Modelling of skin exposure from distributed sources

PII: S0003-4878(00)00035-1

Ann. occup. Hyg., Vol. 44, No. 7, pp. 529–532, 2000  2000 British Occupational Hygiene Society Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain. 0003-4878/00/$20.00

Modelling of Skin Exposure from Distributed Sources CHRISTIAN LANGE FOGH* and KASPER GRANN ANDERSSON Riso National Laboratory, NUK-114, PO Box 49, DK-4000, Roskilde, Denmark

A simple model of indoor air pollution concentrations was used together with experimental results on deposition velocities to skin to calculate the skin dose from an outdoor plume of contaminants. The primary pathway was considered to be direct deposition to the skin from a homogeneously distributed air source. The model has been used to show that skin deposition was a significant dose contributor for example when compared to inhalation dose.  2000 British Occupational Hygiene Society. Published by Elsevier Science Ltd. All rights reserved. Keywords: aerosols; deposition; indoor air pollution; particle size; skin; clearance; radiation doses

INTRODUCTION

Radioactive particles emitting β-radiation deposited on human skin will cause a skin dose. β-radiation can penetrate the outer dead skin layer and a significant fraction of the radiation energy is deposited in the living skin layer causing skin burns and increasing the risk of having skin cancer. As an example, most of the acute fatalities due to the Chernobyl accident were to firemen that suffered severe skin burns due to β-emitting particles deposited onto their protective clothing. In order evaluate the possible skin dose that a population around a nuclear facility might experience in case of an accident a study focusing on skin exposure to particulate airborne matter has been undertaken. The source term can be considered to be a cloud of particles of outdoor origin that will penetrate into houses. The primary pathway for skin exposure was considered to be deposition of airborne particles to the skin. Transfer rates of airborne particles to skin, hair and clothing have been determined for different particle sizes in a research programme supported by the European Commission, Fogh et al. (1999). Based on the obtained parameters a model study has been used to calculate possible skin doses using the framework of the conceptual model for assessment of dermal exposure presented by Schneider et

al. (1999). This paper presents the methods applied and discusses the key parameters that have been identified by this modelling work. Usually airborne releases are referred to by their activity concentration (Bq m⫺3) rather than their mass concentration (mg m⫺3), but to unify terms contamination is referred to by mass in this paper, as the two terms are proportional. THE MODEL

First the source term must be defined. When we consider deposition of airborne substances it is the integral air concentration of the radioactive particles over time:

冕

Integral air fraction = Cin(t)dt

where Cin is the activity concentration in indoor air (mg m⫺3). Simple one compartment models of indoor air dynamics linking indoor and outdoor air concentrations have been presented and discussed in several journal papers by various authors, e.g. Alzona et al. (1979). In a recent paper by Thatcher and Layton (1995) the following generic formula was presented for steady state indoor particle concentration: Cin =

Received 6 April 2000. *Author to whom correspondence should be addressed. Tel.: +45-46-77-41-73; Fax: +45-46-71-41-93; E-mail: [email protected]

(1)

LflAflR + lnPVCout + G Adnd + lnV

(2)

where Lfl is the floor dust loading (mg m⫺2), Afl is the floor area (m2), R is the resuspension rate (h⫺1),

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lv is the air exchange rate, P is the fraction of the outdoor particles penetrating the building structure (dimensionless), V is the volume of the house (m3), Cout is the outdoor concentration (mg m⫺3), G is the indoor particle generation rate (mg h⫺1), Ad is the surface area available for particle deposition (m2) and nd is the deposition velocity (m s⫺1). Resuspension can be ignored during the first phase, where the outdoor cloud passes the house, as the indoor surfaces will be clean (Lfl=0). Also, we can assume that we have no indoor sources. P is assumed to be unity, since Roed and Cannell (1987), in an experiment demonstrated that the indoor reduction could be explained purely by deposition without any filtration. With these assumptions the equation reduces to: Cin =

ln Adnd Cout, where ld = ln + ld V

(3)

Lange (1995) proved that this steady state ratio also is the long-term ratio between the indoor and outdoor integrated air concentrations:

冕

Cin(t)dt =

冕

ln C (t)dt ln + ld out

(4)

This equation thus establishes our source term. The direct deposition to the skin can now be calculated: Msu = Askinnskin

冕

ln C (t)dt ln + ld out

(5)

where Msu is the mass of the substance in the skin surface contamination layer (g), Askin is the exposed skin area (m2) and nskin is the deposition velocity to skin. As it is generally assumed that radiation dose has no lower threshold with respect to the detrimental effect of radiation, the assumption of proportionality between mass (Msu) and dose is justifiable. If we operated with a dose threshold we would need to have knowledge about the outdoor concentration’s time variations and solve the appropriate differential equation numerically. Now the radiation dose can be calculated as follows: 1 Drad = ⌫isotopeMsu lisotope + lskin

(6)

where ⌫isotope is a radionuclide specific dose conversion factor, lisotope is the radioactive decay constant for the radionuclide in question and lskin is the clearance rate of the substance from the skin contaminant layer. If the substance were taken up and accumulated in human body tissue, the dose would be proportional to the ratio between the uptake rate and the clearance rate:

luptake Duptake⬅Msu luptake + lskin

(7)

where Duptake is the dose due to uptake and luptake is the uptake rate (s). For an estimation of resuspension and contact transfer an estimation of the surface contamination inside the dwelling is needed. For an upward facing surface the deposition velocity to the floor will be the parameter determining the mass per unit area:

冕

ln C (t)dt Lfl = nfl ld + ln out

(8)

