Sources of natural (background) radiation are cosmic rays from space, gamma rays from radioactive soil and rocks, alpha particles produced by radon gas, and our own radioactivity within our bodies. Background radiation depends tremendously on geography, altitude, and habitat architecture. The biological effects of radiation are so small that at such low levels of background radiation, no detrimental effects in humans have ever been observed.
Radioactive decay chain of natural radioisotopes within the earth. Starting with uranium-238, with a half-life of 4.5 giga-years (4500,000,000 years-about the same as the age of the earth itself), which has been present since the earth was formed. The decay chain passes through radium-226 and radon-222 (shown in yellow), which the two radioisotopes now responsible for terrestrial background radiation and radon gas exposure.
The world of radiation, radioactivity, and radiation risks is pretty complex and is generally left to the domain of radiological physicists (such as myself) to explain the intricacies of this field; but unfortunately, radiation topics are also frequently discussed in the press and elsewhere by those who do not understand the science, leading to a lot of misunderstandings. We will attempt to rectify this situation!
In this article we will talk about the origins of various components of "natural" radiation--sometimes referred to as "background" radiation. Natural radiation is composed of different kinds of radiation: cosmic radiation from space, influenced by our elevation and our proximity to polar regions, natural radiation from the ground, resulting from the decay products of primordial radioisotopes that were formed during the big-bang, a radioactive gas that oozes out of the earth and building materials and which irradiates the insides of our lungs when we breath it, and naturally occurring radioisotopes trapped in our own bodies. The last of these includes two radioisotopes, created by normal nuclear reactions in our atmosphere, which are incorporated into every organ and tissue in our bodies and irradiate us from within. We will also talk about the relative risks of natural radiation vs. other common everyday activities, such as drinking wine, flying, and just living in New York City. And finally, we will talk about consumer products that contribute a very small fraction of natural radiation; with the exception of one product--radioactivity from cigarettes--which can contribute radiation to the linings of our lungs at a level about four times that of the remaining natural radiation!
What kinds of radiation are we talking about?
During everyday life we are exposed to a variety of different types of “natural” radiations; that is in contrast to our “obligatory” exposure to man-made radiations, such as radioactive fallout from nuclear weapons testing (which largely ended in the 1960s) and exposure to medical sources of radiation, such as diagnostic radiology, nuclear medicine, and radiotherapy.
Sources of natural radiation
Terrestrial radiation emanates from inside the earth--literally under our feet--as well as from soil, sand, and stone-based building materials that surround us. It consists of primarily two types of radiation: gamma rays and alpha particles.
Gamma rays mostly originate from primordial uranium and thorium (ultra heavy radioactive elements) that were originally created and trapped in our planet at the time of the big-bang. Subsequently, the uranium and thorium have decayed into many other radioactive elements, but in terms of the sources of terrestrial radiation, the main ones are radium-226 and radon-222, that radium-226 decays into (see the second picture above, which shows the "decay chain" of uranium-238 into non-radioactive lead-206; the chain passes through radium-226 and radon-222). It is the subsequent radioactive decay of radium-226 in the soil and its derivative building materials that exposes us to gamma rays. Radon-222 is a gas that oozes (good scientific term!) out of anywhere that radium-226 is present, that we are then forced to breath and that exposes us internally to its decay radiation, which consists of alpha particles. Because radon-222 constitutes such a major percentage of our annual natural exposure (55%), we will spend some additional time talking about it and treat it separately from radium-226.
Radon gas inhalation
Radon-222 is formed as one intermediate step in the normal radioactive decay chains through which the primordial isotopes thorium and uranium slowly decay into lead. The picture at the top of this article of one of the primordial uranium decay chains shows where along the decay chain radium-226 and radon-222 are situated. Radon-222 is particularly hazardous as a component of natural radiation because it decays by emitting alpha particles. Radiation dose delivered by alpha particles is deemed to be 10 times more harmful than radiation dose delivered by x-rays or gamma rays. The reason for this is explained below.
