Tehran University of Medical Sciences Case Reports in Clinical Practice 2538-2683 2 2 2017 10 10 46 49 EN Mehran Zarghami Department of Psychiatry, School of Medicine, Psychiatry and Behavioral Sciences Research Center, Addiction Institute, Mazandaran University of Medical Sciences, Sari, Iran. Fatemeh Taghizadeh Department of Psychiatry, School of Medicine, Psychiatry and Behavioral Sciences Research Center, Addiction Institute, Mazandaran University of Medical Sciences, Sari, Iran. Ali Sharifpoor Internal Ward, Imam Khomeini Hospital, Mazandaran University of Medical Sciences, Sari, Iran. 2017 07 23 2017 09 03 Carbon monoxide (CO) concentration in the body will be decreased after smoking cessation. However, confounding factors may influence the results. Exhaled CO (eCO) assessment of lung is a simple, noninvasive tool, but confounder factors such as gas fireplaces might influence results. We thus quantified the effect of them in two smokers with chronic obstructive pulmonary disease (COPD). We described one of these confounder factors in two smokers with COPD in a clinical trial study setting with IRCT201609271457N11 coding in IRCT. The amounts of eCO and carboxyhemoglobin of these patients rose while the average daily smoking decreased (in the first patient) or stopped smoking (in the second patient). We found that they had used the fireplace to heat their home. These measures decreased in both patients by discontinuing the use of the gas fireplace. The gas fireplaces influence the results of eCO assessments in smoking cessation programs. https://crcp.tums.ac.ir/index.php/crcp/article/view/127 https://crcp.tums.ac.ir/index.php/crcp/article/download/127/76