Different types of radon measuring devices have been developed and used in the last twenty years to meet the demand of the radon industry. At first, most of the radon measuring methods and techniques were developed in government laboratories to address the needs of the government programs such as in the characterization of uranium mine atmospheres, in contaminated US Atomic Energy Commission Excess Sites, and in indoor environments. With the discovery of elevated indoor radon concentrations throughout most of the US, a great deal of awareness was generated about the health risks from exposure to radon/thoron and their decay products. This increased interest on the health risk from radon exposure, stimulated instrument research and development to meet the need for short-term and long-term radon measurements. In the US, unlike other countries, most of the radon measurements are performed with short-term passive integrating or continuous monitoring devices over periods of 2-7 days. Although, several types of instruments for measuring the concentration of radon decay products were developed, they are hardly used for routine monitoring of the exposure risk from radon decay products. For this reason, only the sensitivities of radon monitoring devices will be discussed in this paper. During the last 20 years more than 13 million radon measurements were made in the US alone, with results showing that about 8% of the homes have radon levels above the US EPA action level of 150 Bq m(-3) (4 pCi L-1). There are more than 60 millions homes yet to be tested initially. Since, high radon levels have been found in every State, there is a need to encourage testing in all homes. The selection of the radon measuring instrument, is based on the manufacturer's claim what their instrument can do without comparison with other competing instrument developers. Comparison of the radiosensitivity among different radon measuring methods and techniques can make it easier for radon testing firms to choose the most appropriate one for their field measurements. This paper compares the sensitivity of every radon device we could get information from its manufacturer in net counts min(-1) per 150 Bq m(-3). This value was selected from the fact that most of the measurements in the US are below 150 Bq m(-3). In all, nineteen different devices were intercompared. They represent active and passive continuous radon monitors such as scintillation monitors pulse ionization chambers, solid state alpha detectors, current ionization chambers and several types of diffusion barrier passive activated carbon collectors. The sensitivities of short-term electret ion chambers and long-term alpha track detectors are also presented for comparison with the other standard short-term devices used in the US radon industry. The results of the intercomparisons indicate that in most cases the sensitivity of each device is roughly proportional to its sensitive volume or sensitive mass. The sensitivities of most portable continuous radon monitors range from 0.8 to 24.0 net cpm/150 Bq m(-3) whereas the sensitivity of the most frequently used continuous radon monitors is < 5 net cpm/150 Bq m(-3). By comparison, the sensitivities of the six diffusion barrier passive activated carbon collectors range from 48-145 net cpm/150 Bq m(-3). At radon levels below 150 Bq m(-3) most of the continuous radon monitors will provide very poor results if counts are acquired at one minute intervals. For this reason EPA, requires that continuous radon monitors accumulate counts based on one hour basis to improve counting statistics. Passive activated carbon collectors are usually counted for a ten minute interval yielding 480-1450 net counts/150 Bq m(-3). If counted for one hour as the continuous radon monitors are required to do, they yield 2880-8700 net counts/150 Bq m(-3).
