UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON, D.C. 20460

OFFICE OF PREVENTION,

PESTICIDES AND

TOXIC SUBSTANCES



### June 25, 2008

MEMORANDUM

SUBJECT: Mode of Action, Eye Irritation, and the Intra-Species Factor: Comparison of Chloropicrin and MITC

PC Code: 081501 Chloropicrin

068103 MITC

039003 Metam Sodium

035602 Dazomet DP Barcode: DP 293356

MRID No.: None Registration No.: None

Petition No.: None Regulatory Action: None Assessment Type: Single Chemical/ Nonfood Reregistration Case No.: None

T XR No.: 0054860 CAS No.: 137-42-8 Metam Sodium, 76- 06-2 chloropicrin, 533-74-4 Dazomet, 556-61-

6 MITC

FROM: Elissa Reaves, Toxicologist Reregistration Branch II Health Effects Division (7509P)

Anna Lowit, Toxicologist Toxicology and Epidemiology Branch Health Effects Division (7509P)

THRU: Felecia Fort, Branch Chief Reregistration Branch II Health Effects Division (7509P)

TO: Karen Santora, Chemical Review Manager Reregistration Branch II Special Review & Reregistration Division (7508P)

The following document is written in response to recent documents submitted to EPA by TERA on behalf of the registrant, AMVAC, and public comments submitted by the Metam Sodium Alliance with regard to MITC mode of action and intra-species factorThis document also clarifies the scientific data which support reduction of the intraspecies factor for chloropicrin.

Page 1 of 15Background:

EPA is undergoing re-registration of several soil fumigants, including metam sodium, dazomet, and chloropicrin. Metam sodium and dazomet each degrade soon after application to methylisocyante (MITC). Post-application exposure in air to workers and by-standers following off-gassing of chloropicrin and MITC are key pathways of exposure being considered in the human health risk assessments. This memo provides descriptions of important issues for the risk assessment of chloropicrin and MITC such as mode of action, eye irritation, and the intra-species factor.

The Agency has relied on following relevant frameworks and guidance documents in the conclusions presented here:

• Mode of Action Framework 

### as described in U.S. EPA (1999, 2005), SonichMullin et al. (2001), and Meek et al. (2003). This framework, based in large part on the Bradford-Hill criteria for determining causality in epidemiology studies, describes a framework for evaluating mode of action using animal data. In brief, this framework is described by the following steps (extracted from Seed et al, 2005):

o Postulated MOA . A brief description of the sequence of measured events, starting with chemical administration, to the formation of the toxic effect at a given site.

o Key events . Clear descriptions of each of the key events (measurable parameters) that are thought to underlie the MOA.

o Dose-response relationships . Dose-response relationships identified for each key event, and comparisons presented of dose-response relationships among key events and with the toxic effect.

o Temporal association . Sequence of key events over time that lead to the toxic effect. This includes any MOA differences specific to different life stages.

o Strength, consistency, and specificity of association of key events and

the toxic effect . Complete assessment and presentation of the relationships among key events, precursor lesions, and the toxic effect, portrayal of the consistency of observations across studies of different designs.

o Biological plausibility and coherence . Determination of whether key events and the sequence of events are consistent with current biological thinking, regarding both the specific toxic effect in general and the specific chemical under review.

o Other MOAs . Alternative MOAs that may be applicable for the chemical under review. Comparison of their likelihood vis-`a-vis the proposed

MOA.

o Conclusion about the MOA . Overall indication of the level of confidence in the postulated MOA.

o Uncertainties, inconsistencies, and data gaps. Identification of information deficiencies in the case; description of inconsistent findings in the data at large; evaluation of uncertainties; proposal of pointed research that could significantly inform the case.

