CO – Carbon monoxide


Carbon monoxide (CO) is a colourless, tasteless, odourless and non-irritant toxic gas, that is slightly less dense than air. It is produced by the incomplete combustion of carbonaceous fuels such as (natural) gas, petrol, coal, wood and kerosene.
It mixes freely with air in any proportion and moves with air via bulk transport. It is combustible, may serve as a fuel source and can form explosive mixtures with air.
It reacts vigorously with oxygen, acetylene, chlorine, fluorine and nitrous oxide.

Carbon monoxide is not detectable by humans either by sight, taste or smell.

Inhalation is the only exogenous exposure route for carbon monoxide.

Presence in the atmosphere

Carbon monoxide (CO) is present in small amounts (about 60-70 ppbv in the Southern Hemisphere and 120-180 ppbv in the Northern Hemisphere ) in the Earth’s atmosphere, with extremely high levels experienced in in urban and industrial areas (ppm levels).  Natural sources of carbon monoxide occurring in Earth’s atmosphere are the oxidation of methane en VOCs, initiated by the OH radical and from incomplete combustion associated with biomass burning (e.g. forest fires). mall amounts are also emitted from the ocean, and from geological activity because carbon monoxide occurs dissolved in molten volcanic rock at high pressures in the Earth’s mantle.

Most of the rest of carbon monoxide has anthropogenic sources and comes from chemical reactions with organic compounds emitted by human activities, like incomplete combustion of fossil fuels and wood, in particular motor vehicles, oxidation of hydrocarbons, industrial processes, blast furnaces.

Because natural sources of carbon monoxide are so variable from year to year, it is difficult to accurately measure natural emissions of the gas.

Urban pollution

Carbon monoxide is a temporary atmospheric pollutant in urban areas, mainly from the exhaust of internal combustion engines (including vehicles, portable and back-up generators, lawn mowers, power washers, etc.), but also from incomplete combustion of various other fuels (including wood, coal, charcoal, oil, paraffin, propane, natural gas, and trash).

Indoor pollution

In residential buildings, the normal concentration is 0.5 to 5 ppm, with concentrations of up to 15 ppm occurring in the vicinity of gas burners. Carbon monoxide has a density of 96.5% of the density of air, so it is slightly lighter. If there is enough chimney draught, the carbon monoxide produced rises quickly with the warm exhaust gas via the chimney when carbon is incompletely combusted. If the chimney draught of a stove is poor, carbon monoxide can enter the room air and lead to poisoning. There are many reasons for the exhaust gas backflow or its inadequate discharge. For example, wind, outdated or poorly adjusted and maintained boilers, a switched on extractor fan or a switched on central vacuum cleaner system can cause back pressure, which can lead to an exhaust gas backflow. Leaking chimneys and stoves, incorrectly dimensioned chimneys or chimneys blocked by wasps or birds’ nests, for example, can prevent the exhaust gases from being exhausted sufficiently.

Other sources of carbon monoxide in homes are barbecues over charcoal, wood fires or the use of propane gas-fired radiant heaters. Gas stoves, emissions from wood pellets in pellet bunkers, car exhaust from connected garages and the use of combustion engine driven ventilation equipment and generators can also be sources of carbon monoxide.

Tobacco smoke contains significant concentrations of carbon monoxide. Ten smoked cigarettes increase the concentration by about 22 ppm carbon monoxide in an unventilated 30 m³  room.

Incense burning in homes and public buildings such as stores and shopping malls can be a source of exposure to carbon monoxide. Incense burning might be a significant contributor to carbon monoxide exposure in cultures where incense is burned frequently, for example in religious rituals.

What are the sources of CO?

Although the CO content of the outside air, e.g. on busy roads, contributes significantly to the CO concentration in indoor spaces, sources in indoor spaces can ensure that the CO concentration there is several times higher than the outside air concentration. Important indoor sources are gas stoves, which are fired without sufficient ventilation. Leaky chimneys and poorly draughting stoves or chimneys can also contribute to varying degrees to increased concentrations of carbon monoxide in indoor air. Smoking and passive smoking cause additional CO exposure.

What are the health effects of CO?


The most common symptoms of carbon monoxide poisoning are such as

· headache

· nausea

· vomiting

· dizziness

· fatigue

· feeling of weakness

Affected people often believe they are victims of food poisoning. Infants may be irritable and feed poorly.

Neurological signs include:

· confusion

· disorientation

· visual disturbance

· syncope (fainting)

· seizures

Evaluation, guideline and limit values

Commission Directive (EU) 2017/164 of 31 January 2017 establishing a fourth list of indicative occupational exposure limit values pursuant to Council Directive 98/24/EC, and amending Commission Directives 91/322/EEC, 2000/39/EC and 2009/161/EU gives following limit values for work places:

8 hours (Measured or calculated in relation to a reference period of 8 hours time-weighted average (TWA)): 23 mg/m³ or 20 ppm

Short-term (A limit value above which exposure should not occur and which is related to a 15-minute period unless otherwise specified.): 117 mg/m³ or 100 ppm

How can CO be reduced? 

Beside checking and adjusting the burning capacity of the appliances, sufficient ventilation is key to reduce CO in premises.

How do we measure CO?

At Aristoteles Consulting, we measure CO with continuously registering measuring devices. This devices are equipped with an electrochemical sensor. The measured data are saved by a data logger, which can be read out after the measurement to get information about the concentration and to create data tables and graphs showing time dependent variations.

For more information have a look at our CO-measurement page.

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