Interestingly enough, United Airlines was the only airline to comply with FAA's voluntary program of descending all flights to 25,000 feet or lower if radiation levels exceeded 20 uSv/hour, which they did during the last week of October 2003.

So just what does 20 uSv/hour mean anyway?

The international unit dose equivalent is the sievert.  When referencing measurements, divisions of the sievert are used as well, so 1,000 millisieverts (mSv) equal one sievert and 1,000 microsieverts (uSv) equal one millisievert (mSv).   Got that?

Three Kinds of Radiation

Ionizing Radiation
Sub-atomic particles and rays traveling at a high velocity which can theoretically cause changes to human tissue by ionizing (changing the chemistry of) cells in our bodies.

Terrestrial Radiation
That portion of ionizing radiation which comes from radioactive atoms normally present in our bodies and in the earth.

Cosmic Radiation
That portion of ionizing radiation which comes to us through the atmosphere from the sun (solar) and other stars (galactic).

How much 'Cosmic Radiation' do I receive on a flight?

Here are some articles that discuss this:

• Radiation and Air Carrier Crew Members.  Article originally produced by United Airlines.  Not a bad article and easy to follow.
• Cosmic Radiation: Hidden Dangers Above the Clouds.  Article produced by the AFA Department of Air Safety & Health, Washington DC.  Also offers links to good resources.
• World Health Organization Article.  Contains more links to other technical and scientific resources.