Page 2 of 15• IPSC (International Programme on Chemical Safety) Guidance on Chemical Specific Adjustment Factors (CSAFs, 2005 ) . In 2005, the WHO published its guidance for deriving chemical specific adjustment factors (CSAFs). This CSAF guidance describes approaches for use of kinetic and mechanistic data to refine inter-species and intra-species extrapolation factors. The IPSC guidance is based in large part on analyses by Renwick (1993) and Renwick and Lazarus (1998) which describe the use of toxicokinetic and toxicodynamic data as a means of replacing the traditional 10-fold safety factors for human sensitivity and experimental animal-to-human extrapol ation. Although EPA does not yet have guidance for developing CSAFs, also often called data-derived extrapolation/uncertainty factors (DDEFs), the Agency has used these concepts in some risk assessments. Understanding mode of action is an important component of deriving CSAFs in that mode of action provides the foundation for understanding what toxicokinetic and/or toxi codynamic characteristics are critical for evaluating inter- or intra-species extrapolation.

The first part of this memo includes a comparison of the toxicology databases (both animal and human) for chloropicrin and MITC. This is followed by a description of the chloropicrin mode of action and toxicology data along with the scientific support for reducing the intra-species factor. The third section of the memo describes the deficiencies in the MITC database which precludes the refinement of the intra-species factor and discusses in general what information would need to be available prior to any consideration of deriving a CSAF, or DDEF, for MITC with regard to intra-species extrapolation.

I. Comparison of Chloropicrin and MITC Toxicology Databases

The animal database for chloropicrin is robust and especially provides insight into the dose-response, temporal concordance, and progression of respiratory irritation from varying concentrations of chloropicrin. Specifically, the inhalation database consists of durations ranging from 30 minutes to two years (108 weeks) and evaluates acute, subchronic, chronic/cancer, reproductive and developmental toxicity. From these rodent studies, it is apparent that factors such as air concentrations of chloropicrin and duration of exposure are important in the development of port-of-entry effects. Chloropicrin is also a known sensory irritant with a defined mode of action.

An intentional dosing human sensory irritation study is available for chloropicrin. The human study was evaluated by the Human St udies Review Board (HSRB) in June, 2006. The HSRB concluded that the chloropicrin human study was scientifically sound for the purpose of estimating a safe level of inhalation exposure to chloropicrin (HSRB Final Report, October 2006; http://www.epa.gov/osa/hsrb /files/june2006mtgreportfinal100606 ). The chloropicrin study is a controlled three phase human irritation study that provides information on: 1) the odor threshold for chloropicrin; 2) the detection of feel in healthy subjects’ eyes, nose, and throat; 3) the irritation potential from repeated exposures; and 4) the magnitude of variability in subjects’ responses to varying concentrations and durations of exposure to chloropicrin. The concentrations (50 ppb to 1200 ppb) and durations (1-2 seconds to 60 minutes/day for 4 consecutive days) of exposure to chloropicrin varied with each phase of the study. The human irritation study provides concentrations of chloropicrin that are recognized as irritation to eyes but not in the nose or throat and

Page 3 of 15before irritation of the respiratory system. Since recognition of irritation in rodents is obviously not possible, the human study prov ides critical information regarding the recognition of irritation before physical irritation is present. Phase 2 and 3 in the human study evaluated chloropicrin concentrations (0.05 ppm to 0.15 ppm) orders of magnitude below concentrations that produced port-of-entry effects after acute (10.6 ppm), subchronic (1.0 ppm) and chronic (0.5 ppm) exposure in the rodent studies.

The inhalation database of toxicology studies which support the MITC hazard characterization and hazard identification are more limited compared with chloropicrin (Table 1). The inhalation database for metam sodium consists of one subchronic inhalation study. However, there are two subchronic inhalation studies for MITC as well as an eye irritation and odor threshold goggle study in human subjects with MITC.

