detectors. Radon-in-water testing requires a water sample being sent to a laboratory. Retesting is recommended in several situations, for example, before spending money on the installation of a mitigation system. Test results which exceed accuracy tolerances also require re-testing. When a mitigation system installation is warranted, a retest after the system is functional is advised to be sure the system is effectively reducing the radon concentration below the action level, and after any mitigation system repairs such as replacing a fan unit. The US EPA recommends retesting homes with radon problems every two years to ensure proper system function. Due to the vast fluctuation in indoor radon levels, the EPA recommends all homes be tested at least once every five years. Testing in the United States ASTM E-2121 is a US standard for reducing air-borne radon in homes as far as practicable below the action level of 4 picocuries per liter (pCi/L) (148 Bq/m³). Some states recommend achieving 2.0 pCi/L or less. Radon test kits are commercially available and can be used by homeowners and tenants and in limited cases by landlords, except when a property is for sale. Commercially available test kits include a passive collector that the user places in the lowest livable floor of the house for 2 to 7 days. The user then sends the collector to a laboratory for analysis. Long-term kits, taking collections from 91 days to one year, are also available. Open land test kits can test radon emissions from the land before construction begins, but are not recommended by the EPA because they do not accurately predict the final indoor radon level. The EPA and the National Environmental Health Association have identified 15 types of radon test devices. A Lucas cell is one type of device. Retesting is specifically recommended in several situations. Measurements between 4 and 10 pCi/L (148 and 370 Bq/m³) warrant a follow-up short-term or long-term radon test before mitigation. Measurements over 10 pCi/L (370 Bq/m³) warrant only another short-term test (not a long-term test) so that abatement measures are not unduly delayed. Progress has been made regarding radon in the home. A total of 37 states have now passed legislation requiring home-sellers to disclose known radon levels before completing the transaction (although only a handful have introduced criminal penalties for misrepresentation). And over half the legislatures have written radon into their state’s building code. Purchasers of real estate