Is radon gas a problem in Hawaii, particularly on the big island? What causes radon gas to occur?
In answer to your questions about radon gas, "no", it is generally not a
problem for most of Hawaii. I suppose the
best way to explain this is to first explain where radon gas (abbreviated as
"Rn") comes from and how it can accumulate to
hazardous levels in some areas.
Rn is a radioactive gas produced by the decay of uranium. Uranium (U) occurs naturally in most rocks and natural waters, and it is the isotope 238U that decays through a series of daughter elements to Rn. The half-life of Rn is only about 3.5 days, so for it to remain at high levels in a natural system, it must be continually produced from its immediate parent isotope, 226Ra (the latter of which is also a daughter of 238U). Radon itself decays to make another radioactive isotope of the element Polonium which in turn feeds a chain of six main additional radioactive isotopes before the radioactivity stops at stable lead (Pb) 206. The longest-lived intermediate daughter isotope of Rn is 210Pb, which has a 22 yr half-life.
U occurs in most igneous rocks (which are rocks of volcanic origin or "intrusive" rocks such as granites, which are formed from slowly cooling magmas). After the rocks cool, Rn gas produced from uranium builds up in the rock. The higher the concentration of U, the higher the amount of Rn per unit volume of rock. (U also exists in some sedimentary and metamorphic rock types, but these are not important for our discussion of Rn in Hawaii, so I won't discuss them further). The action of groundwater leaching through these rocks strips the Rn gas (and the element Ra) from the rocks and can concentrate these elements greatly over what existed originally in the rocks). If this groundwater seeps slowly out to the surface, situations can occur whereby pockets of Rn gas are trapped in the rock substrate of the crust. These pockets of gas can then find their way into a space of restricted air flow (such as a house) and essentially make the air in the house dangerous to breath. A second mechanism for getting Rn out of the rocks and into a house is by direct seepage of the gas from the rocks. This only occurs in significant quantities when the rock in question is very high in U-content (such rocks are not found in Hawaii).
For Rn to become a hazard, it must not only seep out of the ground in elevated amounts, but it must also become trapped in the restricted airflow of a house or other building. Part of the reason that the Rn hazard is low in Hawaii is due to the fact that typical homes here have very high air exchange rates compared to houses on the mainland (mostly because we use "natural" ventilation and don't commonly seal our houses to stay warm in the winter). Thus, it and it's daughters don't accumulate within the air spaces of the houses.
In places where Rn is high and it does accumulate, it is a hazard mostly because of its daughters, rather than because of Rn itself. Why? Because inhaled radon resides in the lung for a very short time relative to its half-life (and hence makes a relatively insignificant radioactive dose to the lungs). But... it turns out that when radon resides in a closed environment (like a basement) for a sufficiently long time to generate significant concentrations of the daughters, they become attached to dust/smoke particulates in the air. It is these particles that have a significant probability of impacting/depositing at various places in the lung. As the daughter nuclide sits on the surface of the lung tissue, it in turn decays and impacts the tissue. The cumulative effect of the deposited daughter radiation can cause cancer.
So, in general, because the uranium content of Hawaiian rocks is low, because most of our rocks near the surface are relatively porous, and because of our typically well-ventilated homes, it is difficult to find high levels of Rn for most locales in Hawaii. This doesn't mean, however, that elevated levels can't occur here. Don Thomas, a University of Hawaii researcher who specializes in environmental Rn measurements, tells me that elevated Rn does occur in the groundwater of some areas on the older Hawaiian islands (Kauai and Oahu) but even this is not high enough to accumulate in our homes to dangerous levels. He tells me that Rn is also not much of a hazard near active Kilauea volcano even though Rn is enriched by a factor of 4 or 5 in geothermal waters of the area relative to "normal" Big Island groundwater. The thermally-driven groundwater convection system (the same that has caused all the geothermal power controversy in recent years) tends to force large volumes of water through even larger volumes of rock at elevated temperatures, which enriches it in certain chemicals. Dr Thomas has examined soil gases around the East Rift of Kilauea (a focussing point for the geothermal fluids) and he finds that the Rn concentrations are quite low - substantially lower than on Oahu. This is because the permeability of the subsurface rocks on the rift zone are so much higher than the soils here, allowing air infiltration and exchange to dilute the radon.
I hope this answer has been of help to you. Feel free to contact our local expert on Rn in Hawaii at the UH: Dr. Don Thomas (firstname.lastname@example.org, phone is 808-956-6482) if you have additional questions.
or visit the US Environmental Protection Agency's special Radon website.
Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822
with additional info provided by Don Thomas