Table 1. Comparison of animal and human data for MITC and chloropicrin,

MITC Chloropicrin

Animal

Using toxicity information from MITC, metam sodium, metam potassium, dazomet to bridge among chemicals for the oral route No bridging to other fumigants

Limited temporal and dose-response data (only from acute lethality studies) Temporal data from minutes to years in animals

and seconds to minutes in humans Doses/concentrations from 100 ppb to 11 ppm

Incomplete animal database via inhalation -one subchronic inhalation (metam sodium) -two subchronic inhalation & acute lethality (MITC) Robust animal database via inhalation - acute (including RD 50 )

- subchronic - developmental & reproduction (rat & rabbit) - reproductive

- chronic/cancer -mutagenicity

No data to determine susceptibility Developmental and reproduction data demonstrate no susceptibility

Data comparing local vs. systemic data are incomplete Robust local vs. systemic data - good port of entry information (local)—eye recognition and severity eye irritation scores in humans, nose and respiratory parameters -good identification of systemic effects—body weight loss at high concentrations in animals

Human

Human Goggle Study Human Inhalation Study (Chamber)

Eyes only; No nose, throat, respiratory parameters, physiological parameters Eyes, nose, throat, respiratory, physiological parameters

Page 4 of 15No data to evaluate relative sensitivity of eyes to other targets Eyes the most sensitive target, No severe eye, nose, or respiratory irritation at 150 ppb for one hour/day for 4 consecutive days -eye recognition at 100 ppb (40-60 minutes) and 150 ppb (20-30 minutes)

Don’t know day to day carry over effects, not examined in human study Identified no day to day carry over effects, each day a new day

I. Chloropicrin: Mode of action and Intra-species factor

A. Mode of Action

The first step in consideration of refinement of uncertainty factors is the characterization of the mode of action. The mode of action framework, based in large part on the Bradford-Hill criteria for determining causality in epidemiology studies, describes a 9 step framework for evaluating mode of action using animal data. For chloropicrin, the mode of action is sensory irritation, a well documented mode of action. As a sensory irritant, chloropicrin stimulates the trigeminal nerve which mediates sensations in the nose, eyes, throat, and upper respiratory tract. As such, chloropicrin is detected at concentrations that are lower than would be expected to result in port-of-entry related toxic effects (Alarie, 1973, Buckley et al., 1984, Shusterman 2002). Responses from a sensory irritant are distinct from that of chemical stimulation (specialized olfactory or taste receptor cells). Irritation responses from a sensory irritant may include localized and transient chemosensations such as burning, itching, and stinging, as well as associated physiologic (e.g., secretory) responses resulting from surrounding tissues (e.g., eyelid closure, inflammation, etc.). This is a local surface reactivity type of effect which is distinctly different from a systemic type of effect. The parent compound is the key stimulant of the trigeminal nerve and metabolism is not required to produce the irritation response.

1. Key events: Both the rodent database and human study for chloropicrin supports the key events in sensory irritation as the mode of action. Figure 1 below outlines the key events of chloropicrin. Acute air or external exposure to chloropicrin will produce sensations in the eyes through the triggering of the trigeminal nerve. Triggering of the trigeminal

nerve is a surface reaction from air concentrations such that an internal dose is not required for the chemosensation. Triggering of the trigeminal nerve was evident in the chloropicrin human study (MRID 46443801) and quantification of the chemosensations from sensory irritants like that produced from chloropicrin vapor has been determined in mice (RD 50 concentration) (Hoffman et al., 1999; MRID 451178901; Buckley et al., 1984).

Page 5 of 15Figure 1. Key events in the MOA of chloropicrin

Acute Air Chloropicrin Exposure
↓ Trigeminal Nerve Stimulation
↓ Eye Detection (Feeling)
↓ Eye Irritation

2. Dose-Response: Table 2 below highlights the dose-response relationship of the key events highlighted in Figure 1. From this table it is apparent that concentration and duration of exposure are important in the toxicity of the target tissue, onset of effect, and severity of the irritation. As the concentration increases the severity of the port-of-entry effect also increases. Acutely, eye effects are more sensitive than nose (>12x) or lung (>100x).

