Radiation Basics - Measuring Ionizing Radiation

Measuring Ionizing Radiation

RAD and REM

The human senses cannot detect radiation or discern whether a material is radioactive. However, a variety of instruments can detect and measure radiation reliably and accurately. The amount of ionizing radiation, or 'dose', received by a person is measured in terms of the energy absorbed in the body tissue, and is expressed in RAD. One rad is 0.01 joules deposited per kilogram of mass. Equal exposure to different types of radiation expressed as RAD, do not however, necessarily produce equal biological effects. One rad of alpha radiation, for example, will have a greater effect than one rad of beta radiation. When we talk about radiation effects, we therefore express the radiation as effective dose, in a unit called the REM (Roentgen Equivalent Man).  Regardless of the type of radiation, one rem of radiation produces the same biological effect. (100 rem = 1 Sv)  Smaller quantities are expressed in 'mrem' (one thousandth) or 'µrem' (one millionth of a rem). We will use the most common unit, rem, here.

What Are The Health Risks From Ionizing Radiation?

It has been known for many years that large doses of ionizing radiation, very much larger than background levels, can cause a measurable increase in cancers and leukemias ('cancer of the blood') after some years delay. It must also be assumed, because of experiments on plants and animals, that ionizing radiation can also cause genetic mutations that affect future generations, although there has been no evidence of radiation-induced mutation in humans. At very high levels, radiation can cause sickness and death within weeks of exposure - see list below.

But what are the chances of developing cancer from low doses of radiation? The prevailing assumption is that any dose of radiation, no matter how small, involves a possibility of risk to human health. However, there is no scientific evidence of risk at doses below approximatly 5 rem in a short period of time or about 10 rem over a period of one year.

Higher accumulated doses of radiation might produce a cancer which would only be observed several - up to twenty - years after the radiation exposure. This delay makes it impossible to say with any certainty which of many possible agents were the cause of a particular cancer. In western countries, about a quarter of people die from cancers, with smoking, dietary factors, genetic factors and strong sunlight being among the main causes. Radiation is a weak carcinogen, but undue exposure could certainly increase health risks.

On the other hand, large doses of radiation directed specifically at a tumor are used in radiation therapy to kill cancerous cells, and thereby often save lives (usually in conjunction with chemotherapy or surgery). Much larger doses are used to kill harmful bacteria in food, and to sterilize bandages and other medical equipment. Radiation has become a valuable tool in our modern world.

How Much Ionizing Radiation is Dangerous?

Radiation levels and their effects

The following list gives an indication of the likely effects of a range of whole body radiation doses and dose rates to individuals:

• 1,000 rem as a short-term and whole-body dose would cause immediate illness, such as nausea and decreased white blood cell count, and subsequent death within a few weeks.
  Between 200 and 1000 rem in a short-term dose would cause severe radiation sickness with increasing likelihood that this would be fatal.
• 100 rem in a short term dose is about the threshold for causing immediate radiation sickness in a person of average physical attributes, but would be unlikely to cause death. Above 100 rem, severity of illness increases with dose.
  If doses greater than 100 rem occur over a long period they are less likely to have early health effects but they create a definite risk that cancer will develop many years later.
• Above about 10 rem, the probability of cancer (rather than the severity of illness) increases with dose. The estimated risk of fatal cancer is 5 of every 100 persons exposed to a dose of 100 rem.
• 5 rem is, conservatively, the lowest dose at which there is any evidence of cancer being caused in adults. It is also the highest dose which is allowed by regulation in any one year of occupational exposure. Dose rates greater than 5 rem/yr arise from natural background levels in several parts of the world but do not cause any discernible harm to local populations.
• 2 rem/yr averaged over 5 years is the limit for radiological personnel such as employees in the nuclear industry, uranium or mineral sands miners, and hospital workers (who are all closely monitored).
• 1 rem/yr is the maximum actual dose rate received by any Australian uranium miner.
• 300-500 mrem/yr is the typical dose rate (above background) received by uranium miners in Australia and Canada.
• 300 mrem/yr (approx.) is the typical background radiation from natural sources in North America, including an average of almost 200 mrem/yr from radon in air.
• 200 mrem/yr (approx.) is the typical background radiation from natural sources, including an average of 70 mrem/yr from radon in air. This is close to the minimum dose received by all humans anywhere on Earth.
• 30-60 mrem/yr is a typical range of dose rates from artificial sources of radiation, mostly medical.
• 5 mrem/yr, a very small fraction of natural background radiation, is the design target for maximum radiation at the perimeter fence of a nuclear electricity generating station. In practice, the actual dose is less.

