Methodology for the radiological assessment of noble gases in nonhuman

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1 Methodology for the radiological assessment of noble gases in nonhuman biota Nele Horemans and Jordi Vives i Batlle nhoreman@sckcen.be; jvibatll@sckcen.be BVS-ABR scientific meeting: ICRP concept on protection of the environment

2 Radon and Thoron daughters of U- and Th-decay Radon (222Rn) 238U decay chain, half-life of d Thoron (220Rn) 232Th chain half-life of 56 s.

3 Radon and thoron decay products Radon Thoron Radon four first products are very short-lived Thoron decay products are short-lived except 212Pb(10.6 h)

Reasons for noble gas dosimetry For both Rn/Tn the problem is the internal dose by the daughter products Pb

4 General drive to ensure that the environment is protected Species in high background soil exceed the lower end of the DCRL protection bands in burrowing mammals The Rn gas itself gives negligible contribution Reasons for noble gas dosimetry For both Rn/Tn the problem is the internal dose by the daughter products Pb isotopes, alpha emittors Sources and distribution of natural background radiation to world population. Hussein 2008

5 Some history Due to complexity, Rn/Tn dosimetry for biota a scientific challenge. Only a few studies in rodents explicitly model lung deposition (Harley, 1988; Hofmann et al., 2006, Strong & Baker, 1996; Winkler-Heil et al., 2015) Method for radon respiration in burrowing mammals (MacDonald and Laverock, 1998) First assessment tools developed in UK ( ) Ar and Kr method in the EA R&D 128 methodology (Vives et al., 2003). Rn allometric approach published in EA Science report (Vives et al., 2008) Assessments for 222 Rn authorisation limits under UK Radioactive Substances Act (RSA) ICRP work as part of TG74 and TG77 to develop advanced allometric method or 222 Rn and 220 Rn (Vives I Batlle et al., 2017) Extension of the methodology to other noble gases: 41 Ar, 85,88 Kr and 131m,133 Xe doses to non-human biota (Vives i Batlle et al., 2015) To date, no such approaches exist in ERICA

6 Basis of the approach We produced tables with dose coefficients (DCs) for biota due to radon, thoron and daughters

7 Many biological parameters relating to organism structure relate to metabolism and scale according to the Brody-Kleiber law: Y b = A M, b = 0.75 Other parameters scale on the basis of surface exchange, like heat transfer, hence scaling to mass to power of 2/3: SurfaceArea r 2 and Mass V r so S M Allometric scaling For this study we use the following relationships: B M = A Lung BR = M A B BR LM = M ( 8.7 ± 4.4) B LM ± 0.02 Where M is the total mass in kg and B is breathing rate in m 3 h -1 6 M = (1.28 ± 0.72) 10 2 M 1.02± 0.03

8 Aim A model based on allometrically derived respiration rates, designed for calculating 222 Rn daughter dose rates to sensitive tissues and the whole body of terrestrial animals and plants.

9 How to calculate the dose coefficient Breathing induces a constant flow of Rn atoms into the lung while undergoing continuous decay Assumptions: influx and the decay are in balance due to the short half-life of the decay products gas is exhaled but 100% of the decay products are trapped inside the lung dose coefficient DC is: E DC = B g MT Where: B =Respiration (breathing) rate (m 3 h 1 ), E =Potential alpha energy = total energy absorbed in the target tissues due to radiation emitted by the radon progeny until decay to Pb isotopes (µj Bq 1 ), M T =Mass of the target tissue/organ (kg) g = Geometrical factor which takes into account (in)homogeneity of activity deposition in airways/respiratory organs (dimensionless).

10 How to obtain M T, B and E Potential α-energy for radon ( 222 Rn) and thoron ( 220 Rn) progeny calculated using ICRP Publication 107 data Potential α energy Radionuclide Half-life per atom per unit of activity (MeV) (10 12 J) (MeV Bq 1 ) (10 10 J Bq 1 ) Radon ( 222 Rn) progeny 218 Po 3.10 min Pb 26.8 min Bi 19.9 min Po µs Total (at equilibrium), per Bq of radon Thoron ( 220 Rn) progeny 216 Po s Pb h Bi min

11 How to obtain M T, B and E Mass of target tissue approximation as a function of density, surface area and thickness: M S R h h S h 2 T T T = ρt T aw + 2 ρt Raw 2Raw T T General allometric equation of B as a function of mass: B( M) = e β M + β β 0 = ± 0.050, β 1 = ± 0.019

