Radon decay products deposition

The concentration of radon decay products and therefore the factors F and fp are influenced by the basic processes of the attachment, recoil and deposition (plateout) and by room specific parameters of radon emanation and ventilation (Fig. 1). ... 

Surface deposition is the most important parameter in reduction of the free and aerosol attached radon decay products in room air. If V is the volume of a room and S is the surface area available for deposition (walls, furniture etc), the rate of removal (plateout rate) q is vg S/V, always assuming well mixed room air. vg is the deposition velocity. 

Taking into account the results of wind tunnel experiments the average deposition velocities for the free (vq = 2 m h 1) and the attached (v = 0.02 m h"1) radon decay products can be derived... 

Few data are available on the deposition of ultrafine particles (dradon decay products in a rubber latex cast of a human windpipe which extended from the epiglottis to a few cm below the Carina. Martin and Jacobi (1972)... 

A better way to express the activity of the radon decay products is as the Potential Alpha Energy Concentration (PAEC). This quantity incorporates the deposition of energy into the air and is expressed as MeV/nr... 

Another concept relating to the decay products is that of the "unattached" fraction. Although it is now known that the decay product atoms are really attached rapidly to ultrafine particles (0.5 to 3 nm in diameter), there is a long history of an operationally defined quantity called the "unattached" fraction. These decay products have much higher mobilities in the air and can more effectively deposit in the respiratory system. Thus, for a long time the "unattached" fraction has been given extra importance in estimating the health effects of radon decay products. Typically most of the "unattached" activity is Po-218 and the value of unattached frac-... 

It is possible to remove radon decay products from indoor air by filtration. The effects of air cleaning on dose levels are described by Jonassen (1987). However, there are major uncertainties in the effectiveness of air cleaning to remove the decay products because the particles are also removed. When the particles are removed, the "unattached fraction increases and although there are fewer decay products, they are more effective in depositing their dose of radiation to the lung tissue. Thus, there will. be much lower dose reduction than there is radioactivity reduction. It, therefore, may be more protective of health to control the radon rather than its decay products. 

Air with radon is passed into a vessel coated internally with zinc sulphide. Alpha particles from radon in the chamber, and from decay products deposited on the walls, give scintillations which are counted by photomultiplier tubes viewing the chamber through windows (Lucas, 1957). With a chamber of volume 0.11, and a counting time of 1 h the detection limit of 222Rn in air was about 10 Bq m-3, but by concentrat-... 

Mercer, T.T. Stowe, W. A. (1969) Deposition of unattached radon decay products in an impactor stage. Health Physics, 17, 259-64. 

While the major removal of radon gas from air is by radioactive decay, the chemically radioactive decay products (formerly called radon daughters) may additionally be removed by processes such as washout and plating-out on surfaces. In air inside buildings, these radioactive decay products can attach themselves to walls, floors, people, or airborne particles that are inhaled into the Ivmgs. Unattached radon decay products can also be inhaled and, subsequently, can become deposited on the Ivmg tissue. As a consequence, radon decay products are seldom in radioactive equilibrium with radon in the lower atmosphere (near the earth s sruface) or indoors. 

The subject of this particular volume relates to aerosol particle physics including aerosol characterisation, the formation mechanism, the aerodynamic size distribution of the activity and aerosol residence time, instrumentation techniques, aerosol collection and sampling, various kinds of environmental (atmospheric aerosols), particularly radioactive aerosols and the special case of radon decay product aerosols (indoors and outdoors) and the unattached fl ac-tion, thoron decay product aerosols, the deposition patterns of aerosol particles in the lung and the subsequent uptake into human subjects and risk assessment. 

