Carbon monoxide (CO) is a colorless, odorless gas produced by the incomplete combustion of carbon-based fuels such as wood, gasoline and diesel fuel, coal, natural gas, and kerosene in simple stoves, open fires, wick lamps, furnaces, fireplaces. It is found in both outdoor and indoor air.1
Carbon monoxide is a dangerous health threat. The most common symptoms of carbon monoxide poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. These symptoms are often described as “flu-like.”2 At high concentrations carbon monoxide can be fatal because it binds to hemoglobin in the blood, preventing it from carrying oxygen to the body’s tissue. In 2021, unintentional carbon monoxide poisoning killed about 29,000 people in the world.3
Complete combustion occurs when a fuel reacts with sufficient oxygen to produce only carbon dioxide, water, and heat as products. Incomplete combustion happens when a fuel doesn’t burn completely because there isn’t enough oxygen available. The products of incomplete combustion are carbon dioxide, water, less heat, carbon monoxide, particulate matter, and other pollutants.
Our ancestors were the first to produce significant carbon monoxide emissions with the consistent and widespread use of fire about 400,000 years ago. Global anthropogenic carbon monoxide emissions increased sixfold from 1750 to the late 1980s after which emissions began to decline.
The source of emissions mirrors the major transitions in the world’s energy system. Prior to the Industrial Revolution the combustion of biomass such as fuel wood, dried animal dung, and crop residues were responsible for most carbon monoxide emissions. Beginning in the late nineteenth century, emissions from coal and then oil increased rapidly. Note that emissions from biomass also increased due to the ongoing reliance on biomass fuels in many developing nations.
The residential sector accounts for the single largest share of carbon monoxide emissions. Households accounted for nearly all emissions prior to the discovery of fossil fuels, and in 2022 accounted for about 39% of global emissions. Approximately 2.4 billion people worldwide lack access to clean cooking fuels, with the majority relying on biomass (firewood, charcoal, dung) for cooking.4 Those fuels are often burned in open fires, low-efficiency stoves, and in poorly ventilated spaces, conditions that increase carbon monoxide emissions and human health impacts.
Carbon monoxide from fuels derived from oil and burned in internal combustion engines grew rapidly after World War II, and accounted for about 40% of emissions by 1980. Emissions from road transport accounted for about 19% of global emissions in 2022.
About 10 percent of carbon monoxide emissions are so-called fugitive emissions from solid fuels. These are unintended or unplanned releases of carbon monoxide that escape during the production, processing, transport, storage, or use of solid fuels such as coal, wood, charcoal, or biomass. For example, charcoal is produced by thermal decomposition (pyrolysis) in the absence of oxygen at high temperatures. That process releases volatile compounds such as greenhouse gases, carbon monoxide, and nitrogen oxides.5
The production of some metals is another major source of carbon monoxide emissions. In traditional steelmaking using a blast furnace, carbon monoxide is released when coke (a carbon-rich material made from coal) is burned with limited oxygen. In an electric arc furnace, the reaction of oxygen with carbon in the bath produces carbon monoxide which either burns in the furnace if there is sufficient oxygen or is exhausted with potential release to the atmosphere.6
The all-time country leaders in carbon monoxide emissions have various combinations of high rates of fossil fuel use, high rates of biomass energy use, and inefficient technologies that burn those fuels. China is the all-time leader in cumulative carbon monoxide emissions because it has experienced explosive growth in the use of fossil fuels and still has large numbers of people, especially in rural areas, that use biomass fuels in inefficient cook stoves. China, the United States, and India account for more than 50% of emissions since 1750.
Global carbon monoxide emissions peaked in 1989 and declined by about 30% through 2022. The time profile of carbon monoxide emissions varies considerably among countries. Emissions in many countries have peaked and markedly declined, while in other countries emissions continue to rise. Countries can be categorized as follows:
- Early peakers: the Netherlands (1931), United Kingdom (1956), United States (1972), Japan (1977), Croatia (1977), Canada (1980)
- Mid-peakers: Sweden (1989), Australia (1990), Norway (1989),
- Later peakers: Brazil (2004), China (2006), Mexico (2007), South Africa (2008), Vietnam (2015)
- No apparent peak: Pakistan, Bangladesh, Nigeria, Kenya, Cambodia
The decline in carbon monoxide emissions was caused by a combination of government policy, technological, innovation, and fuel switching. Japan’s Motor Vehicle NOx Law (1992 and amendments), Mexico’s ProAire programs (1990 and amendments), the Clean Air Act in the United States (1970 and amendments), and the European Union’s Euro emission standards (1992 and amendments) are policies that required stationary and mobile sources of air pollution to reduce carbon dioxide emissions.
These regulations drove the development of technologies that reduced emissions. In the automotive industry, catalytic converters reduce carbon monoxide emissions by using metals like platinum and palladium to catalyze a chemical reaction that converts carbon monoxide into less harmful carbon dioxide. Electronic fuel injection systems and oxygen sensors optimize air-fuel ratios, resulting in more complete combustion and lower carbon monoxide emissions.
In the industrial sector (power plants, steel mills, cement factories), fluidized bed combustion and catalytic oxidation systems reduced carbon monoxide emissions by converting carbon monoxide to less harmful forms and improving the efficiency of combustion.
A wide ranging suite of fuel-switching behavior also reduced carbon monoxide emissions. Switching from coal to natural gas in electricity generation and industry reduces carbon dioxide emissions in several ways. First, natural gas burns more completely compared to coal due in part to its much simpler molecular structure compared to coal, which has a lot of impurities. Second, gas-fired systems typically allow for more precise control of the air-fuel ratio, allowing plant operators to minimize carbon monoxide emissions.
1 World Health Organization, “Air pollution is responsible for 6.7 million premature deaths every year,” accessed March 23, 2025, https://www.who.int/teams/environment-climate-change-and-health/air-quality-and-health/health-impacts/types-of-pollutants
2 U.S. Center for Disease Control, “Carbon Monoxide Poisoning Basics,” accessed March 23, 2025, https://www.cdc.gov/carbon-monoxide/about/index.html
3 GBD 2021 Carbon Monoxide Poisoning Collaborators. Global, regional, and national mortality due to unintentional carbon monoxide poisoning, 2000–2021: results from the Global Burden of Disease Study 2021. The Lancet Public Health. 6 October, 2023. doi: 10.1016/S2468-2667(23)00185-8.
4 World Health Organization, “WHO Publishes New Global Data on the Use of Clean and Polluting Fuels for Cooking by Fuel Type,” press release, January 20, 2022, https://www.who.int/news/item/20-01-2022-who-publishes-new-global-data-on-the-use-of-clean-and-polluting-fuels-for-cooking-by-fuel-type.
5 European Environment Agency, “EMEP/EEA air pollutant emission inventory guidebook 2023,” https://www.eea.europa.eu/en/analysis/publications/emep-eea-guidebook-2023.
6 “Electric Arc Furnace Process – an Overview | ScienceDirect Topics.” Accessed March 24, 2025. https://www.sciencedirect.com/topics/engineering/electric-arc-furnace-process.
