All isotopes of francium decay into astatine, radium, or radon. The isotopes of francium decay quickly into astatine, radium, and radon. Some researchers have investigated changes in groundwater radon concentrations for earthquake prediction. Radon gas is considered a health hazard. Helium, neon, argon, krypton, xenon, and radon are inert gases. The concentration of radon gases is miniaturized by realtime detectors. These occur for the noble gases helium, neon, argon, krypton, xenon, radon and oganesson. Efforts to reduce indoor radon levels are called radon mitigation. Another method of reducing radon levels is to modify the building's ventilation. Cohort studies are impractical for the study of domestic radon exposure. Not all granitic regions are prone to high emissions of radon. These are helium, neon, argon, krypton, xenon, radon and oganesson. There are relatively simple tests for radon gas. All discussions of radon concentrations in the environment refer to Rn. Optimum conditions for radon release and exposure occurred during showering.