lives by 2020. to complete these goals, representatives from each organization have established the following action plans: embed radon risk reduction as a standard practice across housing sectors, provide incentives and support to test and mitigate radon, promote the use of certified radon services and build the industry, and increase public attention to radon risk and the importance of reduction. the nrap is currently in action, implementing programs, identifying approaches, and collaborating across organizations to achieve these goals. policies and scientific modelling worldwide dose-effect model retained the only dose-effect relationship available are those of miners cohorts (for much higher exposures), exposed to radon. studies of hiroshima and nagasaki survivors are less informative (the exposure to radon is chronic, localized, and the ionizing radiations are alpha rays). although low-exposed miners experienced exposures comparable to long-term residence in high-radon dwellings, the mean cumulative exposure among miners is approximately 30-fold higher than that associated with long-term residency in a typical home. moreover, the smoking is a significant confounding factor in all miners' studies. it can be concluded from miner studies that when the radon exposure in dwellings compares to that in mines (above 1000 bq/m3), radon is a proven health hazard; but in the 1980s very little was known on the dose-effect relationship, both theoretically and statistical. studies have been made since the 1980s, both on epidemiological studies and in the radiobiology field. in the radiobiology and carcinogenesis studies, progress has been made in understanding the first steps of cancer development, but not to the point of validating a reference dose-effect model. the only certainty gained is that the process is very complex, the resulting dose-effect response being complex, and most probably not a linear one. biologically based models have also been proposed that could project substantially reduced carcinogenicity at low doses. in the epidemiological field, no definite conclusion has been reached. however, from the evidence now available, a threshold exposure, that is, a level of exposure below which there is no effect of radon, cannot be excluded. given the radon distribution observed in dwellings, and the dose-effect relationship proposed by a given model, a theoretical number of victims can be calculated, and serve as a basis for public health policies. with the beir vi model, the main health effect (nearly 75% of the death toll) is to be found at low radon concentration exposures, because most of the population (about 90%) lives in the 0–200 bq/m3 range. under this modeling, the best policy is obviously to reduce the radon levels of all homes where the radon level is above average, because this leads to a significant decrease of radon exposure on a significant fraction of the population; but this effect is predicted in the 0–200 bq/m3 range, where the linear model has its maximum uncertainty. from the statistical evidence available, a threshold exposure cannot be excluded; if such a threshold exists, the real radon health effect would in fact be limited to those homes where the radon concentrations reaches that observed in mines—at most a few percent. if a radiation hormesis effect exists after all, the situation would be even worse: under that hypothesis, suppressing the natural low exposure to radon (in the 0–200 bq/m3 range) would actually lead to an increase of cancer incidence, due to the suppression of this (hypothetical) protecting effect. as the low-dose response is unclear, the choice of a model is very controversial. no conclusive statistics being available for the levels of exposure usually found in homes, the risks posed by domestic exposures is usually estimated on the basis of observed lung-cancer deaths caused by higher exposures in mines, under the assumption that the risk of developing lung-cancer increases linearly as the exposure increases. this was the basis for the model proposed by beir iv in the 1980s. the linear no-threshold model has since been kept in a conservative approach by the unscear report and the beir vi and beir vii publications, essentially for lack of a better choice:until the uncertainties on low-dose response are resolved, the committee believes that is consistent with developing knowledge and that it remains, accordingly, the most scientifically defensible approximation of low-dose response. however, a strictly linear dose response should not be expected in all circumstances. the beir vi committee adopted the linear no-threshold assumption based on its understanding of the mechanisms of radon-induced lung cancer, but recognized that this understanding is incomplete and that therefore the evidence for this assumption is not conclusive. death toll attributed to radon in discussing these figures, it should be kept in mind that both the radon distribution in dwelling and its effect at low exposures are not precisely known, and the radon health effect has to be computed (deaths caused by radon domestic exposure cannot be observed as such). these estimations are strongly dependent on the model retained. according to these models, radon exposure is thought to be the second major cause of lung cancer after smoking. iowa has the highest average radon concentration in the united states; studies performed there have demonstrated a 50% increased lung cancer risk with prolonged radon exposure above the epa's action level of 4 pci/l. based on studies carried out by the national academy of sciences in the united states, radon would thus be the second leading cause of lung cancer after smoking, and accounts for 15,000 to 22,000 cancer deaths per year in the us alone. the united states environmental protection agency (epa) says that radon is the number one cause of lung cancer among non-smokers. the general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes 214po and 218po are thought to cause the majority of the estimated 15,000–22,000 lung cancer deaths in the us every year that have been attributed to indoor radon. the surgeon general of the united states has reported that over 20,000 americans die each year of radon-related lung cancer. in the united kingdom, residential radon would be, after cigarette smoking, the second most frequent cause of lung cancer deaths: according