 

neighborhood of twice the 24-hr concentration. This effect is documented in air dispersion theory 

literature. 

Thus, it is inappropriate to make the direct comparison and calculations such as those made by 

Arysta. With respect to the DPR air concentration estimates, it is not possible to make simple 

statements such as “For a given “maximum” 8-hour average concentration, the corresponding 

“maximum” 24-hour average concentration should not be less than one-third of the “maximum” 

8-hour average concentration.” 

This is exactly the stage that DPR is at with Iodomethane. DPR chose to use the screening 

method at this stage of evaluation. 

 

Arysta states: “DPR’s methodology for estimating the 2-week average concentration is based 

on something DPR calls “peak-to-mean” theory. The peak-to-mean theory is based on 

measurements made in the 1950-1960s of the variability in air pollution at sampling points 

downwind of contiuously-emitting smoke stacks...” 

The technique is commonly used in the air pollution regulatory community. The memorandum 

cited (Barry, 2000) is available for reference at the following internet website: 

http://www.cdpr.ca.gov/docs/emon/pubs/ehapreps/analysis_memos/2000_segawa.pdf. The 

theoretical foundation of “peak-to-mean” methods dates back to 1921 in the work of G.I. 

Taylor’s concepts of diffusion by continuous motion. The first major paper addressing the peakto-mean concept was published in 1959 by Gifford. The method was further developed by Hino 

in 1968 and is referenced specifically by Turner in his 1994 text entitled “Workbook of 

atmospheric dispersion estimates. Furthermore, the peak-to-mean adjustment concept is used by 

U.S.EPA in the 1992 document Screening Procedures for Estimating the Air Quality Impact of 

Stationary Sources, Revised (EPA-454/R-92-019) in the same manner as applied by DPR. This 

method has been used routinely to obtain long-term averages from shorter averaging time air 

concentrations. 

Arysta states; “...emission rates for iodomethane dramatically decline after the first 24-

hours after the application, as evidenced in all of the flux studies. Therefore, the primary 

reason for lower concentrations over a 2-week period is the reduction in emission rate. 

Peak-to-mean theory is used to estimate the impact of variability due to changes in the 

wind direction and the looping of the plume. Therefore, it was misapplied to estimate 

season exposures by DPR. 
Randy Segawa 

January 29, 2010 

Page 4 

DPR staff is aware, and agrees that methyl iodide flux declines steeply after the first 24 hours. 

That is why the effective flux used to estimate the 2 week average air concentration is the 2 week 

average 24 hr flux. This flux estimate incorporates the form of the flux profile over 2 weeks and 

is effectively a uniform flux for that period, thus satisfying the continuously emitting assumption 

of the peak-to-mean theory (see the Arysta comment above). It should be noted that the peak-tomean theory does not incorporate the looping of an elevated plume. The theory is derived by 

characterizing the lateral displacement of the plume centerline over one averaging period relative 

to the averaging period of interest. P.J. Barry in Boundary-Layer Meteorology Vol 2(1971): 122-

126 indicates that the basic peak-to-mean ratio holds for any short-period to long-period ratio 

within the bounds given in his paper. The short period bounds are 5 s to 24 hrs; long period 

bounds are 6 min and 5 yrs. The ratio can be applied to obtain long to short or short to long 

adjustments, as discussed in Turner. Peak-to-mean adjustments are routinely used on area 

sources (e.g. manure piles, sewage treatment plants) in odor abatement. 

Worksafe – This set of comments pertains only to farm workers. No air dispersion modeling or 

environmental fate comments were included. Thus, no response is required. 

NRDC – This set of comments has no comments pertaining to air dispersion modeling and one 

comment pertaining to environmental fate. 

Comment 9: “MeI is likely to contaminate groundwater resources, which could pose a 

significant health hazard through contamination of drinking water.” 

The assessment by NRDC is that the ground water iodide estimate should be refined and that the 

issue should be thoroughly studied and resolved before MeI is granted a California registration. 

