Abstract: An in-situ method for monitoring of retrospective long-term radon exposures in dwellings is disclosed. Typically one nuclear track detector is placed on a glass surface to measure the alpha track density due to the glass alpha activity background plus the glass surface alpha activity of 210Po. The second nuclear track detector is placed on the glass surface with removed 210Po surface activity to measure the alpha tract density which is due to the alpha activity background of the glass. The plate-out on glass of short lived 222Rn decay products in the air is the major source of 210Pb and 210Po activities on the glass surface. The retrospective radon exposure is calculated from the alpha track density corresponding to the 210Po activity on the glass surface which is the difference of the alpha track densities measured by the two nuclear track detectors.