Page 6 of 15Table 2. Dose-response and temporal concordance of key events from chloropicrin exposure

Key Event NOAEL LOAEL Timing Reference

Eye Recognition Human 300 ppb EC50 242 ppb seconds

25 seconds ACGIH 2001

MRID 46443801

Eye Recognition 73 ppb Human 150 ppb, no eye or respiratory irritation 20-30 minutes MRID 46443801

Odor Recognition Human EC50 406 ppb Median 356 ppb 1 second MRID 46443801

Nasal Pungency Human

No EC50 or median > 1200 ppb 7 seconds MRID 46443801

Respiratory

Irritation Rat 2.34 ppm

RD 50 4 hours MRID 45117901

Respiratory

Irritation 400 ppb 1,200 ppb rat & rabbit developmental:

mortality, nasal staining 6 hrs/day GD 7-29 (23 days) MRID 42740602/

42740601

Respiratory

Irritation 300 ppb 1,000 ppb

13-week rat & mouse:
⁭ lung weights, histopath of nose and lungs 6 hrs/day 5 days/week

13 weeks MRID 43063201

Respiratory

Irritation 100 ppb 500 ppb

chronic mouse:
⁭ lung weights, histopath of nose, upper respiratory, and lungs 6 hrs/day 5 days/week

108 weeks MRID 43632201

3. Temporal Association: Regarding temporal association, it is apparent from the data in Table 2 that the timing or the onset of the key events is clearly identified in animals and humans. It is clear that chloropicrin at low concentrations (50 – 150 ppb) for up to one hour evokes trigeminal nerve sensations in the eyes without producing nasal or respiratory irritation. As concentration and duration of exposure increases sensations in the eyes evolves into eye irritation as well as sensations in the nose followed by nasal irritation. With continued chloropicrin exposure eye and nose irritation will develop into respiratory irritation. Respiratory irritation of the extrathoracic and pulmonary regions was evident in rodent studies with exposures of 6 to 8 hours per day for 30 days to 2 years. The rodent studies, however, can not provide the timing

Page 7 of 15of recognition of irritation. The human study evaluates concentrations and duration of exposure that are recognized by human subjects without producing adverse eye, nasal, or respiratory irritation. For example, a chamber concentration of 150 ppb for one hour produced eye recognition within 20-30 minutes without severe eye or nose irritation and no respiratory irritation. Respiratory irritation was quantified by a clinical exam of the eyes, nose, and throat, as well as pulmonary function testing including FEV 1 (Forced Expiratory Volume) and FVC (Forced Vital Capacity), rhinomanometry, and nasal cytology. In addition, an assessment was performed based on ocular cytology from cell samples taken from the conjunctival membrane inside the lower eyelid and from the concentration of exhaled nitric oxide sampled from the lung (eNO) and nose (nNO). The rodent database does demonstrate, however, that increased chloropicrin concentrations can produce irritation within several minutes (1 hour) as in the RD 50 mouse and acute rat inhalation study. Concentrations in the human study are orders of magnitude lower than

concentrations evaluated in the rodent studies.

4. Strength, consistency, and specificity of association of key events: The rodent database for chloropicrin demonstrates both the dose-response and temporal relationship of portal-of-entry effects. For example, congestion, bronchiole mucosal edema, necrosis and death were observed within 2 days following a 4 hour exposure to 10.6 ppm (10,600 ppb) in the acute rat inhalation study. Histopathological lesions of the nasal cavity and lungs were observed after exposure for 6 hours/day, 5 days/ week for 13 weeks at 1.0 ppm (1,000 ppb) in the subchronic inhalation studies while severe rhinitis of the anterior nasal cavity were observed at 0.5 ppm (500 ppb) in the chronic rat inhalation study. The robust database for chloropicrin supports the key events outlined in Figure 1. Furthermore both the concentration and duration of exposure are important in the onset of events and the time required between key events. As for the human study, chloropicrin clearly may evoke the sensation of feeling without producing irritation. Four consecutive days of 1 hour exposures failed to produce eye or nose irritation such that recovery was immediate and with no day to day carryover effects. The feeling of chloropicrin in the eyes stopped or recovered with the cessation of the exposure. The point of departure (0.073 ppm) is based on the most sensitive subpopulation of the human population for sensory irritants. The point of departure is 2-fold lower than the concentration at which the most sensitive humans were capable of consistently detecting chloropicrin (0.15 ppm) in the eyes without severe eye, nose, throat, or respiratory irritation. All subjects had normal physiological parameters one hour after exposure. The point of departure (0.073 ppm) is 145-fold lower than the acute rodent inhalation study (10.6 ppm), 14-fold lower than irritation produced in repeat inhalation rodent studies (1.0 ppm) and 14-fold lower than acute eye and respiratory effects observed in incident reports (1.0 ppm).