Response: DPR’s evaluation indicates that methyl iodide is very unlikely to contaminate 

groundwater. The potential for the iodide anion breakdown product to contaminate groundwater 

is unknown. DPR’s assessment includes a worst-case estimate of potential groundwater 

contamination by iodide. In addition, DPR added an alternative estimate of ground water 

concentrations to its assessment, using bromide ion as a surrogate. The U.S. Geological Survey 

detected bromide in 246 of 256 wells in California, with concentrations ranging from 0.01 mg/L 

to 13.3 mg/L, and a mean of 0.30 mg/L. They also detected iodide in 234 of the same 256 wells, 

with concentrations ranging from 0.001 to 2.4 mg/L, and a mean of 0.08 mg/L. These 

groundwater concentration estimates are adequate to estimate the health risk of iodide in water. 

 

A. Wise – This set of comments references DPR air dispersion modeling and environmental fate 

analysis but does not suggest changes in either. Thus, no response is required. 

K. Collins – This set of comments pertains only to toxicology. Thus, no response is required. 
Randy Segawa 

January 29, 2010 

Page 5 

PANNA – Two documents are included in the PANNA submission: Review of the DPR RCD 

and Comments submitted to U.S.EPA. 

Page 8. Suggestion that DPR should use AERMOD rather than ISCST3. 

Response: The new algorithms in AERMOD are not implemented for area sources. For similar 

inputs, similar results will be obtained with both ISCST3 and AERMOD. All previous modeling 

at DPR has been conducted using ISCST3. For consistency, ISCST3 will continue to be used 

since switching to AERMOD for area sources has no benefit. 

Page 8. comment on modeling poisoning incidents. 

Response: DPR staff has published five peer reviewed articles on modeling incidents. All four 

California incidents shown on Page 6, Table 2 of the PANNA comments are subjects of those 

published peer review articles. Shown below are those articles: 

1: Oriel M, Edmiston S, Beauvais S, Barry T, O'Malley M. Illnesses associated 

with chloropicrin use in California agriculture, 1992-2003. Rev Environ Contam 

Toxicol. 2009;200:1-31. 

2: O'Malley M, Barry T, Ibarra M, Verder-Carlos M, Mehler L. Illnesses related to 

shank application of metam-sodium, Arvin, California, July 2002. J Agromedicine. 

2005;10(4):27-42. Erratum in: J Agromedicine. 2006;11(2):91. 

3: Centers for Disease Control and Prevention (CDC). Illness associated with 

drift of chloropicrin soil fumigant into a residential area--Kern County, 

California, 2003. MMWR Morb Mortal Wkly Rep. 2004 Aug 20;53(32):740-2. 

4: O'Malley M, Barry T, Verder-Carlos M, Rubin A. Modeling of methyl 

isothiocyanate air concentrations associated with community illnesses following a 

metam-sodium sprinkler application. Am J Ind Med. 2004 Jul;46(1):1-15. 

5: Barry T, Oriel M, Verder-Carlos M, Mehler L, Edmiston S, O’Malley M. Community 

Exposure Following a Drip-Application of Chloropicrin. Journal of Agromedicine, 15:1–14, 

2010 
Randy Segawa 

January 29, 2010 

Page 6 

Pages 8-9. Request that DPR include the detail of field studies...clarify how differences in 

soil temperature are accounted for by the model.

Response: DPR staff agrees in concept that temperature should have some effect on flux. 

However, other factors also influence flux, such as method of application, soil type, soil 

moisture, and field preparation procedures. These other factors confound the effect of 

temperature on flux, so no relationship can be established with existing data. In addition, it’s 

likely that soil temperature at the depth of injection has a greater influence on flux than air 

temperature. Since most fumigant applications are made at a depth of 12-24 inches, the 

temperature effect is dampened because the soil temperature varies less than the air temperature. 

The relationship between air temperature and flux is not clear. Analysis of MeI flux from 11 

field studies by Reiss and Griffin (2007) found no detectable relationship between temperature 

and flux. 

