w w w.environment-agency.gov.uk CLEA BRIEFING NOTES 1-3 Updates to Report CLR10 SCHO0505BJBB-E-P

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1 w w w.environment-agency.gov.uk CLEA BRIEFING NOTES 1-3 Updates to Report CLR10 SCHO0505BJBB-E-P

2 CLEA BRIEFING NOTE 1 UPDATE ON THE DERMAL EXPOSURE PATHWAY Introduction This note explains the revisions to the way that the CLEA model estimates human dermal exposure to contaminants in soil. The need for this revision of the dermal pathway has been triggered by: a recent public consultation by the USEPA (2001), proposing revisions to its original technical guidance on estimating dermal exposure (USEPA 1992) which is the basis for the current approach in the CLEA model; and preparation work by the Environment Agency to publish further SGV reports for contaminants where dermal exposure is expected to be a significant pathway. The key areas discussed are: use of an adsorbed fraction per event approach to estimating exposure; use of revised soil-to-skin adherence factors; and the values for the maximum fraction of exposed skin. Key principles There are four key principles in estimating dermal exposure using the CLEA model: i) the CLEA model is used to estimate longterm chronic human exposures to contaminants in soil. It cannot be used to derive Soil Guideline Values based on acute or highly localised skin contact exposures. In addition, it does not take into account chronic exposure arising through non-intact skin such as that resulting from damage during such acute exposures; ii) the CLEA model only considers dermal exposure via contaminated soil; iii) the CLEA model estimates dermal exposure as an uptake with units of mass of contaminant per unit bodyweight per day; and iv) in deriving Soil Guideline Values, and in the absence of any clear toxicological information to the contrary, estimated dermal exposure may be compared with health criteria values derived for the oral or inhalation routes where the effects are systemic and not localised. In such cases the estimated uptake via the dermal pathway will be added to intakes via inhalation and ingestion to estimate total exposure. This briefing note discusses the approach used in the current CLEA model (CLR10; Defra and Environment Agency 2002a) and the updated site-specific version of the model which will be available later this year. CLR10 identifies three important questions to be addressed when estimating human exposure to soil contaminants via the dermal pathway: 1) How much contaminated soil comes into contact with exposed skin? The answer to this question depends on many factors including the exposed skin area, the type of activity involved such as digging or playing, the duration of initial contact, and the texture/wetness of the soil. Few studies have estimated soil loading on human skin. The CLEA model addresses this question in two ways: i) in qualitative and quantitative terms by estimating the maximum area of exposed skin from assumptions about the types of clothing worn by adults and children during typical activities such as gardening and playing; and ii) in estimating the degree to which soil adheres to skin after contact. 2) How much contamination does the skin absorb by soil contact? This is affected by the composition and thickness of the skin layer, the type of contaminant being evaluated, the concentration of the contaminant in soil and the soil type. Page 1 of 9

3 CLEA Briefing Note 1: Version 1.1 (March 2005) Table 1: Summary of the updates to the dermal exposure pathway How much contaminated soil comes into contact with exposed skin? How much contamination does the skin absorb through contact with soil? Current CLEA model approach, CLR10 (Defra and Environment Agency 2002) Assumes that; - For young children the hands, forearms and lower legs are exposed; and - for older children and adults the assumption is that hands and forearms are exposed. Average area of exposed skin is approximately one-third of the maximum exposed area. Adopts the reasonable worse case soil-to-skin adherence factor value of 1 mg.cm -2 for adults and children, indoors and outside. The rate at which a chemical penetrates the skin (as represented by a skin permeability coefficient) is estimated from the experimental permeability in either water or a solvent vehicle in the absence of experimental coefficients for soil (often the case). Where no experimental skin permeability coefficient is available for aqueous solution, this is predicted using a relationship between the molecular weight and the log K ow. Generic application of Enrichment Factors (EFs) for dermal exposure for different soil types. Updated CLEA model approach Assumes that; - for young children the hands, forearms, lower legs, face and feet* are exposed - for older children and adults the assumption is that hands, forearms, face and feet* are exposed. *The feet are only included for indoor assessments and assumed to be covered outdoors. For the standard residential and allotment scenarios the qualitative description of the adult gardener has been broadened to include possible exposure of the face, hands, forearms and lower legs. In the standard commercial/industrial land-use, the typical office worker will be exposed only by hands and face. The revised maximum exposed skin fractions and the assumed exposed skin area at various percentiles are presented in Table 2 and appendix 1 respectively. Note that the proportion of exposed skin according to body parts is estimated separately for male and female adults. Differentiates between soil-to-skin adherence factors for indoor and outdoor exposure. An indoor factor of 0.06 mg.cm -2 is used for ages 0 to 16. For outdoor exposure a value of 1 mg.cm -2 is used. There is a lack of underpinning experimental data to adequately quantify the variation in soil to skin permeability rates for many chemicals and soil properties. Therefore an absorbed fraction per event is used. The dermal absorption fraction (ABS d ) is chosen on a contaminant-by-contaminant basis. Where there is an absence of quantitative date for a substance for which dermal exposure may be important a default estimate of 0.1 for the dermal absorption fraction (ABS d ) is used rather than exclude it from exposure estimates. Equation 6.7 in CLR10 (DEFRA and Environment Agency 2002a) has been made (see main text above). In addition, Equation 6.8 and 6.9 have been deleted. Enrichment Factors (Efs) considered on a substance-by-substance basis, depending on the supporting studies. Page 2 of 9