Whereas x-rays and gamma rays are termed “sparsely ionizing” radiation, alpha particles are termed “densely ionizing” radiation. X-rays and gamma rays mostly cause DNA single-strand breaks, which are randomly distributed along the DNA molecule and separated by quite long distances, relatively speaking. Since the DNA molecule is composed of two tightly bound spiral strands, it is unlikely that single-strand breaks will occur exactly opposite each other, thereby causing the DNA to break apart. Individual single-strand breaks can be quite easily repaired by specialized enzymes within time periods of about 2-4 hours.
In contrast, alpha particles also cause single-strand breaks in the DNA, but at much shorter intervals, so it is therefore much more likely that two such single-strand breaks occur opposite each other, causing the DNA molecule to break apart. These are called “double-strand breaks” and are much more difficult, if not impossible, to repair.
In addition to this, alpha particles have a very much shorter penetration depth in tissue than x-rays or gamma rays (in fact about 1,000 times shorter). Therefore, all the energy of an alpha particle is delivered to only a few layers of cells, whereas the same energy delivered by x-rays or gamma-rays is diluted over a much larger volume of many millions of cells.
A useful analogy is a watering hose used to water flowers. When the nozzle is set to “jet”, the water applies much more pressure as it hits the flowers than when it is set to “spray”—even though in both cases the volume of water being delivered (equivalent to the energies of the x-rays, gamma rays, and alpha particles) may be the same. This occurs because the water is concentrated into a small area in the case of the “jet” and a large area in the case of the “spray”.
These two differences between sparsely and densely ionizing radiations accounts for most of the increased damage for the same dose that alpha particles cause to tissues and organs relative to x-rays and gamma rays. The factor used to account for this difference in damage is called the quality factor of the radiation. X-rays and gamma rays are defined as having a quality factor of 1. Relative to this, alpha particles are defined as having a quality factor of 10.
That is why radon gas is especially hazardous when inhaled. When the surfaces of our body are exposed to radon gas, there is no risk, because the range of the alpha particles is so short that it cannot even penetrate the surface layers of our skin. Numerous studies have shown an association between radon gas exposure and lung cancer, which is why radon gas is of such concern as a component of natural radiation. The pie-chart in fig. 1. shows that radon gas constitutes, on average, about 55% of the entire natural annual effective dose we receive. Effective dose, explained a little farther on, incorporates the quality factor of the radiation.
As in the case of gamma ray exposure from radium, which varies greatly depending on the local geological conditions, exposure to alpha particles from inhaled radon gas varies correspondingly with how much radium is present in the soil and building materials, and also on how readily radon gas can diffuse into buildings and how effectively ventilation can remove it.
Typically, houses with basements are a much greater source of radon gas than houses without basements. Houses built of brick are a greater sources of radon gas than houses built of wood. And more modern “energy efficient” houses, with very insulating and weather-tight windows, are also a greater source of radon gas.
Cosmic radiation originates in our sun and partially penetrates our atmosphere, exposing us mainly to gamma rays and charged particles, largely protons. Cosmic radiation varies greatly depending on elevation; the higher the elevation we live at, the higher is the cosmic radiation component because there is less atmosphere to absorb the cosmic rays before they reach us. Pilots and cabin crew receive substantially more cosmic radiation exposure than the rest of us due to the hundreds of hours a year they spend at very high altitudes. In fact, among so-called radiation workers, pilots and cabin crew of long-haul aircraft receive close to the highest occupational radiation doses of any profession. The highest occupational doses are actually received by interventional radiologists working with specialized x-ray machines in hospitals.
As already mentioned, natural radiation varies depending on geographical location and elevation. For example, if we combine annual natural terrestrial and cosmic radiation doses, we find 48 mrem in New York City, 140 mrem in Denver, 300 mrem in Kerala (India), and 500 mrem in parts of the Brazilian coast.