A second example of the lack of demonstrated relationship between air temperature and flux is 

found for chloropicrin (Beard, 1996). A 33.8% mass loss for chloropicrin was found in a 

Washington field study. A 36.5% mass loss was found in a Florida field study. These were both 

broadcast tarp chloropicrin applications. The air temperatures during the Florida study were 15 

to 20 degrees F warmer yet the mass loss results are similar to the Washington study. 

DPR’s work with methyl bromide applications throughout the year found that winter applications 

can show high flux, high emission ratios (flux expressed as the fraction of amount applied), and 

high air concentrations. In fact, analysis of the relationship between Julian date of the application 

(as a surrogate for temperature) and the emission ratio shows no significant relationship between 

emission ratio and day of application. The regression equations for each application of emission 

ratio as a function of Julian date had R-square values of 3%, 7%, and 12% for nontarp deep, tarp 

broadcast, and tarp bed application methods respectively. A measurable temperature effect 

should be clearly discernable in the regression results. Thus, a simple, clear relationship between 

temperature and flux is not supported by the DPR methyl bromide database. More likely many 

factors act together. 

 

A plot of the methyl bromide emission ratios is shown below. Note the complete lack of trend for 

the tarp broadcast data. In particular, the February 13, 1997 application has an emission ratio of 

9.8%. This could be argued to support the low temperature, low flux theory. However, the July 

25, 1998 application shows an emission ratio of 6.8%. It is also clear the tarp bed application 

method shows a high emission ratio no matter when the application is made. In fact, the tarp bed 

applications in December show a 100% emission ratio, similar to those applications made in 

June and October. The methyl bromide database is the largest available and likely reflects broad 

trends in flux and emission ratios for other fumigants. Thus, the relationship between 

temperature and total mass lost is not clear. 
Randy Segawa 

January 29, 2010 

Page 7 

jul date

em ratio

0 100 200 300 400

100

80

60

40

20

0

nontarp deep

tarp broadcast

tarp bed

method

Scatterplot of em ratio vs jul date

Page 9. Request that DPR provide clarification of which field studies were used to develop 

the flux profile and how soil temperatures may differ in those in the study from which the 

flux profile was determined 

Response: 

DPR used 7 flux profiles from the 7 studies available at the time the exposure appraisal was 

developed. The flux profile that resulted in the highest air concentrations was used from among 

those 7 flux profiles. See Barry (2008a) for details of how the air concentrations were 

developed. 

Page 9. Comment 3. Documentation of the exposure model should be explicitly described... 

See Barry(2008a) for details of the exposure modeling. 

Page 12. Comment 5. Subchronic exposure... 

See Barry (2008b) for details of the subchronic air concentration development. 
Randy Segawa 

January 29, 2010 

Page 8 

References 

Barry, T. 2008a. Screening level air concentration estimates for Worker Health and Safety 

Exposure Appraisals. Memorandum to Randy Segawa dated August 21, 2008. 

Environmental Monitoring Branch, Department of Pesticide Regulation, California 

Environmental Protection Agency, Sacramento, CA. 

http://www.cdpr.ca.gov/docs/emon/pubs/ehapreps/analysis_memos/2071_segawa.pdf 

Barry, T. 2008b. Development of sub-chronic air concentration estimates associated with a 

single fumigant application. Memorandum to Randy Segawa dated September 5, 

2008Environmental Monitoring, Department of Pesticide Regulation, California Environmental 

Protection Agency, Sacramento, CA. 

http://www.cdpr.ca.gov/docs/emon/pubs/ehapreps/analysis_memos/2077_segawa.pdf 

Beard, K.K., P.G. Murphy, D.D. Fontaine, J.T. Weinberg. 1996. Monitoring of potential worker 

exposure, field flux and off-site air concentration during chloropicrin field application. HEH 160. 

Chloropicrin Manufacturers Task Force. DPR Registration Data Volume199-072. 

Reiss, R. and J. Griffin. 2007. A review of the flux rate data collected for Iodomethane and 

Midas® 50:50. DPR Registration Data Volume No. 52875-0140.