4 CLEA Briefing Note 1: Version 1.1 (March 2005) How long does soil remain in contact with the skin? Assumes a contact duration of 12 hours, the parameter is used directly to estimate the average absorbed contaminant dose per event. The contact duration is established on a substance by substance basis depending on the relevant experimental conditions used to estimate the dermal absorption fraction. If relevant and authoritative time-dependent data is available then the dermal absorption fraction value for a contact time of 12 hours is chosen as the default for that substance. 3) How long does the soil remain in contact with the skin? This is the amount of time, from first contact, that the soil or indoor dust remains adhered to the skin before it falls away naturally or is removed by washing (USEPA, 1992). Summary of main changes to the dermal exposure pathway in the CLEA model Table 1 provides a summary of the main changes to the dermal exposure pathway within the CLEA model. Note that these changes do not have a significant impact on existing Soil Guideline Values. Current CLEA approach for assessing dermal exposure to contaminants in soil How much contaminated soil comes into contact with exposed skin? Although research continues to address these two issues, there is still a high degree of uncertainty associated with each of them (USEPA 2001). There is increasing evidence that while clothing reduces potential exposures it does not eliminate them (USEPA 2000). However, in accordance with USEPA (2001) it is recommended that clothing is assumed to be protective of soil contact. The CLEA model assumes that for young children the hands, forearms and lower legs are exposed (DEFRA and Environment Agency 2002a). For older children and adults the assumption is that hands and forearms are exposed. These qualitative descriptions are used to estimate the maximum fraction of exposed skin. In order to account for variations in activity, the maximum area of exposed skin that is soiled is treated probabilistically. The shape of this distribution results in the average area of exposed skin being approximately one-third of the maximum exposed area and is intended to take account of the likely variations in soil contact according to different activities. Exposure to soil contamination via the dermal pathway is particularly sensitive to the amount of soil adhered to, or in intimate contact with, the skin over the contact period (DEFRA and Environment Agency 2002a). Despite a number of recent studies, there is still considerable uncertainty in the soil-to-skin adherence factors used to assess dermal exposure (USEPA 2001). Not surprisingly, soil loading is found to be highly variable. The CLEA model adopted the reasonable worse case value proposed by USEPA (1992) of 1 mg.cm -2 for adults and children, indoors and outside. It should be noted that the values in the CLEA model for the soil-to-skin adherence factor are significantly lower than estimates of the adherence factor at mono-layer saturation for different soil types (USEPA 2001). How much contamination does the skin absorb through contact with soil? USEPA (1992) have proposed a predictive model to estimate dermal absorption of a substance from soil through the skin and the CLEA model includes this approach. This is described in detail in CLR 10 (DEFRA and Environment Agency 2002a). Key points are summarised below: the chemical is assumed to diffuse through the skin based on the steady-state assumptions of Fick s first law and the rate at which a chemical penetrates the skin (as represented by a skin permeability coefficient); for a mono-layer of soil in contact with the skin, the skin permeability coefficient for a contaminant in soil can be estimated from the experimental permeability in either Page 3 of 9

5 CLEA Briefing Note 1: Version 1.1 (March 2005) water or a solvent vehicle in the absence of experimental coefficients for soil (often the case). Where no experimental skin permeability coefficient is available for aqueous solution, this is predicted using a relationship between the molecular weight and the log K ow ; and mass balance of the contaminant is maintained by taking into account the reduction in soil concentration over the contact time caused by dermal absorption and volatilisation. There are major uncertainties in our understanding of the extent to which a chemical is absorbed by the skin and in the extent to which it partitions from soil to skin (DEFRA and Environment Agency 2002a; USEPA 1992 and 2001). The CLEA model, therefore, proceeds carefully in assessing dermal exposures using the limited experimental data to make decisions on a case-by-case basis. This approach has already been accepted in the case of assessing plant uptake of inorganic contaminants where data is evaluated on a chemical by chemical basis in the absence of a well-founded unifying model (DEFRA and Environment Agency 2002a). How long does soil remain in contact with the skin? The CLEA model assumes a contact duration of 12 hours and the parameter is used directly to estimate the average absorbed contaminant dose per event (DA event ) in equation 6.7 in CLR 10 (DEFRA and Environment Agency 2002a). Updated CLEA approach for assessing dermal exposure to contaminants in soil How much contaminated soil comes into contact with exposed skin? The estimation of maximum exposed skin areas has been revised in the CLEA model. In the case of children the face and the feet are two notable body areas currently outside of the qualitative exposed area. Given the importance of hand-to-mouth contact in exposure via inadvertent soil ingestion this body area is now included for both adults and children. The feet have now been included, but only for indoor assessments at this stage, and in the absence of UK specific data the feet are assumed to be covered outdoors and uncovered indoors for both adults and children. For both the residential, and in particular, the allotment land-use it is prudent to assume that gardening is a typical activity for adults. In such cases, assuming that only the hands and face are likely to be exposed is not considered to be a reasonable worst-case, especially in the summer months. Therefore, for the standard residential and allotment scenarios the qualitative description of the adult gardener has been broadened to include possible exposure of the face, hands, forearms and lower legs. In the standard commercial/industrial land-use, the typical office worker will be exposed only by hands and face. The revised maximum exposed skin fractions are presented in Table 2. They have been calculated using more recent estimates of the surface area of body parts (USEPA 1997 and 2000) than originally used. Note that the proportion of exposed skin according to body parts is estimated separately for male and female adults. The exposed skin area continues to be treated probabilistically in the CLEA model as described earlier. Appendix 1 presents revised total skin and exposed skin area data, which supersedes Table 5.8 in CLR10. Exposure to soil contamination via the dermal pathway is particularly sensitive to the amount of soil adhered to, or in intimate contact with, the skin over the contact period (DEFRA and Environment Agency 2002a). Despite a number of recent studies, there is still considerable uncertainty in the soil-to-skin adherence factors used to assess dermal exposure (USEPA 2001). Not surprisingly, soil loading is found to be highly variable. USEPA (2000 and 2001) have undertaken a further review of soil-to-skin adherence factors and have revised their opinion on the recommendations made in USEPA (1992). In considering whether the soil-to-skin adherence factors in the CLEA model should be updated we examined the available data to: identify typical contact activities and use reasonable worst-case assumptions (that is, a high-end weighted value from the available studies); and identify a reasonable worst-case activity and use typical assumptions (that is, a central tendency weighted value from the available studies). Page 4 of 9

