Adult bone marrow: alpha-radiation dose from radon in fat cells
Several positive correlations have been demonstrated between indoor radon levels and both adult and childhood leukaemia. It is estimated that natural background radiation accounts for 14% of incidence of childhood leukaemia (COMARE Fourth Report). The target cells for the induction of myeloid leukaemia in adults are understood to be located within the bone marrow, close to the fat cells. Radon gas is 16 times more soluble in fat than it is in other tissues, including blood. This has important implications for the alpha-radiation dose to adult bone marrow which has an age-related fat proportion of up to 60% and more. We have used the topography of bone marrow thin sections with image analysis to calculate the radon-derived dose to adult bone marrow at different ages.
Normal adult bone marrow contains an age-related proportion of fat cells. This can be up to 60% fat in rib and vertebra and over 80% in femur. Radon gas is sixteen times more soluble in fat than in blood, providing a potentially enhanced source of alpha-radiation to the haemopoietic tissue from radon dissolved in the fat cells.
Trabecular bone with marrow removed
In terms of health effects in adult bone marrow, the importance of radon gas comes from its alpha-decay to alpha-emitting decay products 218Po and 214Po. When inhaled, radon gas diffuses across the alveolar membrane of the lung, indeed, radon permeates all cell membranes with ease, including endothelial cells of capillary walls. Blood transport around the human body takes about 60 seconds and the radon concentration in the body tissues is governed by the radon solubility in those tissues.
Fat replacement of trabecular bone
Alpha-particle decays from radon and short-lived decay products 218Po and 214Po within the fat cells deliver most of their energy to the surrounding tissue, although some may be deposited in the fat cell itself.
Fat cell diameters
The target cells for the induction of acute myeloid leukaemia in adults are understood to be located within the haemopoietic tissue, in close proximity to the fat cells. The alpha-radiation dose to the haemopoietic tissue from domestic levels of radon exposure has been calculated using actual situations with actual fat cell sizes and distributions, such as are provided by histologically prepared samples of bone and marrow.
Spatial distribution of fat cells
This was first carried out manually for human rib for an age range of 16-96 years (Allen et al, 1995) to provide information on (i) the size distribution of medullary fat cells, (ii) fat fraction and (iii) the spatial distribution of fat cells within trabecular spaces in normal adult bone marrow. The work has continued using an automated image analysis system and histological slides of adult bone marrow from different skeletal sites - vertebra, rib, sternum, ischium and femur. The information gained has been used to calculate the alpha-radiation dose to the haemopoietic tissue from radon and its short-lived decay products in fat cells at domestic levels of exposure.
Reference
Is indoor radon linked to leukaemia in children and adults? - A review of the evidence.
Henshaw DL and Allen JE, 2001.
Contributions to the International Symposium on Natural Ionizing Radiation
and Health 5th-6th June, Drammensveien 78, Oslo, Norway.
Fat cells in red bone marrow of human rib: their size and spatial distribution with respect to the radon-derived dose to the haemopoietic tissue
Allen JE, Henshaw DL, Keitch PA, Fews AP and Eatough JP, 1995
International Journal of Radiation Biology 68, No.6, 669-678