This article was co-authored with Dr. Rama Hoetzlein, Assistant Professor, Department of Architecture and Media Technology at the Aalborg University at Copenhagen. Dr. Hoetzlein prepared the graphic.
With increasing concern about radiation exposure in Japan and beyond, you might wonder: How much radiation am I getting? It depends...
Note that Dr. Hoetzlein and I are not experts in nuclear engineering; this graphic has been prepared using authoritative source material and is intended as a general guide.
This visualization shows a map of low level ionizing radiation levels received from various sources throughout the year for the average person, along with a time series plot of measured levels in Tokyo from March 15th to March 21st following the Fukushima accident . It shows that the fluctuations in levels recently recorded in Tokyo are below the average accumulated radiation per person in one year, 6.2 mSv/year. Notice that this map displays ionizing radiation only (beta, gamma), received directly from Fukushima, and does not account for the transport of radioactive particles that may be carried in food or in the air.
Reports by the Nuclear and Radiation Studies Board  indicate that the average person accumulates 6.2 mSv/year from cosmic radiation (13%), radon (42%), terrestrial background (16%) and from medical radiation (10%). Living in a high background area such as Denver, CO (cosmic radiation increases with altitude) will increase the terrestrial background amount, while getting more than one mammogram, X-ray or chest CT per year will increase the medical amount. The individual effects of radiation are cumulative. You can estimate your own radiation intake by choosing all the dots in the graph that apply to you, and add up these amounts per year. Thus there is no "safe level" of low level radiation, as all radiation simply adds to your annual total. Smokers who average one and a half packs per day must add 30 mSv/year, as cigarettes contain radioactive lead and polonium .
The idea of going "over your limit" is also suspect since there is no such limit. In Ramsar, Iran, local populations have naturally adapted to levels of 260 mSv/year with no increase in cancer rates , a physical response explained by a theory called radiation hormesis. In this theory, brief bursts of radiation are more harmful than low levels in the long term.
On March 15th, 10:37am and 7:13pm, radiation levels in Tokyo peaked at 14 mSv/year and 9 mSv/year. These peaks are equivalent to getting a single Chest CT scan (6-18 mSv/dose). While this should be avoided, in order to have any significant effect on our radiation intake these peaks would need to be sustained for a full year. The average values from March 15th to 21st have fluctuated between 0.7 and 2.6 mSv/year, which is still within the range of other cities in the world (the X-axis represents time for Tokyo and does not apply to other points).
These results show that, so far, Geiger counter readings taken in Tokyo do not suggest that levels of direct ionizing radiation are at a point that warrant concern. In addition to hereditary predisposition, the two leading causes of lung cancer, smoke and radon, can be higher sources of low level radiation.
It should be noted that particle radiation, radioactive matter physically transported in the air or food from Fukushima, are not investigated here and may represent a greater risk.
Rama Hoetzlein is a media artist and computer scientist working in the areas of computer graphics and artificial intelligence. He completed a BFA in Fine Arts and a BA in Computer Science at Cornell University in 2001, working on robotic sculpture. From 2001 to 2004, Rama co-founded the Game Design Initiative at Cornell University with David Schwartz (CS). In 2007, Rama completed his master's thesis on knowledge organization and visualization with the Media Arts & Technology Program at the University of California Santa Barbara. He has shown work at the 2nd International Beijing Arts & Science Exhibition at Tsinghua University, Centre for the Contemporary Image in Geneve, Switzerland, and Intl. Conference on the Arts & Society in Venice, Italy. In 2010, Rama completed his Ph.D. in Media Arts & Technology on the topic of integrated frameworks for media artists, focusing on interactive procedural modeling. He is currently Assistant Professor in Medialogy at Aalborg University Copenhagen, teaching in the areas of animation, computer graphics, and visualization.
 Tokyo levels: Tokyo radiation levels - daily updates. Geiger counter readings by Chottom, March 21st. http://chottomatte.net/2011/03/16/tokyo-radiation-levels-daily-updates/
Alternatively, you can find up date radiation levels across Japan here.
 ] City levels: Passmore, C. and Kirr, M., A Method to Characterize Site, Urban and Regional Ambient Background Radiation. Radiation Protection Dosimetry. Oct 19, 2010
 Smoking levels: Ravenholt, R.T. Radioactivity in Cigarette Smoke. The New England Journal of Medicine, July 28, 1982. 306(6): 364-365
 Radon & other: University of Iowa Hospitals & Clinics. Radiation Exposure: The Facts vs. Fiction. Created 1996, Updated 2001.
 Unusual cities: M. Ghiassi-nejad, et al. Very High Background Radiation Areas of Ramsar, Iran: Preliminary Biological Studies, 2002. Health Physics Society.
 Airline occupation: Radiation Exposure During Commercial Airline Flights. Health Physics Society. http://www.hps.org/publicinformation/ate/faqs/commercialflights.html
 Natural sources: National Council on Radiation Protection and Measurements. Exposure of the Population in the US and Canada from Natural Background Radiation. 2007
 Three Mile Island: American Nuclear Society. What Happened and What Didn't in TMI-2 Accident. http://www.ans.org/pi/resources/sptopics/tmi/
 Cumulative percentages: Nuclear and Radiation Studies Board. Bier VII: Health Risks from Exposure to Low Levels of Ionizing Radiation. Nat'l Academy of Sciences.
 Other source: H. Vanmarcke. UNSCEAR 2000: Sources and Effects of Ionizing Radiation. http://www.unscear.org/unscear/publications/2000_1.html
This is not very good. The 6.2 mSv accounts for more medical exposure than BEIR VII accounted for. That's why the graph jumps from 2.4 mSv to 6.2 mSv with only about a 15% addition of dose. The 6.2 mSv is accurate and is based on an NCRP study that was done in the same year BEIR VII was published. The NCRP study showed that on average we get more medical exposure than in the past (due to increased technology - primarily computerized tomography).
The use of direct beta, gamma exposure as a basis for comparison is misleading. It is the dose from inhaled or ingested radioactivity that will be the greatest contribution to the average Japanese person's dose.
Also, one must be cautious when making comparisons with whole body doses and doses to specific organs. The risks are different.
Thank you for this perspective, as you raise some valid points.