6 Table 2: Estimating the maximum exposed skin area using the CLEA model current and new options Current New outdoors / indoors Age (children and female adults) * Exposed body parts Max fraction exposed skin Exposed body parts Max fraction exposed skin / Assumes hands, 0.22 Assumes face, 0.26 / forearms and 0.22 hands, forearms 0.25 / lower legs 0.23 and lower legs 0.28 / exposed 0.24 exposed 0.28 / / Assumes hands 8 9 and forearms exposed Assumes face, hands and forearms 0.15 / exposed Assumes only hands exposed 0.05 Assumes face, hands, forearms and lower legs exposed 0.26 / 0.33 * Note that for the standard commercial / industrial land-use, adults in the age range years are assumed to expose only their face and hands with a corresponding maximum fraction of exposed skin of The revisions to the soil-to skin adherence factors (mg of soil per cm 2 skin area) in the CLEA model are presented in Table 2. They have been estimated using the body part weighted average approach set out in USEPA (2001). That is, the average soil-to-skin adherence factor for children and adults reflects the average of the experimental factors determined for each body part (such as hands, face, lower leg etc) taking into account the proportion of the total exposed area accounted for by each body part. Qualitatively, the assumed exposed body areas have been described in Table 2. The new values for the soil-to-skin adherence factor in Table 3 are significantly lower than estimates of the adherence factor at monolayer saturation for different soil types (USEPA 2001). For children, the CLEA model now differentiates between soil-to-skin adherence factors for indoor and outdoor exposure. An indoor factor of 0.06 mg.cm -2 is now used for ages 0 to 16 and is based on the 95 th percentile of studies by Holmes et al (1999) and USEPA (1997). The value chosen is consistent with the recommendation in USEPA (2001). For outdoor exposure we have not revised the current soil-to-skin adherence factor from that recommended by USEPA (1992), that is, for outdoor exposures a value of 1 mg.cm -2 will continue to be used which is considered conservative in respect of exposure to dry soil and protective of above average exposures to wet soil. Table 3 : New soil-to-skin adherence factors for adults and children according to land-use Land-use Soil-to-skin adherence factor for children (0 to 16 years) Soil-to-skin adherence factor for adults (16 to 70 years) mg soil per cm2 skin Mg soil per cm2 skin Residential indoors Residential outdoors Allotments Commercial / industrial Page 5 of 9

7 CLEA Briefing Note 1: Version 1.1 (March 2005) This is a more conservative value than that proposed by the USEPA (2001) of 0.2 mg.cm -2 which is the geometric mean of observed soil adherence factors. In using the geometric mean and not the 95 th percentile value of 3.3 mg.cm -2 from the same study, the USEPA conclude that this was at the high-end of soil contact activity (USEPA 2001). For adults, gardening is assumed to be a typical activity for both the residential and allotment land-uses and therefore the 95 th percentile of a study of gardeners by Holmes et al (1999) has been chosen as a revised soil adherence factor. This value of 0.3 mg.cm -2 is higher than that recommended by USEPA (2001) of 0.07 mg.cm -2. While in both cases the value is derived from Holmes et al (1999), the proposed value for the CLEA model assumes that gardening is a typical activity while the USEPA concluded that it is a highend activity. No indoor studies for adults have been identified by this review other than for specialist groups such as those undertaking Tae Kwon Do (Kissel et al 1996). Therefore, the child soil-to-skin adherence factor has been used. It is recognised that this is likely to be a conservative assumption for adults, albeit much less so than the current default value of 1 mg.cm -2. For the commercial and industrial setting an adherence factor of 0.14 mg.cm -2 has been chosen for indoor and outdoor exposure, which is the geometric mean of weighted adherence factors (excluding feet) from a study of grounds keepers by Kissel et al (1996). There is an absence of relevant data for the typical office worker identified in CLR 10 (DEFRA and Environment Agency 2002a) who may spend their lunch-hour outdoors and it is unlikely that the activities undertaken by groundskeepers in campus grounds or an arboretum are typical scenarios. However, it is considered more prudent to use the 95 th percentile of this data since Kissel et al (1996) reported that the grounds keepers studied intermittently used gloves an unlikely occurrence for the typical office worker on their lunch-break. It is recognised that this assumption is tentative and almost certainly conservative but it is more appropriate than using the current default of 1 mg.cm -2. USEPA 1992 and 2001). The CLEA model, therefore, proceeds carefully in assessing dermal exposures using the limited experimental data to make decisions on a case-by-case basis. USEPA (2001) recognises the need for simplified calculations at the generic screening level and the lack of underpinning experimental data to adequately quantify the variation in soil to skin permeability rates for many chemicals and soil properties. An absorbed fraction per event approach provides a good balance to these two factors and would therefore represent the preferred approach. However, USEPA (2001) recognises that the use of experimental skin permeability coefficients (K ps ) derived specifically from soil is an alternative approach. There may be instances where appropriately-conducted substance-specific studies have derived K ps, rather than dermal absorption fractions. In such instances, the approach in deriving Soil Guideline Values may therefore be reevaluated on a case-by-case basis. The following revision to equation 6.7 in CLR10 (DEFRA and Environment Agency 2002a) has been made. In addition, Equations 6.8 and 6.9 have been deleted. How much contamination does the skin absorb through contact with soil? There are major uncertainties in our understanding of the extent to which a chemical is absorbed by the skin and in the extent to which it partitions from soil to skin (DEFRA and Environment Agency 2002a; Page 6 of 9

