<P> Epidemiological evidence shows a clear link between lung cancer and high concentrations of radon, with 21,000 radon - induced U.S. lung cancer deaths per year--second only to cigarette smoking--according to the United States Environmental Protection Agency . Thus in geographic areas where radon is present in heightened concentrations, radon is considered a significant indoor air contaminant . </P> <P> Residential exposure to radon gas has similar cancer risks as passive smoking . Radiation is a more potent source of cancer when it is combined with other cancer - causing agents, such as radon gas exposure plus smoking tobacco . </P> <P> In industrialized countries, Medical imaging contributes almost as much radiation dose to the public as natural background radiation . Collective dose to Americans from medical imaging grew by a factor of six from 1990 to 2006, mostly due to growing use of 3D scans that impart much more dose per procedure than traditional radiographs . CT scans alone, which account for half the medical imaging dose to the public, are estimated to be responsible for 0.4% of current cancers in the United States, and this may increase to as high as 1.5 - 2% with 2007 rates of CT usage; however, this estimate is disputed . Other nuclear medicine techniques involve the injection of radioactive pharmaceuticals directly into the bloodstream, and radiotherapy treatments deliberately deliver lethal doses (on a cellular level) to tumors and surrounding tissues . </P> <P> It has been estimated that CT scans performed in the US in 2007 alone will result in 29,000 new cancer cases in future years . This estimate is criticized by the American College of Radiology (ACR), which maintains that the life expectancy of CT scanned patients is not that of the general population and that the model of calculating cancer is based on total - body radiation exposure and thus faulty . </P>

Which of the following has an effect on the amount and type of radiation induced tissue damage