12 DCs in µgy h -1 per Bq m -3 for four different body parts: E: bronchial epithelium, TB: Full tracheobronchial epithelium, L: Full lung; WB: Whole body These formulas make a scale conversion from the mass of the lung epithelium (M BE ), bronchial tree (M TB ) and whole body compared with ICRP reference man This approach is only recommended for mammals Calculation of animal DCs DC DC DC DC BE TB L WB = = = = E M E M L E M RM BE RM TB E α M β M M ( M ) M M L RM Total RM Total ( M ) B( M ) B( M ) M: animal mass; M total RM = ref. Man mass B(M): Breathing rate as a function of mass E: Potential alpha energy α L and β L : parameters of the allometric formulae for lung mass Vives i Batlle et al (2012, 2017)

13 On the basis of the following allometric respiration formula for plants Uses CO2 respiration as surrogate for other gases (conservative) 3 BR m s ( ) = M ( kg) 02 = PLANT Simple power functions for DCs in µgy h -1 per Bq m -3 : DC S P = E a PL 2 6h PL am DC = E a M PL PL 1 Key parameters: b b T 1 (plant tissue) (whole plant) a: radius of the plant which is assumed to be cylindrical (m) h T : depth of sensitive tissue = 50 µm Approximation for plants a PL M b PL Vives i Batlle et al (2012, 2017)

14 Parameter or quantity Internal DCs for Rn and Tn (reference animals) Amphibian Reptile Mammal small Mammal big Bird (ICRP Frog) a (ERICA snake) a (ICRP rat) (ICRP deer) (ICRP duck) a M (kg) a (m) b (m) c (m) B (m 3 h 1 ) DCs per air concentration of 222 Rn (µgy h 1 Bq 1 m 3 ) DC B DC TB DC L DC WB DCs per air concentration of 220 Rn (µgy h 1 Bq 1 m 3 ) DC B DC TB DC L DC WB a DC for non-mammals are shown for illustrative purposes only

15 Internal DCs for Rn and Tn (reference plants) Parameter or Lichen & bryophytes Grasses and herbs Trees quantity (ICRP bryophyte) (ICRP wild grass) (ICRP pine tree) M (kg) a (m) b (m) c (m) B (m 3 h 1 ) DCs per air concentration of 222 Rn (µgy h 1 Bq 1 m 3 ) DC SS DC WB DCs per air concentration of 220 Rn (µgy h 1 Bq 1 m 3 ) DC S DC P

16 External DCs for Rn and Tn (animals and plants) DC (µgy h 1 Bq 1 m 3 ) Organism in air in air (h = 500 m) (h = 10 m) on the ground Radon ( 222 Rn) and progeny Amphibian (ICRP frog) Reptile (FASSET snake) Mammal (ICRP rat) Mammal (ICRP deer) Bird (ICRP duck) Lichen and bryophytes (ICRP bryophytes) Grasses and herbs (ICRP wild grass) Tree (ICRP pine tree) Thoron ( 220 Rn) and progeny Amphibian (ICRP frog) Reptile (FASSET snake) Mammal (ICRP rat) Mammal (ICRP deer) Bird (ICRP duck) Lichen and bryophytes (ICRP bryophytes) Grasses and herbs (ICRP wild grass) Tree (ICRP pine tree)

17 From the DC to actual dose (Based on ERICA tool)

18 From the DC to actual dose (Based on ERICA tool) The following formulas are used to calculate doses from the DC s: Internal dose = Air conc DC Soil dose = Air conc CR DC int ernal External dose = Soil dose + Immersion dose Immersion dose = Air conc DC external external Air density OF immersion CR is a factor for converting concentration of radon in the air of the soil pores (Bq m -3 ) to concentration of radon in soil (Bqkg -1 ) = 10 4 m 3 kg 1 Where the air concentration is in Bq m -3, the internal dose is in µgy h -1, the DC is in µgy h -1 per Bq kg -1 and the occupancy factors OF are defined as: OF OF soil = immersion fsoil = organism fair + organism fsoilsurf + organism fsoilsurf organism 2 The reduction factor modifies dose to organisms in air it corrects for reduction of dose received from soil: 0 for α and low-energy β radiation and 0.25 for high energy β+γ radiation. soil Air density OF 2 + fairorganism reduction factorradiation type

19 Validation of the Rn approach with data from MacDonald and Laverock (1998) Organism Mass (kg) DCC (µgy Bq -1 s -1 m 3 ) Dose rate (mgy h -1 ) % diff. B TB L WB Calculated From paper Mole 4.00E E E E E Pocket gopher 2.00E E E E E Ground squirrel 5.00E E E E E Ground hog 3.00E E E E E Badger 8.00E E E E E Our model gives similar predictions for total dose rates to whole body than the more simplified approach derived by MacDonald and Laverock (1998)