Intuitively, one would expect to observe elevated cancer rates, for example, for the populations in Kerala vs. New York City. In fact, such an association is generally not observed, leading to the supposition that for protracted, low-dose radiation exposures, cancer induction may not be elevated, but may in fact be decreased.
Such an inverse relationship between protracted low-dose radiation exposure and cancer induction is also observed in other situations. Neighboring provinces in Mainland China that are culturally identical, but due to geological factors have natural radiation levels differing by almost a factor of three are observed to have a higher cancer rate in the province with the lower natural radiation levels. In the U.K., radiation workers experienced a significantly reduced cancer rate compared to workers in other industries after statistically confounding factors had been accounted for.
Data such as that point to the possible existence of radiation hormesis, which is the phenomenon when increased radiation causes a reduction in observed cancer rates. Radiation hormesis is a topic in radiation biology that has gained some traction after “coming out of the scientific closet” about 15 years or so ago. Although the mechanisms that result in radiation hormesis are still not entirely understood, they are gradually being elucidated.
Internal radiation from naturally occurring radioisotopes originates from inside our bodies and is due mainly to naturally occurring radioactive potassium-40 and carbon-13. Potassium and carbon constitute, respectively, important components of intracellular composition and a large fraction of virtually every tissue and chemical structure in our body. The potassium-40 exposes our bodies to gamma rays, while the carbon-13 exposes our bodies to electrons, often called beta rays. Potassium-40 and carbon-13 are a component of natural radiation that is totally unavoidable.
An interesting byline: On average, men have about 20% more muscle mass than women, so their exposure from their own internal radiation is roughly 20% higher than that of women. Furthermore, since the potassium-40 decays in our bodies producing gamma rays, which are quite penetrating and in fact to some degree escape from our bodies, if we routinely sleep with a partner our natural radiation exposure from potassium-40 is a few percent higher than if we remain celibate, because the escaping gamma rays from the internal potassium-40 in one partner will slightly increase the radiation doses that the other partner receives.
Medical Radiation exposes our bodies to radiation, mainly due to diagnostic x-rays and nuclear medicine procedures. Medical radiation doses dropped significantly in the 1940s with the introduction of screen-film technology, but have gradually risen again due to the increasing dependence on and utilization of high technology x-ray diagnostic systems, in particular CT scanners. Obviously there are some individuals who do not get x-ray exams, while others may get many, so the medical component of natural radiation actually varies tremendously and can only be considered in average terms.
Another article in my Dr. Simple Science blog, titled “Do CT scans kill patients?” goes into this topic a little more deeply.
Summary of natural radiation sources
The pie-chart in fig. 1. below summarizes the sources of natural radiations (including medical radiation) that we are exposed to, showing an average breakdown by annual effective dose and by percentage. The pie chart also shows the contribution of natural radiation from “consumer products”, which will be discussed in more detail later on.
Fig. 1. Pie chart of annual natural and medical effective doses received by the average individual in the U.S.
The doses shown in the pie-chart in fig. 1. are expressed as average “effective doses” . The concept of effective dose will be explained shortly.
To summarize, natural radiation is composed of five main categories, as listed in table 2 below:
Table 2. The 5 main categories contributing to total annual natural radiation exposure in units of annual mrem (divide by 100 to convert to mSieverts) of "effective dose"
CATEGORY ANNUAL EFFECTIVE DOSE (mrem) PERCENT OF EFFECTIVE DOSE
TERRESTIAL 28 8%
COSMIC 27 7%
INTERNAL 39 11%
MEDICAL 53 15%
INHALED RADON GAS 200 55%
REMAINDER 11 3%
TOTAL: 360 mrem (3.6 mSv)/year
But what is effective dose? – permit me a slight digression into physics geek-dome (in blue text).
The concept of effective dose is used extensively when assessing the risks of radiation exposures to individuals and populations.
