All residential radon mitigation professionals and companies must be licensed by June 1, 2020, and start to use mitigation system tags on June 1, 2020. Licensure requirements for radon measurement and non-residential mitigation went into effect on January 1, 2019, and continue to remain in effect. A real estate professional needs a radon license to test or mitigate a property they don’t own or lease. The MDH radon website has more information, including licensing instructions and lists of professionals.