into living areas; installing crawlspace or basement ventilation systems; installing sub-slab depressurization radon mitigation systems, which vacuum radon from under slab-on-grade foundations; installing sub-membrane depressurization radon mitigation systems, which vacuum radon from under a membrane that covers the ground used in crawlspace foundations; installing a radon sump system in the basement; sealing floors and walls (not a stand-alone solution); and installing a positive pressurization or positive supply ventilation system. the half-life for radon is 3.8 days, indicating that once the source is removed, the hazard will be greatly reduced within approximately one month (seven half-lives). positive-pressure ventilation systems can be combined with a heat exchanger to recover energy in the process of exchanging air with the outside, and simply exhausting basement air to the outside is not necessarily a viable solution as this can draw radon gas into a dwelling. homes built on a crawl space may benefit from a radon collector installed under a "radon barrier, or membrane" (a sheet of plastic or laminated polyethylene film that covers the crawl space floor). astm e-2121 is a standard for reducing radon in homes as far as practicable below 4 picocuries per liter (pci/l) in indoor air. in the us, approximately 14 states have a state radon programs which train and license radon mitigation contractors and radon measurement professionals. to determine if your state licenses radon professionals contact your state health department. the national environmental health association and the national radon safety board administer voluntary national radon proficiency programs for radon professionals consisting of individuals and companies wanting to take training courses and examinations to demonstrate their competency. without the proper equipment or technical knowledge, radon levels can actually increase or create other potential hazards and additional costs. a list of certified mitigation service providers is available through state radon offices, which are listed on the epa website. indoor radon can be mitigated by sealing basement foundations, water drainage, or by sub-slab, or sub-membrane depressurization. in many cases, mitigators can use pvc piping and specialized radon suction fans to exhaust sub-slab, or sub-membrane radon and other soil gases to the outside atmosphere. most of these solutions for radon mitigation require maintenance, and it is important to continually replace any fans or filters as needed to continue proper functioning. since radon gas is found in most soil and rocks, it is not only able to move into the air, but also into underground water sources. radon may be present in well water and can be released into the air in homes when water is used for showering and other household uses. if it is suspected that a private well or drinking water may be affected by radon, the national radon program services hotline at 1-800-sos-radon can be contacted for information regarding state radon office phone numbers. state radon offices can provide additional resources, such as local laboratories that can test water for radon. if it is determined that radon is present in a private well, installing either a point-of-use or point-of-entry solution may be necessary. point-of-use treatments are installed at the tap, and are only helpful in removing radon from drinking water. to address the more common problem of breathing in radon released from water used during showers and other household activities, a point-of-entry solution may be more reliable. point-of-entry systems usually involve a granular activated carbon filter, or an aeration system; both methods can help to remove radon before it enters the home's water distribution system. aeration systems and granular activation carbon filters both have advantages and disadvantages, so it is recommended to contact state radon departments or a water treatment professional for specific recommendations. detractors the high cost of radon remediation in the 1980s led to detractors arguing that the issue is a financial boondoggle reminiscent of the swine flu scare of 1976. they further argued that the results of mitigation are inconsistent with lowered cancer risk, especially when indoor radon levels are in the lower range of the actionable exposure level.