8 CLEA Briefing Note 1: Version 1.1 (March 2005) Equation 6.7 DA event = C soil AF ABS d 10 6 kg mg Where: DA event is the dermal absorbed dose per soil contact event (mg.cm -2.event -1 ) C soil is the chemical concentration in soil (mg.kg -1 ) AF is the adherence factor of soil to skin (mg.cm -2.event -1 ) ABS d is the dermal absorption fraction (dimensionless) The dermal absorption fraction (ABS d ) will be chosen on a contaminant-by-contaminant basis on review of the scientific literature. It should be noted that the USEPA (2001) recommends dermal absorption fractions (ABS d ) for only ten substances based on their review of experimental data, reflecting a wider paucity of data for many organic and inorganic chemicals. Where there is an absence of quantitative date for a substance for which dermal exposure may be important CLEA employs a default estimate of 0.1 for the dermal absorption fraction (ABS d ) rather than exclude it from such exposure estimates. This default value is consistent with the USEPA view, however in CLEA the value is used for both volatile and semi-volatile organic chemicals since the dermal exposure pathway is considered when it contributes to more than 1% of exposure (USEPA consider the dermal pathway when it contributes to more than 10% of exposure). Currently CLR10 describes the generic application of Enrichment Factors (EFs) for dermal exposure for different soil types. USEPA (2001) comment that, except in the case of dioxins, there is insufficient information to relate dermal absorption fractions to soil type. In particular, variations in the dermal absorption fraction (ABS d ) because of experimental differences arising from soil type can be taken into account in the final value chosen. When deriving Soil Guideline Values the use of EFs will, therefore, be considered on a substance-by-substance basis, depending on the supporting studies. How long does soil remain in contact with the skin? This is the amount of time, from first contact, that the soil or indoor dust remains adhered to the skin before it falls away naturally or is removed by washing (USEPA, 1992). There is limited data that suggests that absorption of a chemical from soil depends on time. However, the USEPA (2001) have concluded that there is insufficient information to determine whether the effect on absorption is linear or non-linear. For residential settings the USEPA has previously suggested a contact duration related to the period between washings in the range of 8 to 24 h (USEPA 1992). USEPA (2001) suggest a period of 24 hours in order to be consistent with the experimental conditions from which the chemical specific dermal absorbed fraction (ABS d ) are derived. For occupational settings Paustenbach (2000) suggests a period of 4 h. The new approach using the dermal absorption fraction (ABS d ) does not explicitly use contact time. However, contact time is clearly important in establishing ABS d on a substance by substance basis according to experimental conditions. Therefore the contact duration will be established on a substance by substance basis depending on the relevant experimental conditions used to estimate ABS d. If relevant and authoritative time-dependent data is available then the ABS d value for a contact time of 12 hours will be chosen as the default for that substance. Implications for estimating exposures arising from arsenic and cadmium contamination in soil SGV reports have been published for both arsenic and cadmium (DEFRA and Environment Agency 2002b and 2002c). It should be noted that in both cases exposure via the dermal route was excluded on the basis of the discussion of toxicological effects in the relevant TOX reports (DEFRA and Environment Agency 2002d and e). It is however considered prudent to examine the implications for exposure of the consultation guidance issued by the USEPA (2001) which recommended dermal absorption fractions for both arsenic and cadmium. The CLEA model was modified to include the revised approach set out in this note. Chemical specific values of 0.03 and for the dermal absorption fraction (ABS d ) were selected from USEPA (2001) for arsenic and Page 7 of 9

