Activity size distributions of the shortlived radon decay products and their influence on the deposition probability in the human lung

J. Aerosol Sci., Vol. 19, No. 7, pp. 1331 - 1337, 1988 Printed in Great Britain

0021-8502/88 $3.00. + 0.00 Pergamon Press plc

ACTIVITY SIZE DISTRIBUTIONS OF THE SHORTLIVED RADON DECAY PRODUCTS AND THEIR INFLUENCE ON THE DEPOSITION PROBABILITY IN THE HUMAN LUNG

A. Reineking and J. Porstend~rfer Zentrales Isotopenlaber der Georg-August-Universit~t Burckhardtweg 2 D-3400 G~ttingen, F.R.G.

Introduction The inhalation of the shortlived radon decay products in the domestic environment yields the greatest amount of the natural radiation exposure of the human public. The aerosol attached and the unattached radon daughters are deposited in different parts of the human respiratory tract due to the different particle sizes. Since the dose of lung tissues cannot be measured model calculations are necessary to estimate the radiation exposure. In all dosimetric models the calculated dose principally depends on the size distribution of the inhaled radioactive aerosol and the size dependence of the deposition on the airway surfaces within the human lung. In evaluating absorbed doses it must moreover be considered that atmospheric aerosol particles rapidly grow in the nose and the upper bronchi due to their hygroscopic properties and the high humidity in the respiratory tract (Sinclair et al. (1974)). During the last years in our laboratory the entire activity size distributions of the shortlived radon progeny and the properties of the unattached activities were measured in dwellings under various conditions and in the open atmosphere (Reineking et al. (1985), Porstend~rfer el al. (1987), Reineking et al. (1987)). Research has also progressed in the Fields of measuring and modeling aerosol deposition in the human lung. Heyder et al. (1986) and Schiller (1985) published experimentally determined total and regional deposition data for breathing monodisperse insoluble aerosols through nose and mouth. Ferron et al. (1986) performed model calculations on the behaviour of hygroscopic sodium chloride aerosol particles. In this paper the deposition probability of radioactive aerosol particles in the human lung is discussed with regard to the shortlived radon decay products in the domestic environment.

Activity size daughters

distributions

and

unattached

fraction

of

the

shortlived

radon

Activity size distributions of the shortlived radon daughters were measured in dwellings with a low pressure cascade impactor and a device of high volume screen diffusion batteries under various conditions (Reineking et al. (1987)). The collection efficiency of the impactor and the penetration functions of the screen diffusion batteries were determined with monodisperse aerosol particles and comparison measurements were performed with both instruments (Reineking et al. (1986)). The results of the size distribution measurements of the shortlived radon daughters are summarized in table 1 and illustrated in figure I. In low ventilated rooms without additional aerosol sources the activity size distributions can be described by one lognormal distribution (accumulation mode) with an average activity median aerodynamic diameter (AMAD) of 213 nm (range 115 - 570 nm) and an average geometric standard deviation o£ 2.5 (range 1.4 - 4.5). It was measured that the particle size of the aerosol attached activities of the three shortlived radon daughters RaA, RaB and RaC is not significantly different. The mean diffusion equivalent diameter of the unattached activity of the decay products was determined to be 1.2 nm (range 0.5 - 2.0 nm). Additional aerosol sources (electric motor, candle light) sometimes yield to a second maximum of the attached fraction in the diameter size range of I0 - I00 nm (nuclei mode) or the accumulation mode can be shifted to greater sizes (cigarette

1331

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A. REINEKING and J. PORSTENDORFER

