Radiation and Your Environment: A Guide to Low-Level Radiation for Citizens of Florida
This is a public information booklet available at the FIPR library.
The Florida Institute of Phosphate Research (FIPR) was created in 1978 by the Florida Legislature (Chapter 378.101, Florida Statutes) and empowered to conduct research supportive to the responsible development of the state’s phosphate resources. The Institute has targeted areas of research responsibility. These are: reclamation alternatives in mining and processing including wetlands reclamation, phosphogypsum storage areas and phosphatic clay contaminant areas; methods for more efficient, economical and environmentally balanced phosphate recovery and processing; disposal and utilization of phosphatic clay; and environmental effects involving the health and welfare of the people, including those effects related to radiation and water consumption.
Radiation and Your Environment
Naturally occurring radioactive materials to which citizens of Florida may be exposed has been a research priority of the Institute for several years. This activity was highlighted during 1986 when the state legislature designated the Institute as the responsible agency to conduct a state-wide study of natural radiation levels.
FIPR is located in Polk County, in the heart of the central Florida phosphate district. The Institute seeks to serve as an information center on phosphate-related topics and welcomes information requests made in person, by mail or telephone.
Radiation is as old as the universe. The stars, as well as the earth, are radioactive. Since the beginning of their existence, humans have been exposed to ionizing radiation from natural sources. More recently, man-made ionizing radiation such as x-rays and that from numerous radioactive materials have been introduced. In addition, many of our consumer products, such as smoke detectors and luminous dials, contain radioactive substances. Thus, we are all continuously exposed to ionizing radiation from natural and man-made sources.
What Is Radiation?
Radiation in a variety of forms is familiar to all of us. Light is radiation we can see; heat is radiation we can feel. Other kinds of radiation such as ultraviolet, x-rays or gamma rays, we can neither see nor feel. Radiation that has sufficient energy to produce electrically-charged particles upon colliding with matter is known as ionizing radiation. The damage or harmful effect of such radiation depends upon the degree of exposure; that is, the number of ionizations produced in the absorbing material.
Ionizing radiation may be particulate in forms such as alpha or beta particles emitted from radioactive materials, or it may be in the form of waves, such as gamma rays from radioactive materials, or x-rays from a machine.
How Much Radiation Do We Get?
There are several ways to describe or measure ionizing radiation. The quantity of radioactivity present in a substance is measured in units called Curies — one Curie is the approximate radioactivity contained in one gram of radium. This is an extremely large quantity, and in order to measure radiation in the environment the unit of choice is the picoCurie, or pCi, which is one one-trillionth of a Curie. Typically, Florida soils will contain one or two pCi activity per gram. Another unit frequently used is the rem, which is a measure of absorbed radiation dose to the body. Since most of us are exposed only to low levels of radiation, the millirem (abbreviated mrem) unit is commonly used. The mrem is 1/1000 of a rem. To put this unit into perspective, a mrem is the amount of whole-body radiation the average U.S. citizen receives about every one and one-half days from natural background radiation.
Natural Background Radiation
We receive ionizing radiation both from natural and man-made sources. The rate at which a person receives radiation from natural background is a function of geographic location and living habits. Background radiation has two components: (1) terrestrial radiation, resulting from the presence of naturally-occurring radioactive materials in the soil and earth, and including naturally-occurring radioactive materials deposited in the human body itself from the ingestion and inhalation pathways; and (2) cosmic radiation arising from outer space. Doses in mrem presented in the following discussion refer to doses to the whole body.
Depending on where one lives in the U.S., external radiation doses range from about 25 to 250 mrem/year. The average U.S. citizen receives around 40 mrem/year external to the whole body from natural radioactivity in the soil and other surface materials. Some radioactive material also gets into the human body through ingestion of food and water and inhalation of materials in the air. This amount also varies with geographical location and living habits. A radiation dose to the whole body of about 130 mrem/year would be expected for an average internal radiation exposure, with about 100 mrem of that due to inhalation of radon and its decay products.
Cosmic radiation is caused by energetic particles of extra-terrestrial origin that enter the earth’s atmosphere and interact with atmospheric substances to produce radiation. The average citizen of Florida receives around 30 mrem/year to the whole body from this source of ionizing radiation. Frequent air travel would elevate this dose slightly.
In summary, the typical Florida resident receives a total dose of approximately 200 mrem of radiation each year from natural background.
Man-made Radiation Sources
Sources of man-made ionizing radiation to which we are exposed include medical applications, nuclear weapons fallout, consumer products and nuclear power production. The largest source of man-made radiation to which the U.S. population is exposed is the use of x-rays in the healing arts. A typical chest x-ray contributes a dose of about 10 mrem. Some individuals also receive radiation exposure from the medical use of radiopharmaceutical materials and fluoroscopic examinations. On the average, an individual receives around 30-40 mrem/year to the whole body from medical sources. All other man-made sources combined account for a yearly exposure of 8-10 mrem.