5. Biological plausibility and coherence: This section of the MOA framework

outlines whether the MOA is consistent with what is known about the endpoint. For chloropicrin, the temporal concordance and concentration

Page 8 of 15dependence of eye irritation is consistent with the MOA and is biologically plausible. Coherence is addressed by evaluating both the animal and human data for the target tissue. Consistent port of entry effects are apparent in the rodent and human studies. Recognition of eye sensations is not possible in animals and therefore is addressed in the human study. Furthermore, chloropicrin behaves similarly to another sensory irritant such as carbon dioxide (CO 2 ) (Stevens and Cain 1986; Kjaergaard et al 1992; Shusterman et al 2001; Frasnelli and Hummel 2003; Shusterman et al 2003). As with chloropicrin, CO 2 sensitivity decreases with age.

6. Uncertainties, inconsistencies, and data gaps: Chloropicrin has a robust database, however, there are data gaps regarding the concentration and onset of key events beyond a 1 hour exposure in humans. This is due to the scientific and ethical considerations in evaluating humans in a laboratory setting. Concentrations producing severe irritation are not appropriate and therefore data for concentrations above 150 ppb are not available. Information regarding key events beyond 1 hour of exposure would be helpful in acute risk assessment. However, the current temporal concordance data suggest that as duration of exposure continues the sensation of eye feeling would not progress unless the concentration also increased. Current data suggest that as concentrations increase the onset and severity of irritation also increases or occurs earlier. Therefore, it would be expected that an exposure beyond one hour at currently evaluated concentrations (50-150 ppb) would not produce irritation. This is also supported by animal inhalation studies in which 6 hours of exposure per day for 13 weeks do not produce respiratory irritation up to 300 ppb.

B. Intra-species Factor—Using chemical-specific information to refine the factor

The IPSC Guidance document was published by WHO in 2005. This CSAF guidance describes approaches for use of kinetic and mechanistic data to refine inter-species and intra-species extrapolation factors. U nderstanding mode of action is an important component of deriving CSAFs/DDEFs in that MOA provides the foundation for understanding what toxicokinetic and/or toxic odynamic characteristics are critical for evaluating the intra-species factor. As described above, the chloropicrin MOA is well

understood.

When deriving data derived factors, the intra-species factor is separted into toxicokinetic and toxicodynamic components (UF HK and UF HD , respectively). Toxicokinetics (TK) is defined as the determination and quantification of the time course and dose dependency of absorption, distribution, metabolism, and excretion of chemicals (sometimes referred to as pharmacokinetics or ADME). Based on to xicokinetic characteristics, susceptibility is due to higher target tissue concentrations of the toxicant. Based on the mode of action of chloropicrin as a sensory irritant the target tissue is the eye, or stimulation of sensation in the eyes. Eye effects are more sensitive than nose (>12x) or lung (>100x). As demonstrated in the human study, sensation in the eyes is the most sensitive target. Air is the pathway of exposure where surface reaction with the trigeminal nerve evokes sensations in the eyes. Internal dose or tar get tissues are not a concern for chloropicrin since eyes are stimulated by surface concentrations. Therefore, based on the lack of

Page 9 of 15ADME or the toxicokinetics for chloropicrin, the TK component of the intra-species factor (ie, UF HK ) may be reduced to 1X

.