smoke: AMAD = 354 nm). Most o f the outdoor actzwties o f the radon decay p r o d u c t s are a s s o c i a t e d t o the a c c u m u l a t i o n mode w i t h an a v e r a g e AHAD o£ 369 nm (range t73 - 645 rim) and an a v e r a g e s t a n d a r d d e v i a t i o n o f 2.5 ( r a n g e ] . 6 - 4 . 4 ) . Compared w i t h t h e s i z e d i s t r i b u t i o n in c l o s e d rooms w i t h o u t a e r o s o l s o u r c e s ( v e n t i l a t i o n r a t e ~ 0.5 h-±~ aged a e r o s o l ) t h i s median d i a m e t e r o f t h e a c c u m u l a t i o n mode i s s i g n i f i c a n t l y shifted to greater sizes due t o different p l a t e - o u t r a t e s . Sometimes a small f r a c t i o n o f t h e n u c l e i mode lrl the diameter size range o£ lO - tO0 nm arid a small F r a c t i o n in the s tze range o f some m i c r o n s ( c o a r s e mode) were found. The median d i a m e t e r o f t h e u n a t t a c h e d activities ranged between 2.5 - 3.0 nm w i t h a mean v a l u e o f 2.7 nm. In s e p a r a t e s t u d i e s the unattached or free activities o f the s h o r t t l v e d radon progeny were measured 1{i t h e i n d o o r and o u t d o o r e r l v i r o n m e n t by the screen t e c h n t q u e (Reineking et a l . (1985), P o r s t e n d 6 r f e r et a l . (1987)). The unattached firactiorls are mainly influenced by the aerosol partzcIe concentrations due t o t h e h i g h a t t a c h m e n t r a t e s off the p r i m a r y r a d i o a c t i v e i o n s o r c l u s t e r s to the a e r o s o l p a r t i c l e s . In rooms w~thout a d d i t i o n a l a e r o s o l sources about one t h i r d of the decay p r o d u c t RaA i s u n a t t a c h e d arid about 6 % o f RaB. I t was measured t h a t already a t s m a l l a e r o s o l p a r t i c l e c o n c e n t r a t i o r ~ s off about ( l - 5) * 103 cm-3 the unattached f r a c t i o n o f RaC can be n e g l e c t e d ( ~ 1 % ) . N i t h r e g a r d t o l u n g d o s i m e t r y o f radon decay p r o d u c t s off these v a l u e s a mean u n a t t a c h e d f l r a c t i o n off t h e p o t e n t i a l a l p h a e n e r g y fp o f 0.10 can be d e r i v e d . In atmospheres w i t h h i g h e r a e r o s o l p a r t i c l e c o n c e n t r a t i o n s up t o 256"103 cm -3 t h e unattached activities decreased. Only irl rooms w i t h p a r t i c l e s in t h e n u c l e i mode size range ( e l e c t r i c m o t o r , c a n d l e l i g h t ) relatively h i g h u n a t t a c h e d f r a c t i o n s were measured. In t h e o u t d o o r e n v i r o n m e n t a mean fp - v a l u e o f 0.025 was measured ( i . e . RaA: 15%, RaB: 2 %) a t a mean a e r o s o l p a r t i c l e c o n c e n t r a t i o n off 41"103 cm-3

Aerosol deposition

i n t h e human l u n 9

For d o s i m e t r i c model c a l c u l a t i o n s t h e r e s p i r a t o r y t r a c t can be d e r i d e d i n t o t h r e e r e g i o n s . The nasopharymgeal (N-P) r e g i o n i n c l u d e s t h e r e s p i r a t o r y p a r t s from nose t o larynx, the following tracheo bronchial (T-B) r e g i o n s end i n the a l v e o l a t e d a i r w a y s where gas exchange t a k e s p l a c e and t h e a l v e o l a r (A) r e g i o n c o n s i s t s o f alveolar ducts. The s t r u c t u r e and a i r f l o w zrl each r e g i o n d e t e r m i n e the f r a c t i o n a l d e p o s i t i o n o f t h e inhaled aerosol particles and o f the u n a t t a c h e d radon p r o g e n y . In the p a r t i c l e diameter size range s m a l l e r than 300 rim d i f f u s i o n i s the most i m p o r t a n t d e p o s i t i o n p r o c e s s arid i n the s i z e range g r e a t e r than 300 rim d e p o s i t i o n by s e d i m e r l t a t ~ o n arid inertial impaction is dominant. In the l a s t y e a r s r e g l o n a l arid t o t a l d e p o s i t z o n were e x t e n s i v e l y measured by Heyder and coworkers (see Heyder e t a l . (1986) arid S c h i l l e r (1985)) usirlg monodisperse t e s t a e r o s o l s o f s e b a c a t e , i r o n o x i d e arid s i l v e r . Tvp]va] results for nasal breathing are shown i n f i g u r e 2 fior a b r e a t h i n g r a t e s of 0.45 m3h -1 arid a t i d a l volume off 500 cm3 . In t h e d i a m e t e r s i z e range s m a l l e r tharl 50 rim t h e r e g i o n a l deposition values a r e e x t r a p o l a t e d firom v a l u e s off model c a l c u l a t i o n s p u b l i s h e d by Schiller (1985). For the T-B regzon a d e p o s i t i o n p r o b a b i l z t y o f 2 % i s assumed irl the diameter s i z e range between 50 - 2000 nm i n s t e a d o f t h e e x p e r i m e n t a l v a l u e s o f (0 -+2) %. In t h e s i z e range between 50 - 400 rim t h e t o t a l d e p o s i t i o n i s g o v e r n e d by t h e d e p o s i t i o n off a e r o s o l p a r t i c l e s 1o the a l v e o l a r r e g i o n ( A ) . 5±nclair et al. (1974) and P o r s t e n d 6 r f i e r arid Mercer (1978) measured t h a t atmospheric aerosol p a r t i c l e s o f a l l s i z e s i n c r e a s e i n d i a m e t e r by a f a c t o r of' two or more w i t h t h e amount off h y g r o s c o p i c m a t e r i a l p r e s e n t , i f i the r e l a t i v e h u m i d i t y increases to nearly 100 %. Ferror~ e t a l . (1986) c a l c u l a t e d r e g i o n a l arid t o t a l d e p o s i t i o n v a l u e s For non - g r o w i n g p a r t i c l e s and g r o w i n g sodium c h l o r i d e p a r t i c l e s . With the results of t h e s e model c a l c u l a t i o n s the change o f the deposition probability due t o t h e g r o w t h o f a e r o s o l p a r t i c l e s d u r i n g i n h a l a t l o r l was e s t J m t e d from t h e experimental values of Heyder e t a1.(1986). These r e s u l t s are a l s o illustrated in figure [ ( t h i n [ t r , e s ) . Compared t o the n o n - h y g r o s c o p i c p a r t i c l e s irl t h e d i a m e t e r s i z e range o f 300 rim - 2000 rim arl i n c r e a s e o f t h e t o t a l d e p o s i t i o n by a f a c t o r o f 2 -3 i s found.

