PATHOLOGIC EFFECTS 37 



A. Sources 



1 . Human beings have always been exposed to airborne particles which contain naturally 

 occurring radioelements. 



2. The greatest occupational exposures to lungs of individuals have occurred among 

 miners of ores containing radioactive minerals and among some workers in ore processing and 

 in manufacturing industries that utilize naturally occurring radioelements. 



3. Fallout from nuclear weapons testing to date raises the inhalation exposure of human 

 populations to radioactive particles only slightly. 



4. An increased potential for exposure to radioactive aerosols in an expanding atomic 

 energy economy occurs in the processing and use of nuclear fuels and fission products. 



5. Other possible sources are the varied applications of radioelements in agriculture, 

 industry, medicine, and research. 



B. Behavior 



1 . Deposition and retention of inhaled radioactive particles are related to their physical 

 and chemical properties, and the physiological and anatomical characteristics of the host. 



2. For different radioactive particles, retention in the lung, translocation to other tissues, 

 and excretion have not been successfully generalized by a single mathematical model. 



3. The concentration and retention of radioactive particles are often such that higher 

 radiation doses may be delivered to organs other than the lung. 



C. Effects 



1. Biological effects caused by radioactive particles deposited in the respiratory tract 

 depend upon the radiation dose delivered to the tissue, the responsiveness of the tissue, and 

 the type of radiation. 



2. Whether alpha emitters are retained in the lungs or translocated to other sites, the 

 direct radiation effect is confined to cells immediately adjacent to sites of deposition. The 

 more penetrating radiation from beta and gamma emitters will affect tissues at greater radii 

 from sites of deposition. 



3. In humans, the principal effects of radiation to the respiratory tract are pneumonitis 

 and fibrosis; and in experimental animals, also carcinoma. 



4. Recent findings on the accumulation of radioactive material in the tracheobronchial 

 lymph nodes have pointed to their increased importance in evaluating inhalation hazards. 



5. In experimental animals, the mean lung radiation dose that appears to induce tumors 

 is about 1000 rads, assuming uniform distribution. But due to non-uniform distribution, with 

 some loci receiving orders of magnitude more radiation, this point is yet to be clarified. 



6. The effect of age and condition of the lung on retention and translocation of particles 

 and on the sensitivity of lung tissue is not known. These variables may markedly influence the 

 effects of inhaled radioactive particles. 



D. Evaluation 



1 . During the first few weeks following contaminating nuclear detonations, the radia- 

 tion dose from external exposure of the body to beta and gamma emitters in fallout usually 

 far exceeds the internal radiation dose from inhalation of the same radionuclides. 



