Biomedical subjects

Bobby E Leonard

Publications and source records attributed to Bobby E Leonard.

5 recordsLinked to original sources

PubMed2005

Bimodal unattached 218Po Radon progeny and the influence of enhanced deposition effect at high-radon levels.

Unattached fraction (218)Po airborne particle size distributions from graded screen array diffusion batteries are analysed for bimodal constituents. The analysis is for the high (222)Rn concentration range from 10 to 50 kBq m(-3) where the recently reported enhanced deposition effect is in transition. We find that the single unattached fraction is distributed nominally at an arithmetic mean diameter (AMD) of 0.8-1.0 nm, and consists of two (218)Po molecular species with AMD congruent with 0.756 and 1.101 nm, respectively. In unfiltered room air the 0.756 nm species is the least stable, decreases monotonically in concentration magnitude beginning at 12 kBq m(-3) and disappears completely at approximately 32 kBq m(-3) (at a relative humidity of 54%). The AMD congruent with 1.101 nm mode does not become unstable until it reaches approximately 32 kBq m(-3) (222)Rn level and completely disappears at approximately 50 kBq m(-3). Similar behaviour is observed for filtered air but with some distinct differences. This work supports the bimodal findings for the (218)Po unattached fraction of Porstendorfer and co-workers and provides further details of the high-radon enhanced deposition effect.

Air Pollutants, Radioactive↗

PubMed2005

The radon inverse dose rate effect and high-LET galactic hazards.

The lung dose rate per unit 222Rn concentration in enclosed spaces is shown to experience transitions at high radon concentrations. This has implications on the radon inverse dose rate effect. At an air change rate (ACH) of 0.194 h(-1) and relative humidity (RH) of 52.3% in a 0.283 m3 test chamber, the total human lung dose for an adult male in a residential setting (breathing rate 0.78 m3 h(-1)) would undergo a reduction of 2.5 using the ICRP 66 human respiratory tract model and the BEIR VI methodology. Using the same methodology of both Cross (Pacific Northwest Laboratory rat exposures) and Lubin et al. (miners dose rates), adjustments are necessary for effects of RH and ACHs. These adjustments, however, do not affect the reduction behaviour. It is thus shown that the enhanced deposition effect (EDE) must influence the magnitude of the purported inverse dose rate effect (IDRE). In the analysis of animal data, Cross rat exposures in a 2.0 m3 chamber, a reduction in lung dose is estimated to be over a factor of 3 the transition between the 50 and 500 WLM week(-1) dose rate range. For an estimation of the EDE, using a hypothetical 30 m3 enclosure for underground miners, we obtain a factor of approximately 4 in human lung dose reduction. Although the extensive analyses required make these results qualitative, the EDE behaviour is sufficiently conclusive that these estimates show that the radon IDRE for lung cancer must be an EDE dosimetric issue as well as a radiological lung cell dose response issue. The consequence of analysis of other animal data would achieve the same conclusion.

Adult↗

PubMed2005

Adaptive response by single cell radiation hits--implications for nuclear workers.

Several radiobiology mechanisms show an ability of cells to activate protective mechanisms against the deleterious effects of ionizing radiations. The most interesting with respect to possible health benefits to humans is the phenomenon of adaptive response (AR). Fundamentally AR must be shown to be operative at very low doses and dose rates experienced during human radiation environment activities. A microdosimetric model is presented which, by the use of Poisson cell nucleus hit accumulation, enables examination of the single hit range of radiation exposure where the AR radio-protection is first activated. It is found, with the model, that single charged particle tract specific energy hits to the nucleus can activate AR. Both in vitro and in vivo data reported here indicated that AR may provide a reduction in cancer-causing DNA damage, from both radiation and spontaneous events, for nuclear workers and general public diagnostic treatments.

Adaptation, Physiological↗

PubMed2004

Radon progeny size distributions and enhanced deposition effects from high radon concentrations in an enclosed chamber.

Prior work studying radon progeny in a small enclosed chamber found that at high (222)Rn concentrations an enhanced surface deposition was observed. Subsequent measurements for unfiltered air showed minimal charged particle mobility influence. Progeny particle size measurements reported here, performed at the US Department of Energy Environmental Measurement Laboratory (now with Home Security Department), using the EML graded screen array (GSA) system show in unfiltered air that the high (222)Rn levels causes a reduction in the attached (218)Po progeny airborne particulates and formation of additional normal sized unattached ( approximately 0.80 nm) and also even smaller (218)Po below 0.50 nm. At a (222)Rn level of 51 kBq m(-3), 73% of all (218)Po are of a mean particle diameter of about 0.40 +/- 0.02 nm. At this (222)Rn level, the ratio of (218)Po to (222)Rn airborne concentrations is reduced significantly from the concentration ratio at low (222)Rn levels. Similar reductions and size reformations were observed for the (214)Pb and (214)Bi/Po progeny. The particle size changes are further confirmed using the plateout rates and corresponding deposition velocities. The Crump and Seinfeld deposition theory provides the corresponding particle diffusion coefficients. With the diffusion coefficient to ultrafine clustered particle diameter correlation of Ramamurthi and Hopke, good agreement is obtained between EML GSA and deposition velocity data down to 0.40 nm. Strong evidence is presented that the progeny size reduction is due to, as a result of air ionization, the increased neutralization rate (primarily from electron scavenging of OH molecules) of the initially charged progeny. This is shown to increase with the (1/2) power of (222)Rn concentration and relative humidity as well as increased air change rate in the chamber. These results imply that at (222)Rn levels above 50 kBq m(-3), at relative humidity of 52%, a considerable reduction in lung dose could occur from preferential deposition of the progeny in the nasal and oral passages.

Adsorption↗

PubMed2003

Progeny enhanced deposition rates primarily from increased particle diffusivity at high radon concentrations.

Prior work found that high 222Rn concentrations produced an enhanced radon progeny surface deposition effect (EDE) in a 0.283-m3 electrically grounded aluminum test chamber. To study a possible charge mobility effect, we report here additional measurements with the chamber lined with nonconducting, insulated materials. Specifically, with insulated, chemically clean glass we provide results of measurements at a number of 222Rn concentrations past the threshold and transition 222Rn levels (up to 60 kBq m(-3)). For EDE it was found at most a 14% charge mobility deposition effect. Without electrical grounding in the Al chamber with +/- 2,000 DC volts applied showed a 10% increase in 218Po surface deposition. Ambient and hot (37 degrees C) wetted filter paper to simulate lung tissue was measured and showed EDE of the same factor of > or = 2 as for all other materials. It is concluded that the enhanced progeny deposition from high radon concentration is primarily from a decrease in attached fraction, thus reducing progeny mean particle size, and increasing diffusivity and surface deposition.

Air Pollutants, Radioactive↗