Carbon monoxide is produced when cigarettes burn. When carbon monoxide is inhaled, it combines with hemoglobin, producing carboxyhemoglobin, which greatly diminishes hemoglobin’s oxygen-carrying capacity. As a result, small amounts of carbon monoxide can dramatically reduce hemoglobin’s ability to transport oxygen. Common symptoms of carbon monoxide exposure are headache, nausea, rapid breathing, weakness, exhaustion, dizziness, and confusion. Hypoxia due to acute carbon monoxide poisoning may result in reversible neurological effects, or it may result in long-term irreversible brain damage or heart damage effects. So cigarettes increase the risk of heart disease.

Cigarette smoke releases several toxic chemicals and carcinogens including carbon monoxide CO. This study examined the levels of exhaled CO in smokers switching to electronic cigarettes e-Cigs or a tobacco heating system THS and their level of compliance six months after switching.  40 male smokers unwilling or unable to stop smoking were switched to e-Cigs or THSs for six months. Nicotine addiction and levels of carbon monoxide in the exhaled breath eCO were measured. after six months. The two groups showed no difference, but a significant higher addiction score in smokers choosing THS. E-Cig and THS showed significant reduced levels of eCO after six months, which were within the range of non-smoker status. The THS group had a significantly lower reduction in levels of CO vs the e-Cig group.  Both e-Cigs and THSs are capable of significantly reducing eCO at least in the medium term, hence constituting a viable tobacco harm reduction approach in smokers who are unwilling or unable to stop smoking.

references：https://www.ncbi.nlm.nih.gov/pubmed/31618949