Measurement of 238U, 232Th, 222Rn and 220Rn Contents in Optical Contact Lenses: Resulting Alpha and Beta Equivalent Doses to the Eye Tissues of Adult Patients

To estimate equivalent doses received by eye tissues of patients, contents of uranium ( 238 U), thorium ( 232 Th), radon ( 222 Rn), and thoron ( 220 Rn) were determined inside various optical contact lenses used for vision correction. 238 U, 232 Th, 222 Rn, and 220 Rn contents varied between (3.44 +/- 0.24) mBq kg -1 and (18.3 +/- 1.28) mBq kg -1 , (0.57 +/- 0.04) mBq kg -1 and (3.53 +/- 0.25) mBq kg -1 , (3.44 +/- 0.24) mBq kg -1 and (18.3 +/- 1.28) mBq kg -1 , and (0.57 +/- 0.04) mBq kg -1 and (3.53 +/- 0.25) mBq kg -1 , respectively. New external dosimetric models, depending on the cornea eye surface of patients, 238 U, 232 Th, and 222 Rn concentrations inside optical contact lenses, half-life of the emitting radionuclides, and exposure time of patients, have been developed. It has been shown that alpha-particles emitted by the 238 U and 232 Th series inside the studied optical lenses transfer their energies essentially to the cornea tissues whereas the emitted beta-particles may reach and lose their energies in the crystalline lens of eyes of patients. Alpha-equivalent doses received by eye tissues of patients due to the diffusion of 222 Rn and 220 Rn gases present in the considered optical lenses were determined. The higher value of the total (alpha plus beta) equivalent dose to the left and right eyes of adult patients wearing optical contact lenses (14 hours per day) has been found equal to 1.32 mSv y -1 cm - 2 . It is recommended for patients to reduce the wearing period of optical contact lenses to reduce eye disease risks such as cataract.