Toxicodynamics (TD) is defined as the determination and quantification of the sequence of events at the cellular and molecular levels leading to a toxic response. Three main questions are outlined in the IPCS guidance document for the determination of the adequacy of the experimental data for refinem ent of the toxicodynamic component.

1. Relevance of population: The human subjects evaluated in the chloropicrin human study are considered to be the most sensitive subpopulation for sensory irritants. Participants in the human study were between the ages of 18 and 35 (average 23 years). For age, sensory perception (both nasal and eye) is reported to decrease with increasing age (Kjaergaard et al., 1992; Frasnelli and Hummel, 2003; Hummel et al., 2003; Shusterman, 2003; Doty et al., 2004). The human subjects were exposed to the active parent compound via inhalation such that chloropicrin directly produced sensations in the target tissue (eyes). The severity of eye irritation was quantified as well as the response of the nasal and respiratory system. The eyes were clearly the most sensitive target with concentrations up to 150 ppb failing to produce lasting irritation in the eyes and no irritation in the nose, throat, or respiratory system. Therefore, the subjects evaluated in the chloropicrin human study are not only representative of all relevant age groups including both sexes, but the most sensitive subgroup. Furthermore, CO 2 , a sensory irritant, was tested in 158 subjects, including 1 individual with asthma and 9 others with hayfever. CO 2 proved no more sensitive in the upper airways and eyes during nonsym ptomatic periods in these subjects nor did they differ from other subjects in their response to CO 2 (Kjaergaard et al 1992). Likewise, subjects with nasal allergies were similar to individuals of the same age to nasal detection to CO 2 (Shusterman et al 2003). Finally, chloropicrin incident reports support consistent eye irritation across ages (3 months to 63 years) for chloropicrin (1982-2002, D306838; 2002-2004, D305336).

2. Adequacy of concentration-response data: All three phases of the human study provide timing of effects and the most sensitive target, namely the eyes. Specifically, Phase 3 of the human study provides adequate dose-response data

for an acute assessment such that a BMCL 10 which was performed on eye irritation data. The timing or onset of the eye irritation was also demonstrated such that 100 ppb took almost 60 minutes before recognition while 150 ppb produced sensations in the eyes within 20-30 minutes. The results of the human study correspond with the available litera ture regarding lower concentrations of chloropicrin. The data from both the human and animal data suggest that both concentration and duration of exposure are key components in the developmentof irritation, the onset of irritation, and the severity of the irritation. Concentrationresponse data for both human and animal data are outlined in Table 2.

3. Adequacy of number of subjects/samples: The chloropicrin human study evaluated a total of 127 subjects with some subjects participating in more than one phase. This provides sufficient information for the most sensitive subpopulation for sensory irritants. The design of the study was based on the MOA and biochemical basis for this subgroup of individuals. The dose-response

data from Phase 3 were utilized in the BMCL 10 of 73 ppb. There was variability in

Page 10 of 15the ability of these sensitive subjects to detect chloropicrin. This suggests that individuals less sensitive to the feeling of chloropicrin would not be capable of detecting at such low concentrations. At this low concentration, no irritation is produced since concentrations are not high enough to achieve surface reactions in the nose, throat, or respiratory system. The BMCL 10 of 73 ppb is below a concentration detectable by the general population and therefore protective. Furthermore, there no susceptibility concerns since the developmental and reproductive rodent studies failed to identify any susceptible life stages.

