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What Is A Millisievert?

The "collapsed roof" of Dai-ichi Reactor #1

So, you’re sitting there watching the news and the reporter sagely looks at the camera and says, “The current radiation levels recorded at the #2 reactor was measured to be 100 millisieverts”. It sounds bad. The question is, how bad is it really?

In the SI system, a millisievert (mSv) is a measure of radiation that is equivalent to the average accumulated amount of background radiation a person would be exposed to over the course of a year. Now, it’s going to get a little confusing, but I’ll try to be as clear as I can be. A millisievert is also defined as the dose produced by the exposure to 1 milligray (mGy) of radiation. When discussing radiation risk, therefore, we end up talking about a combination of mSv and mGy.

If we’re dealing with medical x-rays and the like, we tend to be dealing with localized exposure. In the case of nuclear fallout, however, we’re looking at whole-body exposure. It is from this perspective that we’ll be working.

When looking at a dose of radiation, we’re concerned with the fact that it’s cumulative. As such, we always think in terms of dosage per hour. From a whole-body exposure, we run into onset of acute radiation sickness at an exposure of ~500 mGy. Significant gastrointestinal distress occurs at ~3000 mGy. Exposure of ~5000 mGy is considered to be lethal for 50% of the population when not treated for 30 days.

It gets to be tricky talking about this stuff because it’s important to always look at the dose per hour. Cumulative exposure means that the longer you’re exposed, the lower the amount of emitted radiation required to cause trouble. This is one of the reasons why they tell you to stay indoors when you’re in an area that is contaminated. By staying indoors, you reduce your exposure and, therefore, the dose.

When you’re facing a dose of ~100 mSv, you’re okay in the short-term. That said, it’s still a hit to your immune system. By the time you get to a dose of 400 mSv and higher, your immune system becomes compromised, white cell counts are reduced and your lymphocytes are depressed.

There are already a number of articles here that discuss optimizing immune system health, and these are all relevant for those of us who face exposure to increased radiation levels. I encourage readers to browse the site and get up to speed on the various ways we can optimize our immune function.

There’s a lot you can do to make exposure to low levels of radiation less harmful. Chief among them is simply this: DON’T PANIC! Worrying only ever makes things worse. There’s a big difference between being informed and making appropriate decisions versus panicking yourself unnecessarily. The more you worry, the more you depress your immune system, which is already a contraindication to good health.

So, don’t worry. Be happy. And stay tuned for more updates on how you can take care of yourself through the coming challenges we’re all facing.

Love and Light to all.


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  1. excellent run-down. i work in the nuclear industry, and i know that when i saw the coverage of everything in japan, i cringed because i knew that people would be whipped into a frenzy over the sensationalist fear-mongering by the press. thanks!

    • Thanks very much for the kind words. These technical articles are a little scary for me to write because I need to make sure that I get it right. Your affirmation of the content is very reassuring. 🙂



  2. This is an article about the millisevert that defines it only in relation to another unit, which is itself not defined. Wikipedia defines the gray as “the absorption of one joule of ionizing radiation by one kilogram of matter (usually human tissue).” A joule is unit of energy, that required to apply a force of one newton through a distance of one meter. A newton is a unit of force, that required to accelerate a mass of one kilogram at the rate of a meter per second squared.

    A millisievert is not “equivalent to the average accumulated amount of background radiation a person would be exposed to over the course of a year,” since that exposure is variable. One would not define a kilogram or a meter as an average fraction of mass or distance.

    • Hi, David.

      Your corrections are valid, but what is important here is that the sievert and gray measure different quantities. Millisieverts measure an equivalent dose. So, while the gray and sievert work with the same dimensions, they can colloquially be described as I have done in this article. You can get as specific as you like, but we still tend to describe things simply, e.g.:

      * Eating a banana: 0.0001 mSv
      * Dental x-ray: 0.005 mSv
      * Mammogram: 3 mSv
      * Chest CT scan: 6–18 mSv

      Thanks for posting. 🙂

  3. information provided in this arctile may not be complete.
    If we look at french nucleor explosions, there fallout at 400 km away island had terrible effects for the populations. Even the French Gov. has recongnised that (40 years after due to lawsuits). Doses in msv were very low (bellow 5) but event then, most people die of cancer from radiations.

    • Hi, inpacts. Thanks for sharing your thoughts.

      It’s important to bear in mind that we’re talking about vast differences in distance. The fallout in the French incident was 400 km versus over 11,000 km between Japan and, say, Los Angeles. It’s not possible to make a direct comparison because of the distances and the prevailing wind patterns.

      While there is some literal risks involved, practical risks are minimal. Most people are at far, far greater risk from health issues from metabolic syndrome, alcohol and tobacco use.

  4. the factor envolved is physical obsorption, alcohol and tobacco use is about as lame as an excuse and a horrific example to radiation contamination. the human biological layout is well designed obviously when contaminated with certain elements as you were can effect the body in different ways Alcohol at an excess will damage the body, and this can be changed done by choice or unpleasant physical damage brought on by the individual…Radiation at high levels can not be adjusted because it is unseen and usually detected too late. As far as Japan I guarantee that cancer will be the topic for the next 20 yrs from infants to adults because of government coverups. and the political process thousands upon thousands will perish prematurely.. Oh heres an example Chernobyl still the truth hasnt touched the surface Do your due dilligence

    • Hi, Robert. Thanks for dropping by and sharing your thoughts. As this article describes the importance of cumulative exposure, we are in agreement on that aspect. That said, I continue to maintain that the current trend in diet and lifestyle choices is a far bigger problem than radiation exposure. You claim that the next 20 years will see cancer being a prevailing topic. I claim that it is already the case, with more than 20% of the population suffering from cancer within their lifetime.

      This is not a play on statistics, it is fact. So, while there’s no denying the grave importance of the nuclear event has it has been unfolding over the last year, it absolutely pales in comparison with the cancer-causing problems we live with on a daily basis and have become so numb towards that we view them and cancer itself as a normal part of growing old.

      I’ve done my due diligence. I live in Tokyo. The food and water supply is compromised by radiation and the younger one is, the more likely there are to be health ramifications later in life. Be that as it may, the cancers caused by the Fukushima incident are going to be a mere blip compared to the ongoing progression of unwellness that is seen throughout 3rd-world countries over the last 30 years.

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