In an incident involving radioactive materials, both the public and emergency responders will need to be assessed for exposure. To understand the hazards associated with exposure, some knowledge of radiation measurement units and tolerable levels of radiation dose is necessary.

First, not all methods of measuring radiation are meant to determine human exposure. Geiger counters, which the public often thinks of when hearing “radiation measurements,” are often used simply to determine if radioactive contamination — dust, dirt and debris containing radioactive elements — is present. For these measurements, the Geiger counter needs only to read out in counts per minute, which are simply the clicks per minute that the Geiger counter is audibly emitting as it encounters radioactivity.

Radioactive atoms disintegrate as they emit radiation; they are literally changing into other atoms with a certain rate of disintegration, or DPM for disintegrations per minute. The Geiger counter, because of its construction and the way it is used, can't detect every DPM. In fact, it detects relatively few of these so that the CPM to DPM ratio is usually rather low, but in many instances this is good enough for finding contamination.

The CPM readings don't tell us directly what the human radiation exposure is, although one can assume that the more CPM measured, the greater the hazard from the radiation. Nevertheless, more measurements or interpretation by a professional health physicist are necessary to obtain an estimate of human radiation dose. The type of radiation measured — gamma rays versus alpha particles — and its energy are required to make this exposure assessment and to identify the organs of the body that could be affected.

Exposure potential

Humans can be contaminated by radioactivity by inhaling particulates into their lungs or by ingesting contaminated food or water. It's also possible to obtain a dose from radiation from a distance if it's gamma radiation.

For example, airborne radioactive material can remain aloft for sometime before settling onto various surfaces such as buildings, automobiles, trees, streets and green spaces. It also can become airborne again if disturbed, or it might be washed by rain into bodies of water.

Fallout from nuclear explosions is an extreme example of airborne radioactivity. Fallout is the melted and aerosolized dirt and debris created at the surface detonation point and sent aloft by the heat of the explosion. It returns to the ground under the influence of wind, which will control the direction of its return, and of rainfall, which will help to wash it out of the atmosphere. Once at the surface, the fallout can enter food sources or water supplies, or it can be resuspended in the air by wind or the mechanical action of machines like automobiles or farm equipment. Some radioactive material in fallout also can deliver a gamma-ray dose at a distance. Therefore, under certain conditions, there would be no need to contact or ingest the radioactive material to become contaminated.

Human exposure doses can be measured using the unit of radiation dose called the “rad.” The “rem” unit is also used. For gamma and x-rays, one rad is equal to one rem. Some survey instruments and even some Geiger counters can read out in fractions of a rad or rem per hour (note that this unit is a dose rate). The fraction most often seen on radiation instruments is the milli-rem, which is one-thousandth of a rem; thus readings usually are reported as milli-rem per hour. To obtain a total dose in mrem, the amount of time spent in the field of radiation (hours) is multiplied by the dose rate.

Dangerous doses

So what are dangerous radiation doses, and what are doses humans can live with? Some background information is necessary for this discussion. First, governmental agencies allow first responders to be exposed to higher levels of radioactivity in the event of a radiological or a nuclear weapon attack, considerably higher than the everyday occupational regulatory limits for those who work with radioactive materials. Further, the non-emergency, occupational regulatory limits are far below the exposures where acutely serious radiation effects become apparent.

The other deleterious effect, cancer, is probable, and its rate of occurrence increases with levels of radiation exposure — much like the probability of lung cancer increases with the greater number of cigarettes smoked. But the risk is such that a limited cumulative dose, say per year, can be attained on the job repeatedly year after year, without deleterious biological effect later in life. Crossing a regulatory limit or guideline isn't a guarantee that a radiation-induced biological effect will occur.

We are continually exposed to background radiation. It emanates from naturally occurring radioactive materials found in the air we breathe, the food and water we ingest and the materials with which we build our homes and work places. It is a constant exposure that occurs throughout our lives. Partial body doses are allowed to exceed whole body doses up to 10 times. This is so because partial body doses, particularly to the extremities where critical organs aren't located, do less biological damage than doses that bathe the whole body in the radiation field.

For example, Americans typically tolerate 0.3 rem annually from background radiation and 0.01 rem from industrial operations without complication. Workers who deal with radioactivity can be exposed to 5 rem to their entire bodies and 50 rem to their extremities, as stated in the Code of Federal Regulations, Title 10, Part 20, Standards for Protection Against Radiation.

Similarly, protective action guidelines values have been developed by the U.S. Environmental Protection Agency. For workers performing emergency services, these rem limits include:

• Protecting valuable property: 10
• Life-saving or protection of large populations: 25
• Life-saving or protection of large populations when rescuer fully understands the risks involved: >25

These PAGS are currently under review and may be modified in the near future.

Harmful results

In a nuclear attack involving fallout or perhaps a radiological attack where radiological materials could be the source of penetrating gamma rays, it would be important to consider exposure rates that could induce potentially life-threatening acute radiation syndrome. Definitive biological changes in humans can be induced by rapidly incurred (acute) doses beginning at about 20 rad.

At these relatively low levels, the biological changes aren't life-threatening. Doses that induce clinical responses to radiation overexposure begin roughly at about 70 to 100 rad. Full-blown ARS affects the tissues and organs that are most radiosensitive. These include those that reproduce most rapidly, such as the blood cell lines and the gastrointestinal lining. Doses in the thousands of rad can fatally affect the central nervous system. At 100 rad, radiation fatalities are minimal but become more probable as the whole body dose increases. The estimated dose that will kill 50% of exposed individuals within 60 days (no medical care) is about 350 rad.

The exposure time of first responders should be limited to keep the total dose as low as reasonably achievable. However, to save life, higher doses may be incurred and tolerated. In a recent publication of the Health Physics Society, mention is made of real life, survivable doses of 25 rem and that doses reaching 50 rem may be acceptable under life-saving situations.

The great difficulty for first responders is determining accurate radiation measurements in mrad/hour and then translating that dose rate into a total dose by estimating the length of exposure time. The ability to make an accurate radiological hazard assessment requires classroom training in radiation science and realistic radiological emergency drills that confront responders with mock radiation exposure rates that could lead to high or perhaps near-lethal total doses. Such training is necessary to overcome inaccurate perceptions about ionizing radiation and to understand the true hazard it presents.

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Mark L. Maiello, Ph.D., received his bachelor's degree in physics from Manhattan College and master's and doctoral degrees from the New York University Institute of Environmental Medicine. He has published several peer-reviewed scientific articles on both subjects. Now radiation safety officer at the Wyeth Pearl River, N.Y. facility, Maiello writes about radiation safety and the radiological implications of Sept. 11. He is a contributing editor to Health Physics News.