Available dose rate estimates for 222 Rn: One

Dose rate similar to predicted no effect dose

20 Available dose rate estimates for 222 Rn: One study in area of Rn rich soils in Canada Radon field studies Whole body dose rate >100 mgy y -1 ( circa 10 µgy h -1 ) for small burrowing animals So Dose rate similar to predicted no effect dose Beresford et al 2012: measurements of Rn in artificial burrows

activity concentration Sites across gradient of

21 Approach Beresford et al 2012 Make artificial burrows Use passive detectors to measure soil gas 222 Rn activity concentration Sites across gradient of expected 222 Rn concentrations Detector changed every 4-6 weeks

22 Measured 222 Rn concentrations in soil gas Range: <0.1 to 14.5 kbq m -3

23 Radon field studies: Beresford et al 2012 Dose rates calculated from measured field soil gas concentration, using the allometric methodology described here Assuming an equilibrium factor F = 0.8 Assuming an α-radiation weighting factor of 10 Beresford et al. (2012)

24 Weighted dose rates Dose rate from 222 Rn to burrowing mammals likely to be at least 10 times higher than previously considered natural exposure sources ( 40 K, Th/U series). In many areas likely to considerably exceed predicted no-effect dose rate benchmarks.

25 A compartment model representing: Radon An advanced plant model Aerosol: free, unattached and attached fractions of 222 Rn, 218 Po, 214 Pb, 214 Bi, 214 Po in the atmosphere Plant uptake: surface interception of unattached and attached daughters, diffusion of radon through stomata, permeation of radon through plant epidermis. Plant turnover: translocation of deposited activity from plant surface to plant interior We derived DCs for internal, surface and external exposure as a function of plant surface area and steady-state concentration at ground level.

26 P o _ _ t o _ u n a t t a c h e d P b _ _ t o _ u n a t t a c h e d B i_ _ t o _ u n a t t a c h e d P o _ _ t o _ u n a t t a c h e d B i_ _ d e p _ u n a t t P o _ _ d e p _ u n a t t P b _ _ d e p _ u n a t t P o _ _ d e p _ u n a t t P o _ _ f r o m P b _ _ f r o m B i_ _ f r o m P o _ _ f r o m _ f r e e _ f r e e _ f r e e _ f r e e B i_ _ f r o m P o _ _ f r o m P b _ _ f r o m P o _ _ f r o m _ u n a t t _ u n a t t _ u n a t t _ u n a t t P o _ _ f r o m _ u n a t t 2 P b _ _ f r o m _ u n a t t 2 P o _ _ f r o m _ u n a t t 2 B i_ _ f r o m _ u n a t t 2 P o _ _ t o _ a t t a c h e d P b _ _ t o _ a t t a c h e d P o _ _ t o _ a t t a c h e d B i_ _ t o _ a t t a c h e d P o _ _ t o _ in t P o _ _ f r o m _ s u r f P b _ _ t o _ in P b _ _ f r o m _ s u r f P o _ _ t o _ in P o _ _ f r o m _ s u r f P o _ _ f r o m P o _ _ f r o m P b _ _ f r o m B i_ _ f r o m _ u n a t t a c h e d _ u n a t t a c h e d _ u n a t t a c h e d B i_ _ t o _ in Bi_ _ f r o m _ u n a t t a c h e d _ s u r f P o _ _ f r o m P b _ _ f r o m _ a t t P o _ _ f r o m B i_ _ f r o m _ a t t _ a t t _ a t t P o _ _ f r o m _ a t t 2 P b _ _ f r o m _ a t t 2 P o _ _ f r o m _ a t t 2 B i_ _ f r o m _ a t t 2 P o _ _ d e p _ a t t P b _ _ d e p _ a t t P o _ _ d e p _ a t t B 1 _ _ d e p _ a t t Po218_clust Po218_att Conceptual model Pb214_clust Pb214_att Free_fraction (bare atoms) Unattached_fraction (0.5-4 nm) Attached_fraction ( nm) Bi214_clust Po214_att Each sub-model contains the decay chain of radon: 222 Rn 218 Po 214 Pb 214 Bi 214 Po. Po214_clust Perm_and_diff_to_plant Rn_222_diff_perm_in Perm_and_diff_from_plant Rn_222_diff_perm_out Bi214_att Perm_and_diff_from_air Perm_and_diff_to_air Stomata_area_ratio Stomata_radius Bi214_dep_u Plant_interior Po214_dep_u Po218_dep_a1 Ext_dose_total Pb214_dep_u Pb214_dep_a1 Equilibrium_factor Po218_dep_u Po218_dep_u2 Po_218_trans Pb_214_transPo_214_trans Bi_214_trans Po214_dep_a1 B1214_dep_a1 Pb_214_dep_u2 Po218_dep_a2 fp Po_214_dep_u2 Pb_214_dep_a2 Total_dose Exchange rates link individual compartments across sub-models, with rate constants linked to the parameter set. B1_214_dep_u2 Plant_surface Po_214_dep_a2 B1_214_dep_a2

Plant leaf geometries Deposition velocity values for different types of surface Surface λ U L v d (m s

90 10-3 Examples of (perennial) plant leaf geometries covering a wide size range Leaf type Major axis

5 10-4 1.2 10-4 9.8 10-9 9.8 10-6 Small 6.0 10-2 3.5 10-2 2.5 10-4 3.3 10-3 2.8 10-7 2.8 10-4 Medium 1.

