Agency for Toxic Substances and Disease Registry Radon Toxicity
Case Studies in Environmental Medicine (CSEM)
The average person in the United States
estimated 625 millirem/year dose from ionizing radiation.
The largest percentage is from medical radiation (48%, 300
mrem), primarily due to the popularity of CT scans and
Nuclear medicine. This is followed by radon (37%, 228
mrem), which is the largest source of background radiation.
While the dose from radon has remained the same over the
years, the percentage that it represents has dropped from
55%, based on 1980s data, to 37% using 2006 data. Due to
the increased use of certain medical procedures, this trend is
expected to continue (NCRP 2009). The dose of ionizing
radiation from radon comes from soil, water, natural gas,
and building materials.
The primary pathway for human exposure to radon is
inhalation from soil vapor intrusion into dwellings and
buildings. Indoor radon levels can, however, also originate
from water usage, outdoor air infiltration, and the presence
of building materials containing radium (EPA 2003).