What is the risk estimate?

According to the Biological Effects of Ionizing Radiation committee V (BEIR V), the risk of cancer death is 0.08% per rem for doses received rapidly (acute) and might be 2-4 times (0.04% per rem) less than that for doses received over a long period of time (chronic). These risk estimates are an average for all ages, males and females, and all forms of cancer. There is a great deal of uncertainty associated with the estimate.

Risk from radiation exposure has been estimated by other scientific groups. The other estimates are not the exact same as the BEIR V estimates, due to differing methods of risk and assumptions used in the calculations, but all are close.

Risk comparison

The real question is: how much will radiation exposure increase my chances of cancer death over my lifetime?

To answer this, we need to make a few general statements of understanding. One is that in the US, the current death rate from cancer is approximately 20 percent, so out of any group of 10,000 United States citizens, about 2,000 of them will die of cancer. Second, that contracting cancer is a random process, where given a set population, we can estimate that about 20 percent will die from cancer, but we cannot say which individuals will die. Finally, that a conservative estimate of risk from low doses of radiation is thought to be one in which the risk is linear with dose. That is, that the risk increases with a subsequent increase in dose. Most scientists believe that this is a conservative model of the risk.

So, now the risk estimates: If you were to take a large population, such as 10,000 people and expose them to one rem (to their whole body), you would expect approximately eight additional deaths (0.08% X 10,000 X 1 rem). So, instead of the 2,000 people expected to die from cancer naturally, you would now have 2,008. This small increase in the expected number of deaths would not be seen in this group, due to natural fluctuations in the rate of cancer.

What needs to be remembered is that it is not known that 8 people will die, but that there is a risk of 8 additional deaths in a group of 10,000 people if they would all receive 1 rem instantaneously.

If they would receive the 1 rem over a long period of time, such as a year, the risk would be less than half this (< 4 expected fatal cancers).

Risks can be looked at in many ways. Here are a few ways to help visualize risk:

1. Health Risks

   One way often used is to look at the number of "days lost" out of a population due to early death from separate causes, then dividing those days lost between the population to get an "Average Life expectancy lost" due to those causes. The following is a table of life expectancy lost for several causes:

   Health Risk	Est. Life Expectancy Lost
   Smoking 20 cigarettes a day	6 years
   Overweight (15%)	2 years
   Alcohol (US Avg.)	1 year
   All Accidents	207 days
   All Natural Hazards	7 days
   Occupational dose (300 mrem/yr)	15 days
   Occupational dose (1 rem/yr)	51 days

2. Job Risks

   You can also use the same approach to looking at risks on the job:

   Industry Type	Est. Life Expectancy Lost
   All Industries	60 days
   Agriculture	320 days
   Construction	227 days
   Mining and quarrying	167 days
   Manufacturing	40 days
   Occupational dose (300 mrem/yr)	15 days
   Occupational dose (1 rem/yr)	51 days

   (These are estimates taken from the NRC Draft guide DG-8012 and were adapted from B.L Cohen and I.S. Lee, "Catalogue of Risks Extended and Updates", Health Physics, Vol. 61, September 1991.)

3. Risks of Common Activities

   Another way of looking at risk, is to look at the Relative Risk of 1 in a million chances of dying of activities common to our society:

   • Smoking 1.4 cigarettes (lung cancer)
   • Eating 40 tablespoons of peanut butter
   • Spending 2 days in New York City (air pollution)
   • Driving 40 miles in a car (accident)
   • Flying 2500 miles in a jet (accident)
   • Canoeing for 6 minutes
   • Receiving 10 mrem of radiation (cancer)

   (Adapted from DOE Radiation Worker Training, based on work by B.L Cohen, Sc.D.)