Since radon concentrations vary substantially from day to day, single grab-type measurements are generally not very useful, except as a means of identifying a potential problem area, and indicating a need for more sophisticated testing.[1] The EPA recommends that an initial short-term test be performed in a closed building. An initial short-term test of 2 to 90 days allows residents to be informed quickly in case a home contains high levels of radon. Long-term tests provide a better estimate of the average annual radon level.[107]