As an example of how the effective dose concept is useful in the case of individuals, assume you have a chest x-ray. The x-rays will expose many organs in your body to varying doses: skin, lungs, heart, bone, bone marrow, etc. Furthermore, these exposed organs in your body will have different sensitivities for developing cancer from those x-ray doses (for example, the same radiation dose delivered to your arm as opposed to your bone marrow would be far less likely to produce a radiation-induced cancer).
These varying sensitivities to radiation are expressed mathematically by the concept of tissue weighting factors in the effective dose calculation method. Similarly, if the same radiation dose were delivered to only half of the bone marrow, it is assumed it would be 50% less likely to cause a radiation-induced cancer than if that same radiation dose were delivered to the whole bone marrow.
Finally, the type of radiation is included in the effective dose calculation as a quality factor. X-rays and gamma-rays are defined as having a quality factor of 1. Other radiations, such as neutrons or alpha particles, have quality factors higher than one, reflecting the greater amount of biological damage they produce for the same dose.
So to calculate effective dose, we determine, 1) which organs receive what doses of radiation; 2) what fractions of each of these organs is exposed; 3) the sensitivity each exposed organ has for developing radiation-induced cancer, i.e., the corresponding published tissue weighting factors; and 4) the quality factor of the radiation. We then sum up all the weighted doses and the result gives us the total effective dose.
The beauty of the effective dose approach is that no matter how a particular radiation exposes an individual, i.e., from which direction, and with what width beam and shape, etc., the effective dose calculation takes into account all of these factors. The total effective dose can then be plugged into an effective dose vs. cancer risk model (such as the commonly used linear-no-threshold (LNT) model) to yield the final probability that an individual will develop a radiation-induced cancer from that diagnostic exam.
For the chest x-ray we postulated in this example, the total effective dose would numerically be approximately 5 times lower than the actual skin dose delivered within the boundaries of the x-ray beam. Correspondingly, the risk for an individual developing cancer from this chest x-ray would theoretically be 5 times lower than if the old-fashioned skin dose assessment were employed. The simple skin dose approach was utilized for assessing radiation risks from diagnostic x-ray procedures until about 15 years ago, but proved notoriously inaccurate in predicting cancer risk.
Here is an example of how misunderstood effective dose is by those who really should understand it. The TSA at U.S. airports is in charge of the operation of threat detection devices such as x-ray backscatter body scanners. Following a request by a congressional committee, the TSA recently released the effective doses delivered to subjects undergoing x-ray backscatter body scans. However, following this disclosure the TSA was vehemently criticized by certain (lay) watchdog groups because, they claimed, the TSA when stating the doses delivered by these devices only gave the effective dose, whereas the dose to the subject’s skin was far higher. This is where the misunderstanding arises; yes, the dose to the skin is indeed be far higher numerically than the effective dose, but what was not understood by the watchdog groups, or the congressional committee, was that the higher skin dose was completely accounted for in the formalism for calculating effective dose, as we demonstrated in the example above of an individual receiving a chest x-ray. The TSA was completely correct in reporting the cancer risk based on the effective dose and not on the skin dose.
How are data needed to perform effective dose calculations obtained?
In order to calculate effective dose, we first have to use a theoretical calculation method called Monte Carlo Simulation. This calculation gives us the distribution of the dose from a specific radiation exposure of the subject on an organ-by-organ basis. The calculation uses realistic mathematical human models of varying sizes and shapes. The Monte Carlo calculation provides the values of all full and partial organ doses, from which the effective dose can be calculated as described above.
We have reviewed the kinds of natural radiations that every one of us is exposed to whether they want to or not. But what is the absolute cancer risk that the effective doses we calculate—say, for a chest x-ray—actually produce? The explanation of how we take this next step is beyond the scope of this article. Another article in my Dr Simple Science blog, titled “How Dangerous is Radiation to Humans—or is it?” explains how effective doses are converted to absolute cancer risk, as well as the many pitfalls of this process. So let us assume that these absolute cancer risks from radiation have been determined, and talk about equivalent risks from various non-radiation-related human activities
Risks of non-radiation-related human activities compared to risks from radiation
Without explaining how the following information was obtained (which is discussed in another of my Dr. Simple Science articles, titled “How Dangerous is Radiation to Humans—or is it?), let us assume that the probability of a single individual getting a fatal radiation-induced cancer due to an effective dose of 1 rem (roughly equivalent, for example, to a CT scan of the pelvis), is 0.05%.