Thus, a typical dose of radiation to an individual from all man-made sources is about 50 mrem per year. Adding this to the dose from natural sources shows that a typical Florida resident can expect to receive about 250 mrem of radiation each year.
Exposure to Indoor Radon
All soils contain uranium, radium, and a number of other radioactive elements, which derive from uranium. One of these elements is radon. Although radon is part of our terrestrial radiation exposure, it is considered separately because of its tendency to build up in dwellings, and because it varies so widely. Radon is a colorless, odorless, and tasteless gas, but it is radioactive. The soils of the earth constantly emit radon.
Sources of Radiation*
In most homes, the soil accounts for about 90% of indoor radon. Radon moves through air spaces in the soil and rode and can enter a building through cracks or openings (sewer pipe, cracks in concrete, wall-floor joints, hollow concrete block walls). The remaining 10% comes from water, building materials, or the outdoor air.
When released outdoors, radon mixes with the outside air and is widely dispersed. Radon entering a building is confined to a relatively small space. Once inside the building, the radon will tend to remain indoors (like an odor or any other indoor pollutant) because most structures are designed to keep heated or cooled air from escaping.
Many different factors affect the radon concentration in a building. Differences in these factors can cause the annual average indoor radon concentration to be as much as 50 times greater in one structure than in another. These variations make it very difficult to predict specifically which homes will have elevated indoor radon concentrations. Some of the more important factors affecting radon concentrations are:
Soil characteristics: The potential for indoor radon increases with increasing soil permeability and radium content of the soil. Clay and water-laden soils, for example, are likely to retard the flow of radon, whereas the gas can move much more freely through sandy soils.
Building type: The type of structure and its design affect the amount of area in contact with the soil, the air exchange rate in the building, and the number and size of entry points for radon. For example, homes with well-ventilated crawl spaces are likely to have less radon than other homes on the same site.
Foundation condition: Cracks and openings in the foundation and floor can serve as entry points for radon.
Occupant lifestyle and weather: Both of these can affect the ventilation rate for a building, and hence, the degree to which radon or any other indoor pollutant is trapped inside.
Radon gas is unstable and decays into radioactive particles of bismuth, lead, and polonium. The primary hazard of inhaled radon is the formation and deposition of these radioactive particles in the lungs. Such particles adhere to the lung lining, where they deposit their alpha energy directly to the lung tissue.
Concentrations of radon gas in the air are measured in picoCuries of activity per liter of air, or pCi/l. A special unit of measurement of the concentration of radon decay products in the air, called the working level (WL), was derived for assessing exposure to uranium miners. At equilibrium, 4 pCi/liter of radon gas is equivalent to 0.02 WL of decay products. The average background of natural radiation from radon decay products in Florida is about 0.004 WL indoors. However, deposits of phosphate ore contain elevated levels of uranium and radium, and hence, more radon. Normally, these ore deposits are located 20 to 50 feet or more below ground surface. Radon escaping to the atmosphere normally is minimal, as the time of passage of the gas through the soil to the surface of the ground is sufficient that most radon decays in the soil and particulate decay products are trapped. If the land has been mined and then reclaimed, there is a mixing of the layers of the soil, and if more of the ore material is left closer to the surface, more radon may escape. This is true also for unmined land with near-surface mineral formations.
On undisturbed, unmineralized lands in central Florida, average home occupants are exposed to about a 100 mrem/year whole body dose equivalent from inhalation of radon. On phosphate-mineralized land or land reclaimed after phosphate mining, dwellers will average a whole body exposure of about 250 mrem/year.
How Dangerous Is Exposure to Low-level Radiation?
Because ionizing radiation cannot be seen, felt, heard or smelled, it occupies a certain mystique. However, this mystique is not justified by the scientific facts. More is known about ionizing radiation and its effects than about almost any pollutant or toxic material of current concern. We can measure incredibly small levels of radioactive substances. Indeed, we sometimes make materials radioactive so we can follow or measure them when otherwise they could not be detected.
We do know that large doses of radiation given at high dose rates can cause cancers and genetic disorders, but we do not know for sure that low doses and dose rates cause these effects. For protective reasons (radiation regulations and standards), we assume that low doses also cause human health effects to a directly proportional, but smaller degree. That is, if we know that a whole body dose of 200 rem could cause 24 cancers in the lifetime of 1,000 persons so exposed, we assume that 100 rem would cause half this number, or 12 cancers among these 1,000 persons, and that exposure to 10 rem would cause about 1 cancer in these 1,000 persons. According to the American Cancer Society, the individual lifetime risk of getting cancer for any member in the U.S. population is 1 chance in 4. This means that in the lifetime of 1,000 persons, we could expect to see 250 cancers (not all fatal). So if every one of the 1,000 persons got 10 rem of radiation dose, and one additional cancer was produced, it would be extremely hard to detect among the 250 produced by all other causes. To put this in other language, we assume that any amount of radiation, no matter how little, causes some effect, no matter how slight. This concept can neither be proven nor disproven, but for the protection of public health we accept this logic in order to “err on the safe side.”
