short mean lives), emitting alpha particles, which have a high LET. Thus radon causes

Transcription

1 Radon What is radon? Radon is part of a chain of decays descending from uranium. Recall that radon is produced as part of the uranium decay chain, discussed in Chapter 16. It is produced in the decay 226 Ra He + Rn Radium-222 decays into polonium Rn He + Po, which decays by another α decay to lead Po He + Pb This lead isotope is radioactive and beta decays to bismuth-214, then again beta decays to polonium-214 or alpha decays to thallium-210. These nuclei decay by α or β and eventually the chain gets to lead-208, which is stable. Radon itself is a gas that is neutral and is not itself a health problem. However, radon deposits its decay products (daughters or progeny) 218 Po, 214 Pb, 214 Bi, 214 Po in the lungs; these all decay quickly (they have short mean lives), emitting alpha particles, which have a high LET. Thus radon causes danger. Radon-222 itself is not dangerous; the progeny are much more hazardous. Because the daughters of radon are positively charged, they are attracted to dust, which can then be inhaled. Internal exposure to alpha particles is very dangerous, so it is common to consider the dose to be that of all alpha-emitting radon daughters together, partly because the half-lives are so short. The exposures are typically given at present in working levels (see Extension 20.2, Nuclear units and applications), which correspond to a bit under 4000 Bq/m 3.

2 Energy, Ch. 20, extension 7 Radon 2 The outdoor radon activity level is 5 to 10 Bq/m 3. Radon in the home is thought to be a great danger to the occupants. The lower air pressure inside relative to outside draws in ground air and radon through cracks in the basement. The average radon level in U.S. homes is 50 Bq/m 3, but one house on Reading Prong measured 100,000 Bq/m 3. (18,85) The radon concentration inside is greater than outside because the air remains so long inside the house; one air exchange per hour is typical. It is estimated that one million U.S. homes have radon levels exceeding 300 Bq/m 3. A study of homes of 453 physicists found an average of 54 Bq/m 3 ; it also found that 0.8% exceeded the 300 Bq/m 3 benchmark. (85) Radon s problems Until the mid-1980s, experts were generally unaware of the extent of exposure to radon in houses and offices. In fact, one of the first indications of the problem s extent came from the United Kingdom, where the National Radiological Protection Board surveyed the situation and found that Britons received over 1 3 of their total annual dose from radon daughters. Inadvertent exposure to radiation has occurred in Tennessee, where the local shale was used to make cement block. (Shale was mentioned as a source of reactor fuel in Chapter 17.) Doses as high as 0.7 Sv/yr could be received by the inhabitants of houses made of the block. (86,87) The discovery of low levels of radiation in some 100,000 homes and schools that were made of brick using gravel from Florida phosphate mines (88) would come as no surprise to health physicists. (86) The case of Stanley Watras is illustrative. He lived in Berks County, Pennsylvania. Stanley J. Watras, a worker helping construct a nuclear generating plant, brought in enough radiation to trigger alarms on site. (89) The Watras living room came in measured at

3 Energy, Ch. 20, extension 7 Radon 3 16 WL (or 59 kbq/m 3 ). (89) The levels of exposure in the Watras living room were so high that lung cancer was a certain result. Even a few years exposure would have produced a high level of risk. A subsequent survey revealed that 40% of Berks County houses had unsafe radon levels. (89) The problem here was that the so-called Reading Prong (running from Reading, Pennsylvania, through Peekskill, New York) has high uranium levels. More than 100,000 people live over the Reading Prong, most in the Morristown area of New Jersey. (89) Radon health effects In the United States, in addition to high levels in Arizona, Colorado, and Pennsylvania, the Gassoway member of the Chattanooga shale has higher levels than background. In Kerala, India, the background radiation gives 100,000 people an annual dose of radiation 5 to 30 times higher than normal (70 mgy/yr). (90-92) Kerala has the greatest population exposed (the population density in this region of Kerala is 2000 people/km 2 ). In addition, Brazil, China, and Iran have high radiation levels. In Brazil, a few people live in an area of monazite sands and get over 10 times the normal dose. (90,93) While the Brazil monazite region does not have a high population, mineral water in Brazil, drunk by many, can deliver as great a dose as 0.33 msv/yr. (93) There are no obvious health effects on any of these people, as is pointed out in Ref. 91: The local Regional Cancer Center director is quoted as saying The cancer incidence in the region is the same as in the whole state. Radon is thought to cause 13,600 deaths per year in the United States. (The range is from 7,000 to 30,000, and it is based on uranium miners experience. Of course, laboring miners breathe more deeply than is typical for someone at home.) (19,94) The National Research Council similarly found that radon associated with breathing radon and its progeny in