9 CLEA Briefing Note 1: Version 1.1 (March 2005) cadmium respectively. The preliminary ADE was then estimated for each of the standard land-uses set out in CLR10 (DEFRA and Environment Agency 2002a) including the dermal pathways. Only in the case of arsenic was the combined indoor and outdoor dermal exposure significant (> 1%), contributing in the order of 5% to the mean of the total exposure from all pathways. In the case of cadmium, soil ingestion was still the most important pathway. At this contribution level, the inclusion of dermal exposure does not have a significant impact on existing Soil Guideline Values. References DEFRA and Environment Agency (2002a) The Contaminated Land Exposure Assessment Model (CLEA): Technical basis and algorithms. R&D Publication CLR10. DEFRA and Environment Agency (2002b) Soil Guideline Values for Arsenic Contamination. R&D Publication SGV1. DEFRA and Environment Agency (2002c) Soil Guideline Values for Cadmium Contamination. R&D Publication SGV3. RIVM Report (2001) Evaluation and revision of the CSOIL parameter set. Proposals for updating the most relevant exposure routes of CSOIL. Rikken, MGJ; Lijzen, J.P.A; and Cornelese, A.A. Spence LR and Walden T (2001) Risk Integrated Software for Clean-ups. User Manual Version 4.0. USEPA (1997) Exposure Factors Handbook. Update to Exposure Factors Handbook. EPA/600/8-89/043 - May 1989 USEPA (1992) Dermal Exposure Assessment: Principles and Applications. Report EPA/600/8-91/011B. USEPA (2000) Child-Specific Exposure Factors Handbook. Report NCEA-W USEPA (2001) Risk Assessment Guidance for Superfund. Volume I: Human Health Evaluation Manual (Part E, Supplemental Guidance for Dermal Risk Assessment) Interim. Review Draft for Public Comment. Report EPA/540/R/99/005 DEFRA and Environment Agency (2002d) Contaminants in Soil: Collation of Toxicological Data and Intake Values for Humans. Cadmium. R&D Publication TOX 3. DEFRA and Environment Agency (2002e) Contaminants in Soil: Collation of Toxicological Data and Intake Values for Humans. Arsenic. R&D Publication TOX 1. Holmes KK, Shirai JH, Richter KY and Kissel JC (1999) Field measurement of dermal soil loadings in occupational and recreational activities. Environmental Research Kissel JC, Richter KY and Fenske RA (1996) Field measurement of dermal soil loading attributable to various activities: implications for exposure assessment. Risk Analysis 16(1) McKone TE and Howd RA (1992) Estimating dermal uptake of non-ionic organic chemicals from water and soil: I. Unified fugacity-based models for risk assessments. Risk Analysis 12(4) x y Paustenbach DJ (2000) The practice of exposure assessment: a state-of-the-art review. Toxicology and Environmental Health, Part B Page 8 of 9

10 CLEA Briefing Note 1: Version 1.1 (March 2005) Appendix 1 Revised total skin and exposed skin area data Values relevant to the critical receptors for the standard land-uses are shaded, and other values are shown for reference purposes only. Male Female Exposed skin area b (residential c ) Exposed skin area b Total skin area a Exposed skin area b (residential c ) Exposed skin area Age b Total skin area a (cm 2 ) (commercial) (cm 2 ) (m 2 ) (cm 2 ) (commercial) (cm 2 ) (m 2 ) Indoor Outdoor Indoor Outdoor Indoor Outdoor Indoor Outdoor (0.4, 0.46) 424 (395, 858) 345 (324, 687) 0.38 (0.38, 0.43) 400 (377, 798) 325 (303, 648) (0.52, 0.59) 566 (530, 1137) 446 (417, 897) 0.49 (0.49, 0.57) 542 (507, 1084) 426 (398, 844) (0.61, 0.70) 646 (605, 1286) 504 (470, 1008) 0.61 (0.61, 0.69) 646 (606, 1291) 505 (474, 997) (0.68, 0.78) 795 (744, 1581) 636 (591, 1274) 0.67 (0.67, 0.77) 776 (732, 1554) 620 (578, 1244) (0.75, 0.86) 870 (816, 1740) 697 (651, 1398) 0.75 (0.75, 0.89) 872 (811, 1727) 698 (655, 1397) (0.81, 0.92) 885 (827, 1767) 696 (654, 1372) 0.80 (0.80, 0.95) 876 (814, 1766) 690 (650, 1386) (0.87, 1.02) 639 (594, 1287) 436 (410, 862) 0.86 (0.87, 1.03) 634 (585, 1279) 432 (399, 868) (0.96, 1.14) 703 (650, 1416) 479 (446, 957) 0.95 (0.96, 1.13) 700 (646, 1413) 477 (445, 959) (1.04, 1.26) 756 (694, 1532) 515 (478, 1049) 1.03 (1.04, 1.26) 756 (700, 1545) 515 (481, 1033) (1.10, 1.32) 802 (743, 1619) 548 (507, 1112) 1.13 (1.15, 1.41) 832 (771, 1694) 566 (523, 1148) (1.22, 1.47) 884 (816, 1785) 603 (559, 1213) 1.20 (1.21, 1.45) 884 (819, 1785) 602 (559, 1214) (1.30, 1.60) 944 (872, 1946) 643 (588, 1313) 1.34 (1.36, 1.68) 981 (901, 2045) 669 (617, 1377) (1.38, 1.65) 1002 (930, 2031) 685 (632, 1395) 1.44 (1.45, 1.73) 1054 (976, 2129) 719 (670, 1452) (1.50, 1.78) 1092 (1004, 2216) 745 (691, 1515) 1.52 (1.54, 1.79) 1115 (1045, 2230) 762 (708, 1542) (1.59, 1.84) 1158 (1077, 2328) 790 (731, 1586) 1.58 (1.59, 1.85) 1154 (1071, 2315) 788 (739, 1577) (1.68, 1.94) 1226 (1141, 2444) 835 (781, 1675) 1.63 (1.64, 1.93) 1195 (1115, 2417) 813 (758, 1634) (1.93, 2.17) 2114 (1977, 4217) 1668 (1564, 3332) 449 (420, 891) 448 (419, 890) 1.76 (1.77, 2.05) 1932 (1801, 3876) 1522 (1427, 3059) 410 (385, 820) 410 (381, 823) (1.95, 2.17) 2127 (1993, 4196) 1679 (1569, 3354) 1.80 (1.81, 2.06) 1975 (1844, 3938) 1557 (1456, 3106) - - Notes a Treated as a secondary probabilistic variable as a function of body weight. Mean (50 th percentile, 95 th percentile) values shown. b Treated as a probabilistic variable using a beta-shaped PDF. Mean (50 th percentile, 95 th percentile) values shown. c Includes allotment land-use Page 9 of 9