Where nfl is the deposition velocity to the floor (m s⫺1). A summary of the parameters applied in the model has been given in Table 1 below. RESULTS AND DISCUSSION

The model was used in a test case study using weekly air activity concentration measurements from Novozybkov in Russia in the weeks following the Chernobyl accident, Fogh et al. (1999). A specific scenario was chosen so that the skin exposure could be compared with other pathways such as inhalation and ingestion. The results showed that the skin dose was a significant contributor to the total dose in the early phase after the release. The contaminants were classified into three categories according to their form during the release: sub-micron particles with Activity Median Aerodynamic Diameter, AMAD, of 0.7 µm (typically elements that have been volatilised during the release and subsequently have condensated on ambient aerosols), coarse mode aerosol with an AMAD of 5 µm (primary combustion particles) and elemental Iodine, that can be considered to be an reactive gas. For each category deposition constants and skin deposition velocities were estimated based on the references listed in Table 1. Especially the skin deposition velocity is connected with some uncertainty, but some values can be found in the literature. As skin deposition velocities increase with increasing particle size the skin deposition was found to be greatest for the coarse particle group. But this increase was countered by a decrease in the I/O ratio due to the higher outdoor deposition constant for the larger particles. Overall, the increase in skin deposition for the coarse group compared with the fine group was only by a factor of three. The dependence on the clearance rate was found to be more significant, resulting in several orders of magnitude variance. As can be seen from Eqs. (6) and (7) this parameter becomes dominant when you have a long-lived radionuclide or a slow uptake rate. Research performed in support of the modelling showed larger variations for 2 and 4 µm particles. For smaller particles there is evidence that the clearance

Skin exposure modelling

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Table 1. Review of parameters used for modelling of skin exposure Parameter

Range

Comments

Reference

ln, ventilation rate

0.25–2.0

Well-investigated parameter compared to most of the other parameters involved. Typical value would be 0.4 h⫺1 for Northern Europe and above 1 h⫺1 for Southern Europe

e.g. AIVC (1994)

nd, indoor deposition velocity

0.5–5×10⫺4 m s⫺1

Some information exists on the indoor deposition velocities. These are very dependent on particle size

Engelmann (1992) and Fogh et al. (1997)

lskin, skin clearance rate

12 h–3 days

Very limited literature available. Depends on particle size, chemical properties of contaminant and to some extent skin characteristics (moisture, hair, etc.)

Fogh et al. (1999)

nskin, skin deposition velocity

苲10⫺3 m s⫺1, dp ⬍1 µm 苲10⫺2 m s⫺1, dp >1 µm

Not much information exists on skin deposition velocities. These depend on particle size and form

Fogh et al. (1999)Schneider et al. (1994)

Askin, exposed skin area

0.1 m2

Can be estimated relatively easily. The area of face and hands were used and estimated to be 苲0.1 m2

‘Simple’ estimate

Ad/V indoor area to volume ratio

1.4 to 1.75

Small houses with many rooms have low surface to volume ratios. For industrial buildings values around 0.6 h⫺1 are often used

Engelmann (1992)

P, building penetration factor

1

Older references attributed the Roed and Cannell (1987) reduction in indoor contaminant concentration to the filter effect and values of 0.2 to 0.8 can be found

R, resuspension rate

10⫺7–10⫺4 h⫺1

Strongly dependent on particle size, Thatcher and Layton increasing with size (1995)

is much slower, Fogh et al. (1999), but this is rather speculative, as very little literature exists on this topic. For contact transfer and resuspension the modelling was of a qualitative nature as the available information on the relevant parameters was even more unreliable. It could be concluded that both pathways were relatively more important for the coarse particle group, as these particles have higher indoor deposition velocities, which will lead to higher indoor surface contamination levels [see Eq. (7)]. Further, resuspension is only significant for particles larger than 1 µm. Deposition to clothing was also calculated for estimation of the exposure to beta and gamma emitters deposited here. If some estimate of the transfer

through ordinary clothing to skin could be found it would be easy to estimate the contribution from this pathway. In summary, the skin dose was estimated to be of the same order of magnitude as the inhalation dose during the passage of the contaminant cloud. The critical parameter was found to be the skin clearance rate, which is deserves further investigation.

Acknowledgements—This work was supported by the European Commissions Nuclear Fission Safety RTD Programme, contract FI4PCT950019. The interpretation of the results in a broader perspective was facilitated by the Dermal Exposure Network, supported by the European Commission Contract SMT4-4CT96-7502 (DG12-RSMT).

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AIVC 44 (1994) An analysis and data summary of the AIVC’s numerical database. Technical note AIVC (Air Infiltration and Ventilation Centre). Alzona, J., Cohen, B. L., Rudolph, H., Jow, H. N. and Frohliger, J. O. (1979) Indoor–outdoor relationships for airborne particulate matter of outdoor origin. Atmospheric Environment 13, 55–60. Engelmann, R. J. (1992) Sheltering effectiveness against plutonium provided by buildings. Atmospheric Environment 26A(11), 2037–2044. Fogh, C. L., Byrne, M. A., Roed, J. and Goddard, A. J. H. (1997) Size specific indoor aerosol deposition measurements and derived I/O concentration ratios. Atmospheric Environment 31(15), 2193–2203. Fogh, C. L., Byrne, M. A., Andersson, K. G., Bell, K. F., Roed, J., Fogh, C. L., Goddard, A. J. H., Vollmair, D. V. and Hotchkiss, S. A. M. (1999) Quantitative measurement of

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