4 Energy, Ch. 20, extension 7 Radon 4 indoor air caused about 19,000 lung cancer deaths per year (between 3,000 and 32,000). (95) Most deaths are among smokers. The exposures in Table E are calculated assuming that individuals spend 75 percent of their time in the house over a seventy year lifetime. TABLE E Lifetime Exposure to Radon Progeny Approximate Estimated Number of Progeny Lung Cancer Deaths Due Concentration Radon Concentration to Radon Exposure, Per (WL) (Bq/m 3 ) (pci/l) 1,000 Persons Source: U.S. Environmental Protection Agency, A Citizen s Guide to Radon (Government Printing Office, Washington, D.C., 1986) OPA A lifetime in a home at 150 Bq/m 3 (the EPA action level) would raise the lung cancer risk (if no one smoked) 1% to 2%. (18,85) Research on the health effects of radon seem to show a reduction in cancer for moderate doses; the rates rise steeply for exposure to a greater activity level. (96) The reason for this is not known but is consistent with other experience.

5 Energy, Ch. 20, extension 7 Radon 5 Many people around the world get very high doses from living on outcrops containing high levels of uranium and thorium. For comparison, the radon level in living areas of homes in the United States is 46 Bq/m 3. (95) Reducing radon levels Houses lying over granite, limestone, or other stone may be at risk to infiltration. Sometimes clayey soils prevent radon migration even when rock underlies the clay; however, most often there are passages for gas through such caps that allow the radon out. If a house sits atop an area through which percolation is possible, the radon readings may be very high. A house not in a percolation path may have very low readings. For this reason, two houses next door to one another may give totally different radon readings. If there is an elevated radon level measured, simple measures may suffice to reduce it. If the house is in a temperate climate, one can leave the windows in the basement and upper floors open at some times of the year, but at other times this may be impossible. Opening the windows usually allows the radon levels to decrease. Such a measure is called a passive reduction technique. Occasionally, though, this method can actually cause increases in radon concentration. If this does not work, the best way to make sure that the indoor radon concentration is reduced is to reduce the possibility of infiltration from the soil. This sounds simple, but is difficult in practice. One may seal all cracks in the basement; this assumes that the cracks are all possible to find. Figures E and E show a basement with the cracks sealed. (97) Sealing is also a passive measure.

6 Energy, Ch. 20, extension 7 Radon 6 The best way to reduce radon levels in the home is to make sure that there are as few pathways for air from the outside to the basement and to reduce the pressure difference between the inside and the outside. Fig. E Drain tile ventilation where water drains to sump hole. (U.S. Environmental Protection Agency)

7 Energy, Ch. 20, extension 7 Radon 7 Fig. E Drain tile ventilation with above-grade discharge. (U.S. Environmental Protection Agency) There are simple inexpensive measures that homeowners who are building a house can use to assure low radon levels. A pipe from the footing, where it abuts the drain tile for the basement, that reaches to the attic area and vents will draw air up because of the pressure difference between top and bottom. Such a pipe can be installed as the soil pipe is being installed for a minimum expenditure. Depending on the perceived severity of the problem, one may put a fan to draw more air up. A plastic vapor barrier should be installed under

8 Energy, Ch. 20, extension 7 Radon 8 the slab. Adequate sealing of the walls and slab floor and between the walls and the slab is fairly inexpensive to assure during construction. Access doors to any crawl space should be sealed as well. A plastic vapor barrier should be installed in the crawl space. Any duct or flue opening to the upper stories of the house should be sealed. Any fireplaces should draw outdoor air, not indoor air. (This also increases fireplace efficiency.) Attic access should be sealed to prevent backdraw into living spaces. It is best to avoid recessed lighting on upper floors abutting the attic. Tight fitting windows and doors will reduce the venturi effect, but may have an adverse effect on ambient air quality by trapping indoor pollutants. Openings through walls and floor for plumbing should be sealed. When passive measures do not suffice, one must adopt active measures. Many possibilities might be entertained. Air exchangers, which are devices that force air out from basements and draw outside air in, act to reduce radon levels in many cases. The exchanger also cools the outgoing air while warming the incoming air. These devices are usually expensive and may not be adequate to deal with the problem. When an existing basement has a sump hole, it is often sufficient to put a plate over the sump hole with a pipe that draws air to the outdoors. In this case, it is necessary to use a fan to provide adequate air draw. In addition to installation cost, which may run to $1,000 to $2,000, fans run 24 hours a day, and therefore consume electricity 24 hours a day. The cost depends on the capacity of the fan, but will probably be in the range of $5 per month. Figure E (97) shows how such a device works off the sump hole. Where the drain tile discharges above grade, a different configuration, shown in Fig. E20.7.2, (97) must be used.

