|Fig. 1: A Geiger counter detecting radiation from a shaker of salt substitute. (Source: Wikimedia Commons)|
While radiation is generally perceived as an exotic threat, many forms of radiation occur in everyday domestic contexts. In many cases, the intensity or type of radiation encountered is harmless, although in certain, specific cases caution is necessary. This paper introduces two cases, salt substitutes and granite countertops, in which radioactivity naturally arises due to the inevitable chemical composition of a required material. In a third case, smoke detectors, the properties of a radioactive material are exploited for a technological innovation that can save lives.
Many commercially-available salt substitutes consist of potassium chloride instead of sodium chloride.  Unlike sodium, potassium is found in nature with an appreciable ambient radioactivity due to the presence of the long-lived isotope K-40, which has a natural abundance of .0117%. This isotope is extremely difficult to isolate from non-radioactive K-39 via standard commercial tools, and so most salt substitutes contain potassium chloride with the native abundance of K-40. This means that the salt-substitute commonly found in grocery stores is weakly radioactive.
We can assess the risk posed by this radiation by comparing it to other sources of radiation that are encountered in everyday life. K-40 nuclei normally undergo beta decay, in which a neutron in the nucleus changes into proton (resulting in Ca-40). The free decay product of this process is a relatively low-energy free electron.  The presence of this decay product does not, in itself, pose a health threat because all individuals at sea-level are constantly exposed to a radiation "background" in the form of secondary decay products from high-energy cosmic rays that enter the Earth's atmosphere.  Most of this radiation occurs at sea-level in the form of muons, which are particles with nearly identical properties to the electrons emitted during the beta decay of K-40. We can thus get an estimate of the background radiation by calculating the flux of muons that would be absorbed by a person at sea-level:
The muon flux at sea level is 1 cm-2 sec-1 = 104 m-2sec-1.  The average energy of these muons is 1 GeV, which is sufficient for most of them to pass through the body without being absorbed, unlike the electrons from salt substitute. A muon passing through water-based soft tissue deposits approximately 10 MeV per meter along its path.  The annual dose is then
Annual Dose = 104 m-2 sec-1 × 1 × 107 eV m-1 × 1.6 × 10-19 joules eV-1 / (103 kg m-3) × 60 sec min-1 × 60 min h-1 × 24 h d-1 × 365 d y-1 × 1 year = 5.05 × 10-4 gray
where 1 J/kg = 1 gray, the SI-derived unit of radiation exposure.
Generally 1 gray per year is the threshold for toxic exposure, suggesting that the total toxicity of background radiation is minor, especially when considering that this calculation assumes that all incident particles are perfectly absorbed by the body, without interference from clothing, cloud cover, rooftops, etc. We can now compare this value to radiation intake due to potassium ingestion.
Because the body already contains a relatively high amount of potassium, we first find that the excess radiation dose due to salt substitute ingestion is negligible, making the radiation dose of a low-salt diet and a standard diet virtually identical. A healthy person contains an average of 2.5 grams of potassium for each kilogram of her body weight.  For a 70 kg average adult, this corresponds to 175 g of potassium contained in the body overall. We observe that the recommended daily sodium intake for an average adult is 1.5 g, which means that even a dedicated low-salt dieter would, at most, consume this amount of excess potassium. Thus the percent excess potassium ingested by a low- salt dieter would be at most 100 × (175 + 1.5)/175 = 2%, which would also correspond to a radiation increase by 2% because radiation level is proportional to the amount of nuclei, which is itself proportional to mass. The actual excess dose would be even smaller than this because the body regulates potassium levels very heavily, and so a large excess of potassium would be metabolized and excreted. Thus the total radiation dose from salt- substitute is negligible compared to the existing dose from potassium in the body, which we calculate here:
A healthy person contains an average of 2.5 grams of potassium for each kilogram of her body weight.  The fraction of K-40 isotopes in this potassium is .0117%, the same amount as all other naturally- occuring potassium sources. 1 g of K-40 has an activity level of approximately 31 disintegrations per second, and the maximum energy of an electron released during one disintegration is 1.33 MeV. 
Annual Dose = (2.5 g/kg) × 31 s-1 g-1 × (1.33×106 eV × 1.6 × 10-19 joules eV-1 ) × (60 s/min × 60 min/hr× 24 hr/d × 365 d) = 5.02 ×10-4 gray
Thus ionizing radiation arising from all bodily potassium is nearly the same as the background exposure from cosmic radiation sources, making the extra radiation accumulated in a low-sodium diet trivial. One noteworthy difference between radiation exposure due to cosmic sources and radiation from potassium is that the latter radiation source is ingested, and so the energetic decay products are able to directly access soft tissues that they can damage. However, the dosage is so low that this effect is negligible compared to other background radiation sources. Moreover, the ingested potassium serves an essential biochemical function in the nervous and cardiovascular systems.
Commercial smoke detectors contain small amounts of Am-243, an isotope that emits low- energy alpha particles when it decays.  In the smoke detector, these isotopes collide with air molecules, resulting in the ionization of individual air molecules and the liberation of free electrons. The presence of ions makes the air weakly conductive, creating a connection in an electrical circuit within the detector. Using an electrical component known as a "normally open" relay, this conducting gap can be configured such that an alarm will sound only when current stops flowing through the air gap. Such a situation occurs when smoke or other contaminants enter the alarm's air chamber, since the molecules in these contaminants do not ionize as easily as air, leading to the breakdown of its conductivity and the triggering of the alarm.
The alpha particles emitted by the Am-243 in a smoke detector are crucial to the functioning of the device because the isotope allows a steady conducting gap to be created more consistently than other means. However, the alpha particles themselves are generally non-penetrating radiation; even a sheet of paper is a sufficient shield for alpha particles. Thus, because of their low energy and large mass, the energy of the alpha particles emitted within the smoke detector drops off exponentially with their distance from the Am-243 source, making the detector source harmless unless it is ingested.  Moreover, the ionized air produced by the detector is harmless and generally non-radioactive, making the risk of contamination low.
Granite countertops are a domestic radiation source that can actually pose a health risk if proper preventive measures are not taken. Most large granite objects, including buildings and paved entryways, contain trace amounts of various radioactive ores. However, recently there have been reports of granite countertops emitting unexpected amounts of radiation.  These occurrences are of special concern because countertops are usually indoors, where air circulation is reduced, and individuals may tend to stand in close proximity to granite surfaces for longer when they are in a household.
Radioactive granite countertops are particularly dangerous because they may emit toxic radon gas, which can cause elevated radiation levels throughout the home.  Generally the levels of radon in the home resulting from installed granite are much lower than ambient radon emitted from the ground beneath the home, but this may not be true for certain samples of granite. Because the amount of natural radioactivity present in a given sample of granite varies widely depending on the specific geological conditions at the source quarry, the best way to avoid obtaining granite countertops with elevated radiation levels is to use trusted sources for building materials, as reputable quarries will periodically test their mined deposits for excessive radiation.
© William Gilpin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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