What, me worry?

There are many misconceptions and strange ideas when it comes to the risks associated with radiation exposure.  Between the movies, cartoons, lawyers, politicians, anti-nuclear groups, and media most people don't know if they will 

Explode

or turn into a Super Hero!

The real risks from radiation exposure are far less exciting!

Units and Limits

Before we talk about risks from radiation exposure, we need to define some units and limits.  If I were to say you can't eat more than 10 apples a day without incurring a certain risk, you would understand what I was saying.  If I were to change that to 10 rem, you might not understand.  So we are not talking "apples and rems" let's review the radiation units we commonly use.

During the early days of radiation use, there were no precise units of radiation dose.  For purposes of radiation protection, a common measuring device was a piece of dental film with a paper-clip attached.  A daily exposure great enough to just produce a detectable shadow, called a "paper-clip unit" was considered a maximum permissible dose.  Another measure was the "erythema dose" or the minimum dose required to cause reddening of the skin.  As you can well imagine, these methods left a lot to be desired!

Today, dose is expressed, or estimated, in basically three ways: estimated based on ionizations in air; expressed as energy deposited in a unit mass; or estimated as relative biological damage per energy deposited per unit mass.

There are basically two standards for units - the old units of Roentgen, rad, and rem and the "new" International units of cc/kg (no official unit), Gray, and Seivert.  We will use the old units because that's what our meters currently read in.

The Roentgen (R)

An old unit that was very useful in the days before accelerators and high energy physics.  It basically was a measure of how many ionizations x-ray radiation caused in air (Unit exposure: 1 electrostatic unit of charge in 1 cubic centimeter of dry air at standard temperature and pressure (273 degrees Kelvin, 760 torr)).  Certain amounts of Roentgens caused certain biological effects.  The Roentgen was also useful because it was a close estimate of how much energy was deposited into biological mass from x-rays.  The unit is mentioned here because there are still some old instruments out there that read in R/hr.

The rad (r)

In 1953, the rad, a unit of absorbed dose, replaced the Roentgen, a unit of exposure.  The name "rad" stood for "Roentgen absorbed dose".  One rad is equivalent to 100 ergs of energy deposited per gram of material from any form of radiation.  For tissue, one Roentgen (air exposure) is equivalent to approximately 0.95 to 0.96 rad.  So, even today, an old R/hr meter can be used to estimate energy deposited in tissue for most x-rays and gamma rays (and we will also find for most measurable beta radiation!).

The rem

So, it sounds like we have solved our problem with the handy unit of absorbed dose.  Well, not exactly.  Remember from our discussion of the different radiations that different particles behave differently when they enter tissue.  Depositing one rad of alpha particles in a gram of tissue will cause many more ionizations than depositing one rad of beta radiation.  Therefore, we need a unit that estimates biological damage from radiation deposited.  That unit is the rem (rad equivalent man).  For this unit, we multiply the rads received by a factor which takes into account the type of radiation. Different organizations have different estimates for what the exact conversion factors should be.  We use the Nuclear Regulatory Commission (NRC) numbers: 

For x-rays, gamma rays, and beta radiation: Multiply by 1

For thermal (slow) neutrons: Multiply by 2

For fast neutrons: Multiply by 10

For alpha radiation: Multiply by 20

For practical purposes, you will only be measuring x-rays, gamma rays, and beta rays.  The reason you need to know the units is so you can have an idea of how the dose you receive equates to the measured dose and the assigned dose.

Dose Limits

There are two types of doses to consider - occupational and emergency doses.  The occupational dose limits take into account that you may only receive exposure to portions of your body and that radiation does more damage to rapidly multiplying, newly forming cells with a long dividing future than it does to other cells in the body.

Occupational Limits

(anyone working with radioactive material or radiation)

Note: Our As Low As Reasonably Achievable (ALARA) limits at the Research Center are established at 10% of these limits.

Risk

First, when looking at any potentially hazardous exposure, radioactive or chemical, we have to look at acute (right now or soon) effects and long-term effects.

Acute effects are not seen for doses below 200 rem.  You should never receive that much exposure!  Most researchers receive less than 0.01 rem per year.

The main risk from low-level exposure to radiation is an estimated increased risk of cancer.  This estimated risk of cancer is based on the cancers that survivors of high radiation doses, such as the survivors of the atom bomb drops in Japan, developed.  We draw a straight line from the bomb doses down to zero dose (above the 0.3 rem we receive each year from background) and assume every exposure carries some, even if very small, risk of cancer.  

This is a hypothesis and the cause for much debate.  Many leading doctors, medical and health physicists, and scientists believe that there is a minimum threshold dose, usually seen as 10 rem, under which there is no harmful effect. 

For radiation protection purposes, we assume the conservative estimates from the bomb data and limit exposures to radiation to as low as reasonable achievable (ALARA).

But what's the risk?

Ok, we'll stop beating around the bush and get into some numbers!

The Nuclear Regulatory Commission has adopted the risk value for an occupational radiation dose (above natural background) of 1 rem as representing a risk of 0.0004 or 4 in 10,000 of developing a fatal cancer.  You must realize that your normal risk of developing a fatal cancer 0.2, or 2,000 in 10,000!

So, lets weigh the risks!

Final note: The chances of receiving multiple rem exposures at this facility are astronomical!  Worry more about the very real dangers of daily life and less about the theoretical dangers of low-level radiation exposures!  Following procedures and being aware of the hazards in your work area will ensure your safety.

Below are some links to current studies being conducted, or that have been completed, to determine radiation exposure risk.  If the link doesn't appear to work, please try "copying and pasting" it into your browser address box: