Carbon monoxide (CO) is a colourless and odourless poisonous gas. Besides smoke from tobacco, known sources of exposure to carbon monoxide include exhaust fumes from cars, gas stoves, wood stoves and heaters. CO is also formed by natural processes and is released into the environment or into the (human) body.

CO is present in varying amounts both indoors and outdoors. Car exhaust fumes, in particular, produce higher concentrations of CO. A poorly calibrated gas stove can lead to a very high concentration of CO  High concentrations of CO can also be found in shisha lounges, cafés where water pipes are smoked, thanks to the heating of charcoal and coal. Standards for air quality and the maximum indoor levels for substances such as CO have been set by European law. The longer the period of exposure, the lower the maximum level. 

CO in tobacco smoke

CO is not added to tobacco but is formed when tobacco is burned incompletely. This happens when there is too little oxygen present to convert all of the carbon in the tobacco into harmless carbon dioxide. Cigarette smoke can contain large quantities of CO. Water pipes are also a major source of exposure to CO. More than 90% of the CO in water pipe vapours comes from the charcoal and coal used to heat the water pipe tobacco.

Adverse health effects

Adverse health effects occur as a result of CO binding to haemoglobin (Hb) in the blood, taking the place of oxygen. As a result, lower amounts of oxygen can reach the organs and muscles in the body. This can cause various health effects.  When CO binds to haemoglobin in the blood, it forms carboxyhaemoglobin (COHb). In non-smokers, the concentration of COHb in the blood is usually 1%. In smokers, this percentage fluctuates between 3% and 8% and can even reach 15% in chain-smokers. 

Starting from a COHb concentration of 5%, healthy individuals can experience reduced exertional capacity. For people with cardiovascular diseases, symptoms can worsen with a COHb concentration as low as 2%. For pregnant women, exposure to COHb concentrations starting from 6% can lead to a decreased birth weight of the unborn child. At a COHb concentration of 10%, symptoms of CO-poisoning can occur. These symptoms resemble the flu: a slight headache, nausea, vomiting and fatigue. Other possible symptoms include confusion, sleepiness and an increased heart rate. Serious symptoms of CO poisoning are unconsciousness, coma and death. 

The amount of CO inhaled depends on many factors, such as the type of cigarette, how many cigarettes are smoked, and the smoking behaviour. When cigarettes are smoked, the concentration of CO in the blood does not usually reach levels where symptoms of CO poisoning occur. However, there are cases where smoking a water pipe, even just once, has led to serious CO poisoning. 


Agency for Toxic Substances and Disease Registry (ATSDR)
Chemical Safety Information from Interngovernmental Organizations (INCHEM)

Blumenthal I. Carbon monoxide poisoning. J R Soc Med 2001; 94(6):207-2

European Commission