11 CLEA BRIEFING NOTE 2 UPDATE ON ESTIMATING VAPOUR INTRUSION INTO BULDINGS Introduction The CLEA model incorporates a range of pathways for modeling human exposure to soil contaminants. One such pathway is the inhalation of indoor air polluted by the migration of soil vapour into buildings from the underlying ground. Recently the approach used to estimate exposure via this pathway has been revised from that reported in R&D Publication CLR10 (Defra and Environment Agency 2002). The need for this update was triggered by the result of an Environment Agency commissioned review of the physico-chemical modelling of soil vapour intrusion into buildings and the screening approach used in the CLEA model (Environment Agency 2002). This peer reviewed research recommended that the Johnson and Ettinger model (Johnson and Ettinger 1991) should replace that of Ferguson et al. (1995) and Krylov and Ferguson (1998). This briefing note explains how this change in approach has been implemented in the CLEA model. It is written for technical professionals who are familiar with assessing and managing the risks to human health from land contamination but who are not necessarily experts in exposure modelling. The approach and algorithms described here replace existing guidance in Defra and Environment Agency (2002) including paragraphs , associated Tables and Equations 6.16 and 6.17, and also the default soil properties in Table 5.2. Key Principles Many organic chemicals have an appreciable vapour pressure under ambient conditions. Therefore the transport of vapour into buildings and its inhalation by adults and children from indoor air represents an important mechanism for human exposure to soil contaminated with such chemicals. R&D Publication CLR10 (Defra and Environment Agency 2002) describes the technical principles of the CLEA model and set out the default parameters and fate and transport algorithms that are used to estimate exposure in deriving the Soil Guideline Values. In predicting human exposure to the indoor inhalation of contaminant vapours from soil, Defra and Environment Agency (2002) posed three key questions: How much chemical vapour is released from sources within the ground? How do such vapours migrate in the subsurface, enter into buildings, and ultimately concentrate in indoor air? How much is inhaled by children and adults living and/or working in the building? Vapour transport in unsaturated soils depends on partitioning of contamination into the vapour phase, and transport of vapours to the surface and into indoor air (Environment Agency 2002). In common with many other exposure models, the CLEA model assumes an equilibrium between the relevant phases of contamination which includes the sorbed component, the dissolved aqueous component, and that found in the vapour phase within air-filled pore spaces. In reality, however, an equilibrium may not be achieved because the unconfined nature of the soil means that pore spaces will be in contact (and may exchange air and water with neighbours). In addition, contamination may be present as free product, that is undissolved liquid or solid contamination. Movement of free product, water, vapours and gases in the subsurface can occur by two processes: diffusion and, where there is a pressure gradient, advection (Environment Agency 2002). Diffusion relates to the fundamental movement of molecules of a gas or vapour that is commonly described as Brownian motion. While diffusion is a random process, over time there will be a net movement of a contaminant from zones of high concentration to those of lower concentration (that is, along a chemical gradient). This can be described using Fick s Law, with the diffusion coefficent in free air or

12 CLEA Briefing Note 2: Version 1.1 (July 2004) water reduced to the effective diffusion coefficient in soil to account for its tortuosity, following the method of Millington and Quirk (Environment Agency 2002). Advective transport occurs in response to pressure gradients in the soil (either barometric or those resulting from differences in air temperature or density). Advective movement of chemicals within unsaturated soils is controlled by its effective air permeability which depends on its properties including porosity, hydraulic conductivity, and water content (Environment Agency 2002). Vapour intrusion into buildings, through its foundations, occurs as a result of diffusion through dustfilled s and bulk building layers such as concrete slabs and by advection through drains, service penetrations, expansion joints, and floor and perimeter s (Environment Agency 2002). In most cases, advection is considered the dominant mechanism for vapour entering a building. Advection through s and openings occurs because of the negative indoor air pressure (relative to atmospheric pressure) created by building construction, temperature differences between inside and outside, wind loading on walls, and mechanical circulation such as by air conditioning (Environment Agency 2002). The indoor air concentration of a chemical depends not only upon its rate of infiltration into the building but also on its loss from the living space. Key processes include the exchange of indoor and outdoor air through natural and mechanical ventilation of the building via gaps in doors, windows, air bricks, chimneys and air conditioning vents; and deposition or absorption of chemicals from the air by building materials and household furnishings (Environment Agency 2002). Summary of Main Changes to the CLEA Model Environment Agency (2002) carried out a review of ten existing soil vapour transport models and recommended that the CLEA model adopt the approach of Johnson and Ettinger (1991). The implementation of this approach is described subsequently but the key changes are summarised below: Revisions to the chemical partitioning model to take into account solubility and saturated vapour limits and the effect of soil temperature, in estimating the amount of chemical in the dissolved and vapour phase in soil Replacement of the Ferguson et al. (1995) and Krylov and Ferguson (1998) with the Johnson and Ettinger (1991) approach for estimating the attenuation coefficient for soil vapour concentration to indoor air concentration Revision to the soil parameters used to estimate its vapour transport characteristics Revision to the building parameters used to estimate its vapour intrusion characteristics (see CLEA Briefing Note 3 Update of Supporting Values and Assumptions Describing UK Building Stock ) It should also be noted that the following exposure parameters that are relevant to the assessment of human exposure to the vapour intrusion pathway have not been revised (see Defra and Environment Agency 2002 for further details): Exposure frequencies and time spent in active and passive respiration for the standard land-use scenarios as set out in Section 4 of R&D Publication CLR10 Breathing and respiration rates for children and adults as set out in paragraphs 5.31 and 5.32 and associated Table 5.9 and Equations 5.6 and 5.7 in R&D Publication CLR10 New CLEA approach for assessing vapour intrusion into buildings The original work of Johnson and Ettinger (1991) is one of the most widely applied models for estimating vapour ingress into buildings and it is important to distinguish the basic equations described in the original paper from its subsequent application in practice (Johnson 2002). In many cases such as USEPA (2003) and ASTM (2000), the initial approach has been supplemented with additional guidance on factors including source zone partitioning and estimation methods for soil characteristics. Although this increases the complexity of the basic approach, the generic application of the CLEA model to derive Soil Guideline Values necessitates the need to replace some site measured parameters in the original Johnson and Ettinger (1991) paper with calculated estimates of partitioning and soil characteristics. In revising the CLEA model, the approach introduced by USEPA (2003) has been used as the basis for implementing the Johnson and Ettinger (1991) equations. Page 2 of 20

