Research Objective: To inspect whether the duration of exposure to Sun radiation significantly affects the positive COX-2 finding and to inspect the correlation of exposure to Sun radiation with the strength of positive test results in observed conjunctival tissues. Study Design: Prospective, case-control study Participants and Methods: 189 adults operated at the Department of Ophthalmology at University Hospital Centre Osijek in the period 2010 - 2015. The first group consists of 119 (63%) subjects who underwent 3rd and 4th degree primary pterygium of the eye conjunctiva surgery. The second (control) group consists of 70 (37%) subjects who underwent senile cataract surgery. The conjunctival tissue was taken during the procedure, and the samples were immunohistochemically analyzed for COX-2 activity. Data on exposure to Sun radiation was complemented with meteorological data. Results: There is no significant difference in exposure to Sun radiation and COX-2 activity analysis among the groups nor are there differences in COX-2 activity by gender or group. Significant is the match of result values in the pterygium tissue and in the paired sample of conjunctival tissue (McNemar-Bowker test, P < 0.001). There is a significant correlation of the years of exposure (Spearman's rank correlation coefficient  = 0.173 P = 0.04) and a very good correlation of exposure hours per year of exposure with the strength of a finding, respectively. The increase in exposure hours increases the strength of positive COX-2 activity findings (Spearman's rank correlation coefficient  = 0.527 P < 0.001). Using logistic regression, the influence of several factors on the likelihood of COX-2 being positive in the observed subjects was assessed. The model consists of eight variables (gender, age of participants, exposure to Sun in hours and years, influence of place of residence, hypertension, diabetes, and cardiomyopathy). The strongest predictor is the duration of exposure in years, then in hours by year of exposure. ROC analysis shows that a better diagnostic indicator is exposure in hours per year than the years of exposure (sensitivity = 80.9, specificity = 85.7, P <0.001) and determines a cut-off point (point of differentiation) of 183.7 hours per exposure year or 45 years of exposure in total. Conclusion: The duration of exposure to Sun radiation (in years and hours per year of exposure) significantly affects the positive findings of COX-2 activity and correlates with the strength of positive findings in the observed conjunctival tissues.