9 Energy, Ch. 20, extension 7 Radon 9 A few years ago, I measured the radon levels in my home. In my basement, there were radon levels of 8.7 kbq/m 3, far beyond the EPA action level of 150 Bq/m 3. I sealed the basement cracks and pump air continuously from the sump hole to the outside. The pressure under the concrete is now quite a bit lower than my basement, so not much radon enters. My basement radon levels have been reduced to about 150 Bq/m 3. The Environmental Protection Agency recommends that homeowners measure the ambient radon levels and consider taking action if the reading exceeds 150 Bq/m 3 (or 4 pci/l or 0.02 WL). (98) Remedial action to reduce radon levels is strongly recommended for any house or building with a reading exceeding 750 Bq/m 3 (20 pci/l or 0.1 WL). The readings should be taken in the late fall through early spring, when basements are least likely to exchange air with the outside. Meteorological conditions also affect the radon level measured; pressure differences between the basement [or ground floor] and the ground can change dramatically as weather changes. Radon readings tend to be higher on rainy days. If there is a net pressure difference to pull soil gases into a basement, radon will enter with the gases if it is present in the soil. Such pressure differences are greatest in winter when furnaces force air into the upper parts of the house from the lower parts of the house. This creates an underpressure in the basement, which can draw radon in from the surrounding soil. Clothes dryers can also create pressure differentials. The levels of radon in dwellings obviously varies considerably, even in houses next door to one another, as the underlying clay and rock can fracture, or open gaps, randomly and allow gas a path from underground into dwellings. A typical expectation is shown in Fig. E20.7.3, from a survey of 5,790 German homes. The distribution is expected to be similar everywhere. Note that the maximum of the distribution occurs for an activity concentration of 30 to 35 Bq/m 3. Nevertheless, well over half the houses had a concentration greater than this. The EPA recommends remediation, recall, for

10 Energy, Ch. 20, extension 7 Radon 10 concentrations greater than 148 Bq/m 3. Very few homes (in Germany, at least) exceed that action level. Fig. E Radon concentration frequency in 5,790 homes in Germany. (Umweltbundesamt, Daten zur Umwelt (Erich Schmidt Verlag, 1998), Fig. 11.3) About half of the total dose experienced by people, about 2 msv/yr, comes from living in enclosed spaces. UNSCEAR attributes a somewhat higher occupational dose of 4.8 msv/yr to people who work in buildings due to radon entrapped. (8) But even in the open air, there is radon and its progeny and cosmic rays. The effective outdoor dose rate from cosmic rays and neutrons is 460 nsv/yr. (8) Radon in water Some radon gets into houses through the public water supply. About half the drinking water in the U.S. comes from wells that depend on groundwater. Groundwater may move through rock that contains uranium or thorium that releases decay-chain radon to the water. The National Academy of Sciences studied the problem. (95) According to that study, The average concentration of radon in public water supplies derived from ground

11 Energy, Ch. 20, extension 7 Radon 11 water sources is about 20 becquerel per liter but some wells have up to 400 times the average. By this they meant that some wells have levels of radon of 10 MBq/L, primarily in mountainous regions. The EPA has responsibility for regulating water systems under the Safe Drinking Water Act, passed in The EPA delayed many times, due to inadequate knowledge. Since the EPA must set a maximum contaminant level in cases of cancer-causing substances, in 1991 it finally set a maximum contaminant level of 11 kbq/m 3, or 11 Bq/L, for water. (95) Radon in the water itself might lead to stomach cancers, but the National Academy committee estimated that only 23 deaths a year resulted from this source, compared to about 500 times that many from natural causes (about 13,000 deaths per year). (95) There are about 160,000 deaths per year from lung cancer, most among smokers. The committee estimated that 19,000 of these were due to inhalation of radon and determined that such exposure is a small part of the radon problem. (95) In particular, the study estimated that inhaled radon daughters from water might cause 160 deaths a year, while breathing outdoor air would cause 720 cancers every year. Much water is mixed with air, as when we take showers. This allows entrapped radon to get free and be breathed in. The EPA by law must also set an alternative maximum contaminant level to make sure that the average radon concentration in indoor air from water exposure is less than or equal to the average level in outdoor air, which is about 15 Bq/m 3. (95) The Academy determined that about half the radon in water did escape to air, but that it contributes about 10-4 to indoor air because of dilution (this is known as the transfer

12 Energy, Ch. 20, extension 7 Radon 12 coefficient : the study actually determined that it was between 8 x 10-5 and 1.2 x 10-4 ). As a result, the committee set a level of 150 Bq/L as the alternate maximum contaminant level for radon in water. If either the maximum contaminant level, recommended to be left at 11 Bq/L, or the alternative maximum contaminant level is overstepped, the water system would be obliged to decrease the levels. The cost of remediation would be stupendous, since it would affect so many water systems. The National Academy Committee found that prior estimates overstated the risk of entrained water: The committee s analysis results in a modest reduction of the overall risk associated with radon in drinking water compared with the two previous analyses conducted by the EPA. (95) Overall, the Committee found that the risk from radon-222 is 1.8 x 10-8 cancers/(bq/m 3 ). Of this risk, the great majority, 1.6 x 10-8 cancers/(bq/m 3 ), is due to inhalation of radon progeny. Only 5 x cancers/(bq/m 3 ) are due directly to radon-222. The remainder of the risk is due to ingestion.