Radioactivity - The Changing Arctic Environment: The Arctic Messenger - page 71

when it comes to evaluating when specific anthropogenic sources may represent present or
future environmental and human health risks on local, regional or larger geographic scales.
Here are the four main sources and global levels of annual effective dose from natural
sources:

· Cosmic radiation: This varies with altitude but has a global average of 0.30 mSv.

It is increased by air travel at a rate of about 0.4 mSv for every 100 hours of fly-
ing.

· Terrestrial radiation: This is highly variable depending on local and regional geo-

logy. Doses can be as high as 260 mSv in northern Iran or 90 mSv in Nigeria. In
Canada, the estimated highest annual dose is approximately 2.3 mSv measured in
the Arctic (Northwest Territories). The global average is about 0.5 mSv a year.

· Dose from inhalation: Earth also contributes to our levels of exposure in a highly

variable way that reflects local geology. Radon gas, which is produced by the
uranium decay series, irradiates the lungs when inhaled. Radon naturally disperses
as it enters the atmosphere from the ground, but concentration can build up in un-
ventilated buildings. The global average annual effective dose of radon radiation is
approximately 1.2 mSv. People who smoke tobacco can greatly increase the dose
at “hot spots” in their lungs. Radon gas decays into a series of highly radioactive
particulate metals (radon decay products) that can cling to the sticky hairs (trich-
omes) on tobacco leaves. The particles are not water soluble and make their way
into tobacco. In the lungs, the radon decay products - lead-210 (half-life = 22.3
years) and polonium-210 (half-life = 138 days) - are retained in the bronchioles. If
a person has been smoking for many years, the concentration of these radionuc-
lides directly on tissues of the bronchioles can become very high and intense local-
ized radiation doses can occur at the bronchiole “hot spots”.

· Dose from ingestion: Several sources of natural radiation penetrate our bodies

through food ingestion, drinking and breathing. If we ignore radon decay, potassi-