The corresponding non-radiation-related activities that carry the same actuarially determined risk of death are listed in table 3 below.
Table 3. Non-radiation-related activities that carry the same actuarially determined risk of death as a single CT scan delivering an effective dose of 1 rem (10 mSv)
BEHAVIOR / SITUATION CAUSE OF DEATH
1 rem (10 mSv) effective dose Induced cancer from radiation
(assuming the LNT hypothesis)
Smoking 28 packs of cigarettes Cancer, heart disease
Drinking 200 liters of wine Cirrhosis of the liver
Spending 400 hours in a coal mine Black lung disease
Spending 1,200 hours in a coal mine Accident
Living 2 years in New York or Boston Air pollution
Traveling 40 hours by canoe Accident
Traveling 70 hours by bicycle Accident
Traveling 20,000 miles by car Accident
Flying 400,000 miles by jet Accident
Flying 600,000 miles by jet Cancer from cosmic radiation
Living 7 years in Denver Cancer from cosmic radiation
Living 17 years in a stone/brick building Cancer from natural radiation
500 chest x-rays or 20 mammograms Cancer from radiation
Living 33 years with a cigarette smoker Cancer and heart disease
Eating 1,600 tablespoons of peanut butter Liver cancer from aflatoxin-B
Living 500 years at the boundary of a nuclear Cancer from radiation
Drinking Miami water for 400 years Cancer from chloroform exposure
Eating 40,000 charcoal broiled steaks Cancer from benzopyrene exposure
Included in the pie-chart in fig. 1. are slices of pie that are not part of naturally occurring radiation exposure. For example, there is a slice that corresponds to radiation from consumer products. Although, as already said, the latter is not part of natural radiation exposure (after all, we are not forced to use such products), it is quite interesting to consider which consumer products in fact do produce radiation exposure—however large or small.
Radiation exposure from consumer products
In addition to being subject to natural radiation as is the population at large, cigarette smokers receive, on average, an annual effective dose of about 1,300 mrem. No, that is not a typographical error. 1,300 mrem (13 mSv)! This is about 4 times the true natural effective dose of 360 mrem (3.6 mSv) per year. The reason for this perhaps puzzling fact is that the tobacco plant contains two naturally occurring radioisotopes, polonium-210 and lead-210. These radioisotopes actually originate in the fertilizer that is used in the growing of the tobacco plant. Subsequently, these two radioisotopes become trapped in tobacco smoke particles that are inhaled by smokers. Although the absolute concentration of the polonium-210 and lead-210 in tobacco smoke is very low, the tar in cigarette smoke traps the smoke particles in small passages in the lungs called bronchioles so that the polonium-210 and lead-210 remain in contact with the walls of the bronchioles for extended periods of time causing substantial radiation dose to be delivered to the cells of the bronchial walls. Furthermore, both polonium-210 and lead-210 are alpha particle emitters, and we have already explained why alpha emitting radioisotopes have a relative effectiveness for producing cancer that is 10 times higher than that of x-rays or gamma-rays.
The average annual actual dose to the lining of the bronchioles of the average cigarette smoker (in contrast to the 1,300 mrem effective dose) is in fact about 10,000--11,000 mrem (100--110 mSv)! Since it is impossible to remove the polonium-210 and lead-210 from tobacco, it is not clear what proportion of the greatly elevated cancer rate observed in cigarette smokers is due to the chemicals present in tobacco and what proportion is due to the associated high radiation dose.
