Radon-219, ultrafine particle

Cohen, B.S., Deposition of Ultrafine Particles in the Human Tracheobronchial Tree A Determinant of the Dose from Radon Daughters, this volume (1987). 

In order to examine the process of ultrafine particle formation, a joint series of experiments were conducted at the Denver Research Center of the U.S. Bureau of Mines. In the Denver radon chamber, the activity size distribution of the ultrafine mode was measured using the mobility analyzer designed by Chu and Hopke (1985), the separate single screen method (Holub and Knutson, 1987), and the stacked single screen method (Holub and Knutson, 1987) for various relative humidities and for various concentrations of SO. The results... 

It has been found that the "unattached" fraction is an ultrafine particle aerosol with a size range of 0.5 to 3 nm. In order to initiate studies on the formation mechanism for these ultrafine particles, a series of experiments were made in the U.S. Bureau of Mines radon chamber. By introducing SO into the chamber, particles were produced with an ultrafine size distribution. It has been found that the particle formation mechanism is supressed by the presence of radical scavengers. These experiments suggest that radiolysis following the decay of Rn-222 gives rise to the observed aerosol and the properties of the resulting aerosol are dependent on the nature and the amount of reactive gas present. 

It has been reported for many years that condensation nuclei can be produced by ionizing radiation. Recent studies have improved the measurement of the activity size distribution of these ultrafine particles produced by radon and its daughters (Reineking, et al., 1985 Knutson, et al., 1985). It seems that the Po-218 ion is formed by the radon decay, is neutralized within a few tens of milliseconds, and then attached to an ultrafine particle formed by the radiolysis generated by the polonium ion recoil. Although there will be radiolysis along the alpha track, those reactions will be very far away (several centimeters) from the polonium nucleus when it reaches thermal velocity. The recoil path radiolysis therefore seems to be the more likely source of the ultrafine particles near enough to the polonium atom to rapidly incorporate it. 

The hydroxyl radicals produced by water molecule radiolysis react promptly with oxidizable species such as SOg in air and form a condensed phase. These molecules further coagulate and become ultrafine particles. The radon concentration in the... 

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)... 

This paper will present some results of a set of experiments carried out in the hollow airway cast system with ultrafine particles which are or particular interest for the calculation of the dose to the bronchial epithelium from the short lived radon daughters. Detailed deposition efficiencies and intrabronchial distributions are presented elsewhere (Cohen, et al., 1986). 

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-... 

Indoor radon in most houses come primarily from soil gas infiltrating into the house because of pressure-driven flow. The radon decays into a series of decay products to which most of the health effects are attributed. These decay products begin attached to ultrafine particles that either plateout on surfaces such as walls, furniture, etc., or become attached to larger particles that are present in the indoor air. The nature of those particles depends on the kinds of sources that exist in the house such as smokers, gas stoves, etc. 

The amount of particles determine the quantity of decay products that stay in the air (equilibrium fraction, F) and the fraction of activity associated with the "unattached or ultrafine mode of the size distribution (fDot) These decay products are certainly harmful at high concentrations but we cannot yet detect the effects at normal levels because the vast majority of lung cancer death are due to smoking. Models predict that potentially 9000 lung cancer deaths per year in the United States are due to indoor radon. Methods are currently available and new methods are being developed and tested for lowering the levels of radon in indoor air. 

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