13 CLEA Briefing Note 2: Version 1.1 (July 2004) Revisions to Source Partitioning Section 5 of R&D Publication CLR10 (Defra and Environment Agency 2002) described the simple steady-state approach used by the CLEA model to estimate the concentration of a contaminant in soil solution and soil vapour using the soil-water partition coefficient (K d ) and the water-air partition coefficient (K v ). Partitioning of organic contaminants between the solid, liquid and vapour phases is estimated assuming equilibrium linear partitioning between the soil, water and soil vapour phases and assumes that this does not contain a residual-phase (that is, a non-aqueous phase liquid or solid). Although the same basic principles are still used in the revised source partitioning equations, the following changes have been incorporated: the vapour concentration of the contaminant in unsaturated soil is estimated using the approach described in USEPA 2003 (see Equation 1, Appendix 1), the water-air partition coefficient (H TS ) is adjusted for soil temperature in accordance with USEPA 2003 (see Equations 2 and 3, Appendix 1) the estimated vapour air concentration is checked against the saturated vapour air concentration of the pure compound and is not permitted to exceed it, the saturated vapour pressure of the pure compound is obtained from the literature and is corrected to the ambient soil temperature (that is, 283K or 10 o C) by the Antoine Method and other approaches on a substance-by-substance basis (Boethling and Mackay 2000); and the saturated vapour air concentration is estimated using the approach described in ASTM (2000) (see Equation 4, Appendix 1). Revisions to Fate and Transport Equations Paragraphs , associated Tables and Equations 6.16 and 6.17 of R&D Publication CLR10 (Defra and Environment Agency 2002) described the use of the fate and transport models by Ferguson et al. (1995) and Krylov and Ferguson (1998) to estimate indoor air concentrations of a contaminant. Following the recommendations of Environment Agency (2002), these have been replaced by the Johnson and Ettinger (1991) model. Key differences between Johnson and Ettinger (1991) and either Ferguson et al. (1995) or Krylov and Ferguson (1998) are: Johnson and Ettinger (1991) accounts for vapour transport through the soil into the zone of influence of the building foundation. Neither Ferguson et al. (1995) nor Krylov and Ferguson (1998) accounted for transport through the soil and assumed that the contamination was directly adjacent to the building foundations. However it should be noted that for the purpose of deriving Soil Guideline Values, it is assumed that the ground contamination is only 1m below the surface Johnson and Ettinger (1991) considers only a simplified foundation design similar to the ground bearing slab while Ferguson et al. (1995) and Krylov and Ferguson (1998) took account of several different types of building foundation and floor design to model vapour entry into the building including a ground bearing slab, timber suspended floor and proprietary beam-andblock Johnson and Ettinger (1991) readily allows consideration of a finite source in addition to an infinite source. The original work of Johnson and Ettinger (1991) is one of the most widely applied models for estimating vapour ingress into buildings (Johnson 2002). It is a screening model with a number of simplifying assumptions regarding contaminant distribution and occurrence, subsurface characteristics, transport mechanisms and building construction and design (USEPA 2002 and 2003). It calculates an attenuation factor that relates steady-state vapour concentrations at the source to indoor air concentration based on soil and building characteristics. Page 3 of 20

