Smoking has long been known to contribute to a number of respiratory disorders, both etiologically and prognostically. Many subjects still smoke nonetheless, despite knowing better. Based on the literature review, the number of people who compiled with advice to quit smoking grew further as a result of the usage of a carbon monoxide (CO) monitor to show an immediate and potentially detrimental consequence of smoking. Levels of CO in exhaled air has been widely used as an indicator of smoking cessation. Although other sources of pollution, such as exhaust gases, induce increases in the fractional concentrations of CO in expired air, measures of exhaled CO level may offer an immediate, non-invasive technique of determining smoking status. Several research studies have been conducted using breath analyser to measure end expiratory CO concentrations in smokers and non-smokers and concluded that breath analysis was rapid to asses the risk of CO poisoning. This paper will review the various research conducted on CO levels in smokers and non-smokers