Carbon Monoxide

Primary reference(s)

WHO, 1999. Environmental Health Criteria 213: Carbon monoxide. Second Edition. International Programme on Chemical Safety, World Health Organization (WHO). Accessed 2 December 2019.

Additional scientific description

Carbon monoxide (CO) is one of the most common and widely distributed air pollutants. It is a colourless, odourless and tasteless gas that is poorly soluble in water. Carbon monoxide has a slightly lower density than air. In the human body, it reacts readily with haemoglobin to form carboxyhaemoglobin. Small amounts of carbon monoxide are also produced endogenously. Carbon monoxide exposure is still one of the leading causes of unintentional and suicidal poisonings, and causes a large number of deaths annually (WHO, 2000).

It is a product of the incomplete combustion of carbon-containing fuels and is also produced by natural processes or by biotransformation of halomethanes within the human body. With external exposure to additional carbon monoxide, subtle effects can begin to occur, and exposure to higher levels can result in serious symptoms and death. The health effects of carbon monoxide are largely the result of the formation of carboxyhaemoglobin (COHb), which impairs the oxygen-carrying capacity of the blood (WHO, 1999).

The total annual global emissions of carbon monoxide into the atmosphere have been estimated to be as high as 2600 million tonnes, of which about 60% are from human activities and about 40% from natural processes. Anthropogenic emissions of carbon monoxide originate mainly from the incomplete combustion of carbonaceous materials. The largest proportion of these emissions are produced as exhaust gases from internal combustion engines, especially by motor vehicles with petrol engines. Other common sources include various industrial processes, power plants using coal, and waste incinerators. Petroleum-derived emissions have greatly increased over the past few decades. Some widespread natural non-biological and biological sources, such as plants, oceans and oxidation of hydrocarbons, give rise to the background concentrations outside urban areas. In indoor environments, space heaters fuelled with oil, gas or kerosene, gas stoves and some other combustion appliances (e.g., wood stoves), and tobacco smoking are also responsible for significant emissions of carbon monoxide (WHO, 2000).

Metrics and numeric limits

Indoor air: 100 mg/m3 (87 ppm) for 15 minutes, 35 mg/m3 (30 ppm) for 1 hour, 10 mg/m3 (8.7 ppm) for 8 hours, 7 mg/m3 (6.1 ppm) for 24 hours (WHO, 2010).

Emergency response and acute exposure: Emergency Response Planning Guidelines (ERPG) (NOAA, 2016) and Acute Exposure Guideline Values for Airborne Chemicals (AEGLs) (US EPA, no date) exist for carbon monoxide.

Examples of drivers, outcomes and risk management

Carbon monoxide is produced when fossil fuels burn without enough oxygen. The most important source of exposure to carbon monoxide for the general public is from cooking or other fuel-burning appliances which are poorly installed, faulty or used inappropriately (including inadequate ventilation). For example, home boilers that are installed incorrectly, and the use of BBQs and portable generators inside homes, caravans and tents. Inhaling smoke from a house fire may lead to carbon monoxide exposure. For smokers, cigarettes are the major source of carbon monoxide. Use of shisha/hookah pipes may also lead to exposure. Exposure to low levels of carbon monoxide can occur outdoors, as it is produced by vehicle exhausts and industrial processes (PHE, 2019).

In emergency situations where power is lost, using an improperly vented generator inside a home or building or using gas grills, charcoal grills, or hibachis indoors can lead to dangerous levels of carbon monoxide. Breathing high levels of carbon monoxide can be fatal. Breathing lower levels of carbon monoxide can permanently harm the heart and brain. Carbon monoxide tends to be more harmful to people with heart or lung disease (ATSDR, 2012).

Risk management

• Encourage means to reduce exposure in the home and indoor environment by discouraging the use of solid fuels, such as charcoal and biomass and encouraging cooking in well ventilated spaces.
• Educate emergency services, volunteers and others to eliminate the use of generators in confined spaces where people may be exposed. This is particularly relevant during emergency and disaster response and recovery activities where main power supplies are lost.
• Consider regulating for safe installation of indoor heating appliances.
• Separate dwelling spaces from vehicle garages.
• Discourage the indoor running of combustion engine vehicles or require adequate ventilation where this is not possible.
• Avoid cigarette smoke and smoking (particularly pregnant women and children) as these are a source of carbon monoxide (and other hazardous chemicals).