Papastefanou and Bondietti (1991a, 1991b) performed experiments on the diffusive deposition of aerosol particles on wire screens and, in particular, used Pb deposition as a measure of the collection efficiency of the screens for aerosol-associated attached radionuclides in outdoor air, at Oak Ridge National Laboratory, Oak Ridge, Tennessee (35 58 N, 84 17 W) during the summer period. Stainless steel wire screens (60, 200, as well as 40 and 100 mesh/inch) were used in the experiments to collect the unattached species of radon decay products in ambient aerosols. Glass fibre filters were used as back-up to collect the radon decay products which passed the wire screens. The screens were separated from the back-up filter by a spacer screen (4 mesh/inch) to prevent contamination by the filter deposit (e.g., " Pb atoms) via a-recoil. 

The data show that the unattached fraction of " Pb atoms is independent of the Pb concentration in air in agreement with theoretical predictions based on the interactions occurring between radon decay products and aerosols (Raabe, 1969). It is now evident that the deposition of Pb 3% in the upper stages of a 1 ACFM normal cascade impactor is not due to unattached species, as Mercer and Stowe (1971) stated, but is from other particle loadings (e.g., resuspended material) (Papastefanou and Bondietti, 1987). 

The largest proportion of the natural radiation dose to humans results from the inhalation of the short-lived radon decay products Po, Pb, and " Po. During respiration these radioactive decay products of radon are deposited within different regions of the lung and can lead to substantial local doses there. For this reason, the natural radiation dose receives strong consideration in radiation protection. Thus, the surveillance of radon exposure in homes and in workplaces has emerged as an important operation. 

In all dosimetric models, the dominant parameter related to dose is the activity size distribution of the radon decay products in air as the original deposition destination and amount of... 

National Radiological Protection Board, NRPB (1990). Human exposure to radon in homes. Doc. NRPB 1, 17-32. National Research Council Committee on the Biolt ical Effects of Ionizing Radiations, NRC (1988). Health Risks of Radon and Other Interrudly Deposited Alpha Emitters (BEIRIV). National Academy Press, Washington, DC. Nero, A.V. (1988). Estimated risk of lung cancer from exposure to radon decay products in U.S. homes A brief review. Atmos. Environ. 22, 2205—2211. 

When radon ( Rn) and its short-lived decay products are inhaled, the alpha particles emitted by the deposited decay products dominate a radiation dose to lung tissue these products, especially those attached to small size aerosols or that remain in an unattached form, cause damage to sensitive lung cells, thereby increasing the probability of developing cancer. The Radon acts mainly as the source of its decay products, which actually deliver the dose to the lungs. However, as a convenient abbreviation, the health effects of radon decay products are often referred to simply as the health effects of radon [1-3]. 

Total exposures vary considerably with human activities as well. Frequent flyers, for example, receive higher doses of radiation because the intensity of cosmic radiation is significantly greater at high altitude than it is at ground level. Residents in locations such as Montana and Idaho, where there are uranium deposits, receive higher doses of radiation from radon, one of the radioactive decay products of uranium. 

This paper deals with the plate-out characteristics of a variety of materials such as metals, plastics, fabrics and powders to the decay products of radon and thoron under laboratory-controlled conditions. In a previous paper, the author reported on measurements on the attachment rate and deposition velocity of radon and thoron decay products (Bigu, 1985). In these experiments, stainless steel discs and filter paper were used. At the time, the assumption was made that the surface a-activity measured was independent of the chemical and physical nature, and conditions, of the surface on which the products were deposited. The present work was partly aimed at verifying this assumption. 

Each liter of air normally contains a few atoms each of 218Po, 211+Pb, 211+Bi and 211+Po, which are the short-lived decay products of the radioactive noble gas radon. When inhaled, these atoms can be deposited on the lining of the respiratory tract, causing irradiation of the tissue due to further radioactive decay. This irradiation accounts for about one half of the average persons dose... 

The dry deposition velocity of lead-212, a thoron (thoron or radon-220 itself originating from thorium-232) decay product has been reported to be in the range 0.03-0.6 cm/sec (Bigu 1985 Rangarajan et al. 1986). These low deposition velocities indicate that the thoron daughter, stable lead, may have a long residence time in the atmosphere with respect to dry deposition. 

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