14 CLEA Briefing Note 2: Version 1.1 (July 2004) Figure 1 is a simplified diagram showing how soil vapour intrusion is modelled. At the top boundary of contamination, molecular diffusion moves the soil vapour towards the soil surface until it reaches the zone of influence of the building. Convective air movement within the soil column transports the vapours through s between the foundation and the basement slab floor. This convective sweep effect is induced by a negative pressure within the structure caused by a combination of wind effects and stack effects due to building heating and mechanical ventilation. Figure 1: Simplified diagram showing how soil vapour intrusion is modelled in CLEA The Johnson and Ettinger model uses a one-dimensional analytical solution to diffusive and convective transport of vapours and assumes that (Johnson and Ettinger 1991, US EPA 2003): the source of contamination is homogeneously distributed within the ground and lies directly beneath the building. The chemicals present are assumed to be at a concentration below their aqueous solubility limit, their soil saturation concentration, and/or their pure component vapour concentration; the model does not account for spatial horizontal variation in subsurface stratigraphy. Vertical stratigraphy can be described as one or more horizontal soil layers with isotropic properties. The issue of convective flow within the soil zone has been considered further by the Environment Agency (Barraclough 2003). A review of relevant literature has found no reported studies of pressure gradients in soil. Although potentially a significant mechanism, Barraclough (2003) concluded that convective flow should not be included at this time because there is a need for stronger evidence that the driving force for such flows exist and that this difference can be sustained long enough and at high enough pressure differences to have an overall effect (See Appendix 2). Diffusion through soil moisture is considered to be insignificant. Vapour flow is described by Darcy s Law, that is, flow through a porous media and does not take into account the presence of dual porosity systems and preferential flow channels; the model assumes that migration of soil vapour through building s and openings in the walls and foundations is the dominant intrusion process and this in turn is affected by the number of s or openings within the building foundations including floor and wall slabs. It accounts for both convective and diffusion driven transport. Convection through the building floor-slab is modelled assuming that vapour flow can only occur through an edge between the wall and the edge of the floor-slab (that is, the perimeter of a building basement or ground floor slab). Soil gas flow through the is simulated using an analytical solution for vapour through soil to a horizontal cylinder, or drain, as proposed by Nazaroff (1988). It assumes that uniform convective transport is likely to be most significant in the region very close to the foundations and vapour velocities are assumed to decrease rapidly with increasing distance from the building. Preferential flow through service ducts, open drains or other pathways is not modelled; and Page 4 of 20

15 CLEA Briefing Note 2: Version 1.1 (July 2004) the indoor air concentration of a contaminant depends not only on the rate of vapour intrusion but on the structure and ventilation performance of the building, as dilution with cleaner outdoor air reduces the contaminant concentration. Johnson and Ettinger (1991) assumes a simple box model with uniform and instantaneous mixing and dilution of chemicals within the air inside the building. The height and footprint of the building define the dimensions of the box and the air exchange rate controls the dilution of the contaminant entering into it with ambient air (Hers et al. 2003). It does not take into account potential building hot spots. No contaminant sources or sinks in the building are considered. Neither sorption nor biodegradation is accounted for in vapour transport through the soil and into the building. Because of its popularity as a screening tool, the Johnson and Ettinger (1991) equations have been the subject of several sensitivity studies (Environment Agency 2002, Johnson 2002 and Hers et al. 2003). The objective of these reviews has been to target those parameters that have the greatest influence on the estimated indoor air concentration to enable better targeted data collection and to enable more meaningful comparison between predicted concentrations and those observed at a number of case-study sites. For example, Hers et al. (2003) reviewed the Johnson and Ettinger model, evaluating the sensitivity and uncertainty in the model from both a theoretical basis and through comparison with several published case studies of petroleum hydrocarbon and chlorinated solvent contaminated sites. The relative sensitivity of the soil, building and chemical parameters used in the Johnson and Ettinger model depends critically on the scenario being conceptualised (Hers et al. 2003, Johnson 2002, and US EPA 2002). Scenarios can be defined on the basis of: Depth below ground to source (that is, a shallow or deep source zone). Whether the contamination is in the vadose zone or at the water table (that is, a soil or a groundwater source). Whether building depressurisation is assumed (that is, flow into the building is based on convective and diffusion driven flow rather than diffusion only). For the purposes of deriving Soil Guideline Values, a reasonable worse case scenario is assumed that consists of a shallow source term in the vadose zone above the water table that lies beneath a depressurised building. For buildings without a basement, the depth beneath the floor to the top of the contamination is assumed to be 100 cm. On the basis of this conceptual scenario, the relative sensitivity of the key parameters in the Johnson and Ettinger model (as observed by Johnson 2002, Hers et al. 2003, and USEPA 2002) are qualitatively shown in Table 1. It should be noted that the sensitivity of the parameters for other conceptual scenarios might be significantly different from those shown and this is particularly true if the source term lies at or close to the watertable where the soil moisture content factors are increasingly important (USEPA 2003). For the conceptual scenario outlined above there is a general consensus on the critical parameters for the Johnson and Ettinger model amongst the studies reviewed (Hers et al. 2003, Johnson 2002 and USEPA 2002). It is important to note that the term critical parameter is not meant to imply large variations in the estimated indoor air concentration with small changes in input value as in most cases the Johnson and Ettinger model responds linearly to changes in input value (Johnson 2002). The equations used to implement the Johnson and Ettinger (1991) model are detailed in Appendix 1 and are based on its implementation by USEPA (2003). Equations 5 11 describe the implementation of the infinite source solution. This critical assumption is that the amount of contaminant present in the soil over the period of exposure does not reduce (even though the mass of chemical lost through the transport of vapours from the soil into the building increases with time). It is however consistent with the level of uncertainty associated with the development of Soil Guideline Values where the source of contamination can only be described in generic terms. Although the default approach assumes an infinite source, the CLEA model does include checks to evaluate whether the rate of removal would exceed the amounts present in a finite source situation (see Equation 9 in Appendix 1). Equations describe two methods for implementing a finite source solution, depending on whether the supply of contamination present in the ground is likely to be exhausted over the duration of exposure. Page 5 of 20