Most residential smoke detectors contain americium-241 radioisotope sources. Americium-241 is an alpha particle emitting radioisotope. With proper use, no significant radiation is measurable outside the unit, but if the smoke detector is trashed, the americium-241 source can fracture and surrounding objects may be contaminated, leading to the possibility of human contamination.
Luminous watches and clocks
Modern watches and clocks sometimes use small quantity of the radioisotope hydrogen-3 (called tritium) or promethium-147 as a source of luminosity. Some older watches and clocks, right up to the 1960s, used radium-226 as a source of luminosity. As mentioned earlier, radium-226 radioactively decays by producing radon gas, which could be inhaled. Furthermore, if these watches or clocks are trashed, the radium-226 would contaminate surrounding objects and contaminate anyone handling these objects. In the early part of the 20th century, workers who painted the numerals on the dials of luminous watches and clocks were known to lick the ends of their paint brushes to make a nice sharp point; but in doing so, they absorbed dangerous amounts of radium-226, and many of them developed cancers as a result.
Ceramic materials, such as tiles and pottery, and in particular orange-colored Fiesta-Ware, often contain elevated levels of naturally occurring radioactive uranium, thorium-232, and/or potassium-40. In most cases, the radioactivity is concentrated in the glaze. While it is less common than it once was, some brands of lantern mantles incorporate thorium-232. In fact, it is the heating of the thorium by the burning gas or liquid that is responsible for the emission of light. Such mantles are sufficiently radioactive that when discarded they are often used as check sources for radiation survey meters.
Glassware, especially antique glassware with a yellow or greenish color, can contain detectable quantities of uranium. Such uranium-containing glass is often referred to as canary or vaseline glass. Collectors are also attracted to uranium glass for the attractive glow that is produced when the glass is exposed to black (ultraviolet) light. Even ordinary glass can contain high enough levels of potassium-40 or thorium-232 to be detectable. Older camera lenses (1950s-1970s) often had coatings of thorium-232 to alter the index of refraction.
Antique radioactive “curative” devices
In the past, primarily 1920s through the 1950s, a wide range of radioactive products were sold as curative devices. For example, radium-containing pills, pads, solutions, and devices designed to add radon to drinking water. Most such devices were relatively harmless (as well as being useless), but occasionally one can be encountered that contains potentially hazardous levels of radium-226.
Commercial fertilizers are designed to provide varying levels of potassium, phosphorous, and nitrogen. Such fertilizers can be measurably radioactive for two reasons: potassium is naturally radioactive due to its radioisotope potassium-40 (as explained earlier in the section on internal natural radiation), while the phosphorous can be derived from phosphate ore that contains elevated levels of uranium-238 and radium-226. The radioactivity of fertilizers is most important due to the polonium-210 and lead-210 that is transferred to plants, in particular the tobacco plant, which (as explained earlier) can be highly hazardous to smokers.
Food contains a variety of different naturally occurring radioactive materials. Although the relatively small quantities of food in a home at any one time contain too little radioactivity to be readily detectable or hazardous, bulk shipments of food have been known to set off the sensitive radiation monitors at border crossings. One exception would be low-sodium salt substitutes that often contain enough potassium-40 to double the level of natural radioactivity.
Beautiful granite countertops are radioactive to a small extent. The granite continually releases radon-222 gas into the air due to the presence of radium-226 in the granite. Although the amount released can vary considerably from one type of granite to another, the radon concentrations in most kitchens tested are much less than the EPA's "safe" limit of 4 picoCi/liter. While the radioactive material in the granite can produce a reading on a sensitive radiation detection instrument, the levels of radiation are well below the level that would result in any harm to humans; so don't replace your granite countertops on account of the minuscule quantities of radon produced, but develop a geeky party patter to tell others about it!
