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CONTEXT: The most prevalent cytosolic protein in erythrocytes, hemoglobin (Hb), autoxidizes to its met form on its own and produces superoxide, a powerful source of intracellular reactive oxygen species. The rate of auto-oxidation is directly associated with the physicochemical characteristics of Hb. The rate of auto-oxidation was found to be higher in cigarette smokers. AIM: In this study, we emphasize on the dose-dependency of Hb auto-oxidation and the possibility of metHb reduction to its ferrous form in different grades of smokers. MATERIALS AND METHODS: We have recruited 30 smokers (10 each of light, moderate, and heavy smokers) and 20 nonsmokers. We assayed auto-oxidation as well as co-oxidation of Hb, metHb percentage, and erythrocyte membrane-bound Hb, and also measured NADH metHb reductase and glucose-6-phosphate dehydrogenase (G6PD) activities. STATISTICAL ANALYSIS USED: One-way ANOVA was chosen to assess the differences between the mean values. Results were expressed as mean ± standard error of the mean, and P < 0.05 was considered statistically significant. RESULTS: We observed that the rate of metHb formation by both auto-oxidation and co-oxidation increased with a smoking habit. Moreover, NADH reductase and G6PD activities were decreased accordingly; hence, the possibility of reversing metHb back to its normal ferrous form is less. We found that membrane-bound Hb was also higher in smokers based on their smoking habits. CONCLUSION: Cigarette smokers have an augmented rate of methemoglobin formation during the auto-oxidation process and a decreased competence of NADH methemoglobin reductase enzyme activity and G6PD activity, which aid in maintaining Hb in its functional state and protecting it from the oxidative insult by scavenging free radicals.