27 Plant leaf geometries Deposition velocity values for different types of surface Surface λ U L v d (m s -1 ) A v d (m s -1 ) Soil Grass Wheat Examples of (perennial) plant leaf geometries covering a wide size range Leaf type Major axis a (m) Major axis b (m) Minor axis c (m) Area (m 2 ) Volume (m 3 ) Mass (kg) Tiny Small Medium large

28 DCs for plant leaf geometries Internal DCC (total), µgy h -1 /Bq kg -1 Nuclide Tiny leaf Small leaf Medium leaf Large leaf Average ± S.D. 222 Rn 3.2E E E E E E Po 3.5E E E E E E Pb 7.6E E E E E E Bi 5.8E E E E E E Po 4.4E E E E E E+00 Surface DCC (total), µgy h -1 /Bq kg Rn 1.6E E E E E E Po 1.7E E E E E E Pb 3.2E E E E E E Bi 5.0E E E E E E Po 2.2E E E E E E-11 DCs are almost independent of size except for the smallest leaves External DCC (total), µgy h -1 /Bq kg Rn 2.3E E E E E E Po 7.0E E E E E E Pb 2.4E E E E E E Bi 1.2E E E E E E Po 4.8E E E E E E-11

29 Model output Model output for the three atmospheric compartments (free, unattached and attached) Model output for the two plant compartments (outer surface and interior)

30 Model output Unweighted dose rates for every radionuclide, corresponding to an initial 222 Rn activity of concentration of 1 Bq m -3 Total dose rates (summing the different radionuclides)

31 Comparison with simpler allometric model Internal + surface dose rates for the present model are consistent with the internal dose rates for the allometric approach. External dose rates for the current model are a factor of two below those for the allometric approach. This is not surprising, given that the allometric model adopted an equilibrium factor of 1, whereas the present model uses 0.5. Total dose rates given by both methods are comparable. Source Dose per unit activity Rn (µgy h -1 per Bq m -3 ) Organism Time Internal Surface Int + surf External Total This work Plant leaf Night Plant leaf Day EA (Vives i Batlle et al. 2008) Fungi All N/A N/A Herb All N/A N/A Lichen All N/A N/A Seed All N/A N/A Shrub All N/A N/A Tree All N/A N/A Average All N/A N/A

32 Conclusions

33 Radon and thoron method Conclusions A method to calculate radiation doses from radon and thoron progenies to ICRP Reference Animals and Plants is now available. Relatively simplified in terms of assuming simple geometries, uniform distribution of radionuclides in the biota, absorbed doses averaged to the whole organism, etc. The implications of the contribution that 220,222 Rn makes to wildlife doses and effects needc to be explored with reference to: the application of the ICRP derived consideration reference levels (DCRLs) for wildlife (ICRP, 2008a) The (as yet unavailable) effects data.

34 Radon (advanced plant model) Conclusions The predominant component of dose is surface-deposited 214 Po and (to a lesser extent) 218 Po activity. Doses to plant surface tissue are x 10 higher than the surface deposition dose averaged to the whole plant. Differences with respect to the allometric model due to combination of surface and internal dose and the equilibrium factor of 1 in the latter. Radon exposures in mammals Radon levels in burrows exceeding background levels and noeffects benchmarks for non-human biota. Advised benchmark dose rates need to be better put into context with background dose rates, including exposure to 222 Rn Context depends on purpose of benchmark & assessment level.

35 Thank you for your attention

36 Perspectives for future work Integrate Ar, Kr, Rn assessment in a single tool (or incorporate into ERICA). Perform additional investigations of allometric radon dosimetry for insects and plants. Seek evidence for dose rates that would cause stochastic effects in the lung using more detailed lung modelling (if appropriate). Consider how to extend the dose assessment for 226 Ra in soil. Review benchmark values in context of background and radon levels in the natural environment.

ICRP & Alliance, Integrated Protection of People and the Environment

ICRP & Alliance, Integrated Protection of People and the Environment 4 th International Symposium on the System of Radiological Protection October 10-12, 2017, Paris, France Kathryn A. Higley ICRP Committee