Long-haul airline flights
Long-haul airline flights cause the passengers and cabin staff to incur higher cosmic radiation doses than the rest of us who remain on terra firma. This occurs for two reasons: first, at cruising altitudes of 30,000-40,000 ft, there is very little atmosphere left to shield the traveller from cosmic rays; second, much of the cosmic rays are deflected by the earth’s magnetic field before they reach ground level, but near the poles the magnetic fields are in an unfavorable orientation to provide optimum deflection of cosmic rays, and since long-haul flights often cross over the poles to exploit the shorter distance of the great circle routes, the pilots, crew, and passengers get double-indemnity with regard to increased radiation levels.
The lowest dose-rates measured during a long-haul flight are approximately 0.3 mrem/hour (3 µSv/hour) during a Paris-Buenos Aires flight totaling approximately 13 hours, resulting in a round-trip additional effective dose of 8 mrem (80 µSv).
The highest dose-rates measured during long-haul flights are approximately 0.66 mrem/hour (6.6 µSv/hour) on Paris-Tokyo flights totaling approximately 12 hours, and 1 mrem/hour (9.7 µSv/hour) on the same route in the Concorde, totaling approximately 4 hours. For the Paris-Tokyo flight, this would result in a round-trip additional effective dose of 16 mrem (160µSv); or 8 mrem (80µSv) in the Concord.
For long-haul pilots, who typically fly 700-1,000 hours a year, these additional cosmic ray exposures could add roughly 400-600 mrem/year (4-6 mSv/year) to their “ground-based” natural exposure average of 360 mrem/year (3.6 milliSv/year).
[Adapted from reference: https://www.hps.org/publicinformation/ate/faqs/commercialflights.html]
In this article we have discussed the sources of natural radiation which we are all exposed to whether we choose to be or not. These are, terrestrial, cosmic, internal, medical, & radon inhalation. Percentage-wise, these constitute, respectively, 8% (terrestrial), 7% (cosmic), 11% (internal), 15%, (medical) and 55% (radon inhalation), for a total annual effective dose of 360 mrem (3.6 mSv).
Expressing doses as effective doses does away with any need to make corrections for radiation type or exposure geometry and other exposure conditions; effective doses can then be directly plugged into radiation dose vs. cancer risk models such as the frequently used linear-no-threshold (LNT) model.
The substantial annual radon inhalation component (55%) in the natural effective dose is due to a number of factors, but most importantly to the radiobiological properties of radon-222 as an alpha particle emitter and the associated high quality factor of 10.
Long-haul pilots, crew, and passengers are typically exposed to additional effective doses of approximately 0.5 mrem/hr (5µSv/hr), due mainly to the reduced protection against cosmic rays from the decreased atmospheric protection at high cruising altitudes and reduced protection from the earth’s magnetic field on great-circle routes over the poles.
Many consumer products are manufactured with radioisotopes of various kinds as necessary components of the product. During normal use, these consumer products are completely safe, but following their disposal there is frequently the possibility of hazardous contamination.
Tobacco smokers, on average, incur an additional annual effective dose of 1,300 mrem (13 mSv), due to polonium-210 and lead-210, both alpha-particle emitting radioisotopes, that are absorbed from fertilizer by the tobacco plant and become trapped in bronchioles by the tar in the cigarette smoke. This maintains these radionuclides in intimate contact with the lining cells of the bronchioles for extended periods of time, and with the high quality factor of 10 for alpha particles, produces actual doses as high as 10,000—11,000 mrem (100-110 mSv). But due to the inability to remove the polonium-210 and lead-210 from fertilizer, it is impossible to conclude whether the high incidence of cardiovascular disease and lung cancer observed in smokers is due to the presence of polonium-210 and lead-210 or to other factors connected with smoke inhalation.
Some radiobiological studies have shown decreased incidence of cancer with increasing effective dose in the range of 0 - 10 rem (0 - 0.1 Sv). This inverse relationship is called radiation hormesis. The scientific facts explaining radiation hormesis are being gradually elucidated, and it appears that the existence of a radiation hormetic effect in the range of typical diagnostic x-ray doses is real.
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