to models, 83.9% of deaths are attributed to smoking only, 1.0% to radon only, and 5.5% to a combination of radon and smoking. the world health organization has recommended a radon reference concentration of 100 bq/m3 (2.7 pci/l). the european union recommends that action should be taken starting from concentrations of 400 bq/m3 (11 pci/l) for older dwellings and 200 bq/m3 (5 pci/l) for newer ones. after publication of the north american and european pooling studies, health canada proposed a new guideline that lowers their action level from 800 to 200 bq/m3 (22 to 5 pci/l). the united states environmental protection agency (epa) strongly recommends action for any dwelling with a concentration higher than 148 bq/m3 (4 pci/l), and encourages action starting at 74 bq/m3 (2 pci/l). epa recommends that all homes should be monitored for radon. if testing shows levels less than 4 picocuries radon per liter of air (160 bq/m3), then no action is necessary. for levels of 20 picocuries radon per liter of air (800 bq/m3) or higher, the home owner should consider some type of procedure to decrease indoor radon levels. for instance, as radon has a half-life of four days, opening the windows once a day can cut the mean radon concentration to one fourth of its level. the united states environmental protection agency (epa) recommends homes be fixed if an occupant's long-term exposure will average 4 picocuries per liter (pci/l) that is 148 bq/m3. epa estimates that one in 15 homes in the united states has radon levels above the recommended guideline of 4 pci/l. epa radon risk level tables including comparisons to other risks encountered in life are available in their citizen's guide. the epa estimates that nationally, 8% to 12% of all dwellings are above their maximum "safe levels" (four picocuries per liter—the equivalent to roughly 200 chest x-rays). the united states surgeon general and the epa both recommend that all homes be tested for radon. the limits retained do not correspond to a known threshold in the biological effect, but are determined by a cost-efficiency analysis. epa believes that a 150 bq/m3 level (4 pci/l) is achievable in the majority of homes for a reasonable cost, the average cost per life saved by using this action level is about $700,000. for radon concentration in drinkable water, the world health organization issued as guidelines (1988) that remedial action should be considered when the radon activity exceeded 100 kbq/m3 in a building, and remedial action should be considered without long delay if exceeding 400 kbq/m3. radon testing a radon test kit there are relatively simple tests for radon gas. radon test kits are commercially available. the short-term radon test kits used for screening purposes are inexpensive, in many cases free. in the united states, discounted test kits can be purchased online through the national radon program services at kansas state university or through state radon offices. information about local radon zones and specific state contact information can be accessed through the environmental protection agency (epa) map. the kit includes a collector that the user hangs in the lowest livable floor of the dwelling for 2 to 7 days. charcoal canisters are another type of short-term radon test, and are designed to be used for 2 to 4 days. the user then sends the collector to a laboratory for analysis. both devices are passive, meaning that they do not need power to function. the accuracy of the residential radon test depends upon the lack of ventilation in the house when the sample is being obtained. thus, the occupants will be instructed not to open windows, etc., for ventilation during the pendency of test, usually two days or more. long-term kits, taking collections for 3 months up to one year, are also available. an open-land test kit can test radon emissions from the land before construction begins. a lucas cell is one type of long-term device. a lucas cell is also an active device, or one that requires power to function. active devices provide continuous monitoring, and some can report on the variation of radon and interference within the testing period. these tests usually require operation by trained testers and are often more expensive than passive testing. the national radon proficiency program (nrpp) provides a list of radon measurement professionals. radon levels fluctuate naturally. an initial test might not be an accurate assessment of a home's average radon level. transient weather can affect short term measurements. therefore, a high result (over 4 pci/l) justifies repeating the test before undertaking more expensive abatement projects. measurements between 4 and 10 pci/l warrant a long-term radon test. measurements over 10 pci/l warrant only another short-term test so that abatement measures are not unduly delayed. purchasers of real estate are advised to delay or decline a purchase if the seller has not successfully abated radon to 4 pci/l or less. since radon concentrations vary substantially from day to day, single grab-type measurements are generally not very useful, except as a means of identifying a potential problem area, and indicating a need for more sophisticated testing. the epa recommends that an initial short-term test be performed in a closed building. an initial short-term test of 2 to 90 days allows residents to be informed quickly in case a home contains high levels of radon. long-term tests provide a better estimate of the average annual radon level. mitigation transport of radon in indoor air is almost entirely controlled by the ventilation rate in the enclosure. since air pressure is usually lower inside houses than it is outside, the home acts like a vacuum, drawing radon gas in through cracks in the foundation or other openings such as ventilation systems. generally, the indoor radon concentrations increase as ventilation rates decrease. in a well ventilated place, the radon concentration tends to align with outdoor values (typically 10 bq/m3, ranging from 1 to 100 bq/m3). radon levels in indoor air can be lowered in several ways, from sealing cracks in floors and walls to increasing the ventilation rate of the building. listed here are some of the accepted ways of reducing the amount of radon accumulating in a dwelling: improving the ventilation of the dwelling and avoiding the transport of radon from the basement, or ground,