Activity size distributions of the shortlived radon decay products

1333

Results The results of the convolution of the deposition probabilities with measured activity size distributions are summarized in table 2. Considering the size distribution of the indoor measurements without additional aerosol sources (accumulation mode: AMAD = 211 nm) show that the total deposition values range between 0.18 and 0.25. The difference of hygroscopic and non-hygroscopic particles and the influence of different breathing rates (0.45 m3h -I and 1.35 m3h -l) on the deposition of radon daughters in the human lung is minor. The calculations show that with regard to the regional deposition most of the activity is deposited in the alveolar part of the respiratory tract. The total deposition of outdoor aerosol particles indicate an increase by a factor of 1.5 comparing non-hygroscopic with hygroscopic aerosol particles, and also an increase with increasing breathing rate is found. Compared to the indoor results the total deposition of outdoor radon daughters (0.24 - 0.42) are approximately two times higher. This increase of the total deposition is mainly determined by the aerosol attached activities in the nuclei mode size range and in the coarse mode size range. For this reason the deposition values of the N-P region (0.07 - 0.19) is nearly in the same order than those of the alveolar part of the human lung (0.15 0.24). The deposition values of the T-B region amounts to 0.02. Various aerosol sources ( e . g . kerosene heating or cooking) produce small aerosol particles in the nuclei mode size range. For example the results of aerosol particles from a burning candle are shown in table 2. The total deposition ranged between 0.22 and 0.27. Due to the significant nuclei mode (AMD = 8.7 nm) the regional deposition of the T-B part increased up to 0.07 compared to 0.02 for the indoor (without aerosol sources)and the outdoor environment. This is important with regard to dosimetry of radon daughters because the radiation exposure of the human lung is mainly determined by the deposited activity in the T-B region.

Acknowledgement This work is supported by the Commision of the European Communities under contract BI6-F-I3OD.

References

Ferron G.H., Haider B. and Kreyling W.G. (1986), "Deposition of Sodium Chloride Aerosol Particles in the Human Respiratory Tract", In: Aerosol, Formation and Reactivity, Proceedings of the 2nd Int. Aerosol Conference, West-Berlin, 236 -239 Heyder J., Gebhardt J., Rudolf G., Schiller C.F. and Stahlhofen W. (1986) "Deposition of particles in the human respiratory tract in the size range 0.005 - 15 um, J. Aerosol Sci. 17, 811 - 825 Porstend6rfer J. and Mercer T.T. (1978) "Influence of Nuclei Coneerltration and Humidity upon the Attachment Rate of Atoms in the Atmosphere", Atmospheric Environment 12, 2223 - 2228 Porstend6rfer J., Reineking A. and Becker K.H. (1987), "Free Fractions, Attachment Rates, and Plate-Out Rates of Radon Daughters in Houses", In: Radon and its Decay Products: Oeeurence, Properties and Health Effects (Edited by Hopke P.K.), ACS Symposium Series 331, 285 Reineking A., Becker K.H. and Porstend6rfer J. (1985), "Measurements of the Unattached Fractions of Radon Daughters in Houses", The Science of the Total Environment 45, 261 Reineking A., Porstend6rfer J. and Becker K.H. (1986), "Comparison Measurements of Radon Daughter Activity Size Distributions with Cascade Impactors and High-Volume Screen Diffusion Batteries", In: Aerosol, Formation and Reactivity, Proceedings 2nd Int. Aerosol Conference, West-Berlin, i158-i163

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Reineking A., Becket K.H. and Porstendarfer J. (1987), "Measurements of the Activity Size Distributions of the Short-Lived Radon Daughters in the Indoor and Outdoor Environment", Fourth International Symposium on the Natural Radiation Environment, Lisboa, Portugal, December 7-i1 Schiller C.F. (1985), "Diffusionsabscheidung von Aerosolteilchen im Atemtrakt des Menschen", Dissertation, Universit~t, Frankfurt am Main Sinclair D., Coutness R.J. and Hoopes G.S. (1974), "Effect of relative humidity on the size of atmospheric aerosol particles", J. Aerosol Sci. 1111-1116