Incident data also provide key information to the toxicodynamics of chloropicrin. A report issued by the California Department of Pesticide Regulation (CDPR) and the Kern County Agriculture Commissionare (KACA) describes an investigation into illnesses associated with the offsite drift of chloropicrin in Kern County in 2003. Approximately 35 households and a day care center had a total of 172 persons present during the exposure period. A total of 165 persons experienced symptoms consistent with chloropicrin exposure. Age groups ranged from 3 months to 63 years of age. The number of individuals from each age group was similar with approximately 13% (22/165) 0-5 years, 10% (17/165) 6-9 years, 14% (23/165) 10-14 years, 9% (15/165) 15-19 years, 11% (18/165) 20-09 years, 13% (21/165) 30-39 years, 16% (26/165) 40-64 years, and 14% (23/165) age unknown. Nearly all individuals (99%; 163/165) reported symptoms of eye lacrimation or pain/burning, including children. Approximately 50% (85/165) of individuals experienced irritant symptoms of the upper respiratory system such as cough (53/165), dyspnea (27/165), upper respiratory irritation (22/165), chest pain (8/165) or asthma exacerbation (6/165). Exposure concentrations were retrospectively estimated by using a standard air dispersion model which indicated the peak-to-mean concentration may have exceeded 1 ppm (1,000 ppb). This concentration was approximately 20-fold higher than concentrations evaluated in the chloropicrin human study (2005, MRID 46443801).

The symptoms reported in the 2003 Kern County chloropicrin incident are concordant with the sequence and onset of symptoms observed in the human study and animal database for chloropicrin. Specifically, the 2005 Cain human study demonstrates that even at low concent rations the eye is the most sensitive target for chloropicrin. Nearly all individuals in the Kern County incident, both young and old, reported eye irritation. Furthermore, the animal database demonstrates that as concentrations increase the potential for upper respiratory irritation increases. Individuals of the 2003 Kern County incident were exposed to concentrations greater than 20-fold higher (1 ppm or 1,000 ppb) than the Cain 2005 human study (50-150 ppb or 0.05 – 0.15 ppm). As would be expected at these higher chloropicrin concentrations, upper respiratory irritation was reported in approximately 50% of individuals. Asthma exacerbation was only reported for approximately 6 of 165 persons (4%). This exacerbation was likely due to the high chloropicrin concentrations causing upper respiratory irritation. According to the American Lung Association (May 2006), Kern County reported approximately 2.6% pediatric asthma (19,004 children /724,846 total population), 5.3% adult asthma (39,061 adults/ 724,846 total population), and 2.8% chronic bronchitis (20,617 individuals/ 724,846 total population). Therefore, the 4% of individuals reporting asthma exacerbation in Kern County is not above the County average, suggesting that chloropicrin did not induce asthma at a higher rate. Overall, the

Page 11 of 152003 Kern County reports symptoms consistent with those for chloropicrin, namely eye irritation being the most sens itive followed by upper respiratory irritation at higher concentrations. The similar number and type of symptoms among children and the older population suggest that individuals are capable of detecting chloropicrin and respond similarly to chloropicrin.

Based on the known MOA for chloropicrin and the consistency and specificity of eye irritation in both young and old individuals from incident and literature studies, robust dose-response and temporal concordance of eye and irritation effects from the chloropicrin database, the TD component of the intra-species factor (ie, UF HD ) may be reduced to 1X . Therefore, the overall intra-species factor for chloropicrin is reduced to 1X.

III. MITC: Mode of Action and the Intra-Species Extrapolation Factor

Recently, TERA, on behalf of AMVAC, submitted two reports:

• Benchmark concentration modeling on the effects of acute exposure to Methyl Isothiocyanate (MITC). November 2007.

• “Supplemental Report on the Effects of Acute Exposure To Methyl Isothiocyanate (MITC).”

The benchmark concentration (BMC) report pr ovides an analysis estimating benchmark concentration lower limit (BMCL) of 0.20 ppm based on a 4-hour human exposure from the human eye irritation study (Russell and Rush, 1996). The Agency appreciates the submission of this analysis and supports the use of benchmark approaches in risk assessment. The BMCL value provided by TERA of 0.20 ppm is similar to the point of departure of 0.22 ppm being used by EPA. As such, the BMCL analysis supports the Agency’s current approach.

Following the submission of the BMC report, TERA and AMVAC met with EPA to discuss the BMC analysis and their proposal to reduce the intra-species factor to 1X. At that meeting, EPA staff requested additional information, particularly with regard to the mode of toxic action for MITC. In response to this request, TERA developed the second report noted above, herein called the “supplemental report.”