Another way to think about risks is to consider radiation doses specific to the lung at various levels of radon gas or radon decay products and compare these with risks from such activities as smoking or getting chest x-rays. The following chart shows how exposure to radon over a lifetime compares to these other risks.
Radon Risk Evaluation Chart
How Can Radon Levels in a Home Be Measured?
Not only are there a number of ways to measure indoor radon, but there are now numerous companies and agencies that offer testing services to the public. The two most frequently used sampling devices are both easy to use and non-mechanical. Both detectors are exposed in the dwelling for a certain period of time and then returned to the company for an analysis and report of the results. The first device is a small container of charcoal, which is opened and left in the home for a few days, usually three, then resealed and sent to the testing company. The other detector is a small alpha track device, which is left in the home for anywhere from one month to one year, then returned to the company for a reading. Either device should be located in the center of a frequently occupied area of the home, away from windows, doors or fans. When the charcoal canister is used, the home should be dosed as much as feasible during the time of the test and for about 12 hours preceding the test.
It should be re-emphasized that radon levels vary widely over time, and are influenced by many factors, including home ventilation, indoor and outdoor air temperature, wind speed, rainfall, time of day, season of year, barometric pressure, and others. It must be remembered that risks are estimated on the basis of long-term, average exposures, and so the best test is one conducted over an extended period of time. A three-day test is only a screening test, helpful in determining whether or not additional study is needed. Major decisions about risks, or about the need for radon reduction, should only be made on the basis of long-term tests, preferably a year, but at least 2-3 months in length. In general whenever a screening measurement is in excess of 0.02 WL (or 4 pCi/l), longer-term studies are recommended.
When Should I Take Action?
If the results of longer-term tests indicate the radon levels in your home are elevated, the Institute recommends that you take action to reduce them as much as possible.
1. If your results are above 0.1 WL (or 20 pCi/l), these levels are considered to be significantly above average. You should take action to reduce them in the immediate future, preferably within a few months.
2. If your results are between 0.02 WL and 0.1 WL (4 pCi/l to 20 pCi/l), these levels are above average for residential structures. We recommend you take action in the near future, preferably within a year.
3. If your results are below 0.02 WL (4 pCi/l), these levels are above average or slightly above average for residential structures. You may consider remedial action, but reductions of levels this low often may be difficult.
Remember, the higher the radon levels in your home, the faster you should take action to reduce those levels.
How Can Radon Levels in a Home Be Reduced?
To prevent radon entry into a new home, one (or a combination) of five general techniques are generally advised as the building is constructed:
1. Build the home with a well-ventilated crawl space underneath. This means a space at least 18 inches between grade level and floor, with a vent on at least three sides of the structure. The recommended minimum vent area should be one square foot of opening for each 150 square feet of wooden flooring.
2. Build with an improved monolithic slab, preferably with post-tensioning. This technique has been used to minimize the possibility of cracking of the concrete, and intact concrete is an effective barrier to soil gas penetration. It is important to be sure all slab penetration points are well-sealed.
3. Cover the earth beneath the building with a tightly-sealed polymeric soil gas barrier.
4. Place sub-slab ventilation pipes in a gravel bed in the soil beneath the home. These must be exhausted through a wall pipe to the atmosphere above the roof of the home.
5. Excavation and removal of soil at the construction site. Replace with fill of low radioactivity.
In all these techniques, it is important to seal cracks and openings (such as plumbing drains and utility openings) in the building foundation with flexible sealants, such as caulking, epoxy, or asphaltic materials.
To reduce radon levels already present in an existing home, several techniques are of general use:
1. Increase the ventilation rate in the home, thus diluting any indoor radon concentrations. This process usually is expensive from an energy standpoint, though the cost may be lowered somewhere by use of air-to-air heat exchangers.
2. Maintain a slightly positive air pressure in the home relative to outdoor air pressure. This will reduce the influx of soil gas into the structure. This can be accomplished by ventilation fans and/or by supplying external air to fuel-burning appliances in the home.
3. Sealing cracks and openings in floors or other surfaces which touch the soil can block the pathways for soil gas to enter the home.
4. Block wall ventilation is usually very expensive, but can be used to either draw radon gas from walls before it can enter the house, or used to pressurize the walls to prevent radon from entering the block.
5. Additional techniques are currently under development. It should be noted that there is currently considerable doubt that air filtration techniques alone actually reduce the hazard to health.
Where Can I Get Further Information?
Should you have questions or desire further information about radon and other sources of radiation, please contact the Florida Institute of Phosphate Research, your local county health department, or the Office of Radiation Control of the Florida Department of Health and Rehabilitative Services.