The supplemental report describes a hypothesis for a mode of action by which MITC would affect the trigeminal nerve. Specifically, describes a hypothesis that MITC interacts with a group of cation channels known as the transient receptor potential (TRP) family (see also Bandell, Story, Hwang, et al., 2004). In particular the report suggests that TRPA1 may be the target. The Agency also acknowledges the work by TERA to characterize the human incidence information from California. This work is an important component of the weight of the evidence for MITC.

Since the submission of the above reports, EPA has met with registrants and/or TERA on several occasions, most recently on June 18, 2008. At the June 18, 2008 meeting, Dr. William Cain provided the Agency with a publication by Jordt (2004). This publication includes data which shows MITC as a TRPA1 substrate. The Agency has not yet formally reviewed this study but it appears to provide by confirmation for the hypothesis described in the TERA reports. Based on further review, if this is indeed the case, then the Jordt study provides an important step forward in the hazard identification and characterization of MITC. Dose-response concordance is a critical element in the

Page 12 of 15MOA Framework. Evaluation of dose response of key events and the relative sensitivity of different target organs provides the foundation for deriving points of departure for risk extrapolation and for deriving DDEFs, like that described for chloropicrin above.

In the event that the MITC registrants decide to perform additional studies with MITC, the Agency suggests that the following key guidance/frameworks are consulted. These provide a roadmap for how the Agency will evaluate and ultimately use the mode of

action information.

• Mode of Action Framework 

### as described in U.S. EPA (1999, 2005), SonichMullin et al. (2001), and Meek et al. (2003). This framework, based in large part on the Bradford-Hill criteria for determining causality in epidemiology studies,

describes a framework for evaluati ng mode of action using animal and human data. This framework was used above for chloropicrin. In brief, this framework includes a statement regarding the postulated MOA and related key events followed with a discussion of dose-response and temporal considerations. Specifically, an understanding of the dose-response and relationships of eye and respiratory effects is a key area of consideration for MITC. An important component of any MOA analysis is the summary of strength, consistency, and specificity of key events, toxic effect s, and biological plausibility and coherence. The analysis also considers the likelihood of alternative MOAs. In the case of MITC, relevance the postulated MOA for eye effects to other targets, such as the lung, should be considered.

• IPSC Guidance on Chemical Specific Adjustment Factors (CSAFs, 2005 ) . This guidance describes approaches for use of kinetic and mechanistic data to refine inter-species and intra-species extrapolation factors. The CSAF guidance is based on the principles 1) that TK and TD features of a compound both influence toxicity and 2) these TK and TD features can then be used to better inform inter- and intra-species extrapolation compared with the use of default factors. As stated above, understanding mode of action is an important component of deriving CSAFs in that mode of action provides the foundation for understanding what TK and/or TD characterist ics are critical for evaluating interor intra-species extrapolation. In deriving CSAFs, each MOA–endpoint combination is individually evaluated. Thus, for MITC, based on the results of the dose-response evaluation in the MO A analysis (described above), EPA may develop CSAFs for any relevant effects (e.g., eye and respiratory).

Conclusions The Agency considers all relevant information when performing risk assessments, especially with the fumigant compounds. For chloropicrin, the available data is outlined in the MOA framework and IPCS guidance on CSAFs in consideration of refinement of the intraspecies factor for eye irritation. For MITC, the body of information is growing on mode of action and human incidence. However, at this time, the dose response data needed for a concordance analysis and for DDEF derivation are not available. The Agency encourages the registrants to pursue this information. This information, in combination with qualitative MOA information and the human information, may provide opportunities to, consider refinements to the extrapolation and uncertainty factors which apply to MITC and ultimately to the margins of exposure used to estimate buffer zones.

.

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Buckley et al., 1984. Respiratory tract lesions induced by sensory irritants at the RD50 concentration. Toxicology and Applied Pharmacology. 74: 417-429.

Doty et al., 2004. Assessment of upper respiratory tract and ocular irritative effects of volatile chemicals in humans. Critical Reviews in Toxicology. 34(2): 85-142.

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