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TL;DR: Carbon Monoxide, formed by the incomplete burning of carbon-based fuels, is invisible, odorless, and poisonous. It has an indirect but adverse effect on climate change. Although global CO levels have declined over the last few decades, it is still considered a criteria pollutant and is regulated by several international and national governmental bodies. Measuring CO is crucial and is especially useful during wildfires.
Carbon monoxide gas (CO), which consists of one carbon atom and one oxygen atom, is colorless, odorless, and poisonous.
Carbon monoxide is formed when carbon-based fuels, such as kerosene, natural gas, wood, coal, and oil, burn incompletely or inefficiently. Many things, including everyday items, produce CO. Various kinds of space heaters, furnaces, stoves, fireplaces, gas appliances, charcoal grills, generators, and more produce CO. Automobiles and fires can furthermore be significant sources of carbon monoxide air pollution. In the United States, transportation was responsible for more than 60% of human-caused CO emissions in 2017. Volcanoes and certain photochemical reactions in the atmosphere also naturally produce CO gas.
In the lower atmosphere, carbon monoxide is spread by winds. Fires, especially those from agricultural burning, contribute to seasonal CO air pollution in Africa and other regions of the Southern Hemisphere. Fires are the main source of carbon monoxide in Africa, South America, and Australia.
In the United States, Europe, and Eastern China, on the other hand, urban areas tend to have the highest levels of carbon monoxide air pollution—except during wildfires. In urban areas, carbon monoxide pollution primarily comes from vehicle and industrial sources.
NASA's satellites have found that carbon monoxide is not inhibited by continental barriers. CO can persist in the lower atmosphere for about a month, long enough for air pollution to travel across oceans and have a large impact on continents where it did not originate.
How carbon monoxide air pollution impacts human health
Carbon monoxide gas reduces the blood’s ability to carry oxygen when inhaled. It does this by attaching to the hemoglobin in the red blood cells, a protein responsible for transporting oxygen throughout the body. By depriving the body of oxygen’s correct use, carbon monoxide can damage organs, particularly the brain and the heart.
Symptoms of carbon monoxide poisoning can resemble those of the flu or food poisoning, so it can be easy to misinterpret their source. Breathing in low levels of CO gas can result in headaches, nausea, fatigue, dizziness, weakness, confusion, and disorientation. Breathing in low levels of carbon monoxide regularly may cause permanent mental or physical problems such as sleep disturbances, unexplained vision problems, numbness, and impaired memory and concentration. Low levels of carbon monoxide can still be highly dangerous.
While outdoors, you are unlikely to experience very high levels of carbon monoxide. However, low levels of CO can still be particularly harmful for people with certain heart diseases, who already have more difficulty delivering enough oxygen to their hearts. Carbon monoxide can also be especially concerning for pregnant individuals since it can lead to adverse developmental effects in unborn babies. Infants and elderly people are additionally at increased risk.
Carbon monoxide can be especially dangerous indoors, where the gas builds up from malfunctioning and improperly ventilated household appliances. Breathing in high levels of CO results in nausea, sleepiness, anxiety or depression, confusion, vomiting, impaired vision, impaired coordination, disorientation, and loss of consciousness. Without immediate treatment, very high levels of carbon monoxide poisoning can result in seizures, coma, and death within a few minutes.
Carbon monoxide poisoning results in approximately 430 deaths and over 50,000 emergency room visits in the United States each year.
The immediate treatment for carbon monoxide poisoning is clean air. However, it can take a very long time for the body to clear the carbon monoxide taken in. Doctors can provide respirators and pure oxygen to aid recovery, and follow-up appointments may be needed.
Environmental impacts of carbon monoxide
Carbon monoxide is classified as a short-lived climate-forcing agent. It does not have much of a direct effect on global warming. However, it does indirectly affect temperature significantly by interacting with greenhouse gases.
CO reacts with hydroxyl (OH) radicals in the atmosphere, reducing their numbers. This is important because OH plays a vital role in removing other gases that more directly contribute to climate change, such as methane and tropospheric ozone. With less OH in the atmosphere, the levels of these climate-forcing gases stay higher than they would be otherwise, exacerbating the greenhouse effect and contributing to global warming.
Methane is 28 times more effective at trapping heat than carbon dioxide. Tropospheric ozone (aka ground-level ozone) negatively impacts both respiratory and ecosystem health in addition to trapping heat.
By removing OH radicals, carbon monoxide also increases the lifetime of other trace gases such as hydrochlorofluorocarbons, methyl bromide, and methyl chloride. These gases may deplete stratospheric ozone, which is crucial in protecting living organisms from excessive ultraviolet radiation from the sun.
In a 2010 integrated science assessment for carbon monoxide, the EPA determined that “a causal relationship exists between current atmospheric concentrations of CO and effects on climate” (pg. 171) indicating carbon monoxide’s role in climate change.
Regulation of carbon monoxide air pollution
On the global scale, the World Health Organization set the air quality guidelines for CO at 4 mg/m3 averages over a short-term 24-hour period, with three to four exceedance days per year. WHO also offers an interim target of 7 mg/m3 per 24-hour average concentrations. These guidelines were last updated in 2021. On a smaller scale, the European Union has established its own air quality standards, which include a maximum daily 8-hour mean of 10 mg/m3 for carbon monoxide.
At the national level, the Clean Air Act requires the United States EPA to set National Ambient Air Quality Standards (NAAQS) for six particularly prevalent and harmful pollutants known as criteria air pollutants. Carbon monoxide is one of these six major pollutants regulated in the U.S.
The Clean Air Act generally sets two national ambient air quality standards for criteria air pollutants: the primary standard, which aims to protect public health, including particularly vulnerable populations, and the secondary standard, which aims to protect public welfare and ecosystems.
Because ambient levels of CO do not have much of a direct ecological effect, there is no secondary standard for carbon monoxide. The primary standard for carbon monoxide includes 1-hour averages of 35 ppm and 8-hour averages of 9 ppm. These averages are not to be exceeded more than once per year. These CO air quality standards have been retained since 1971 and last reviewed in 2011.
While the U.S. EPA does not regulate indoor air quality, they do offer guidelines for what levels of carbon monoxide are considered unhealthy indoors and how to mitigate risks. Average levels of carbon monoxide in homes without gas stoves vary from 0.5 to 5 ppm. Near properly adjusted gas stoves, levels of CO are often 5 to 15 ppm. Near poorly adjusted stoves, these levels may be 30 ppm or higher.
Carbon monoxide measurement and prevalence
While some CO air quality monitoring equipment uses metal oxide sensors or even color-changing gel, the most common carbon monoxide detectors use electrochemical sensors — which is the technology that Clarity uses to measure CO with our Multi-Gas Module.
Electrochemical gas sensors work by having the gas, in this case CO, diffuse through a membrane where it is either reduced or oxidized at an electrode. The electrode converts the reaction into a measurable electrical signal. The degree to which it is oxidized or reduced allows us to determine how much of the gas is present.
Carbon monoxide is often measured in either parts per million (ppm) or parts per billion (ppb). The former reflects the number of CO molecules per million air molecules, while the latter reflects the number of CO molecules per billion air molecules.
CO can even be measured by satellites. In 1999, NASA launched the Terra satellite. Terra carries a sensor known as MOPITT, which takes measurements of pollution in the troposphere. This sensor can scan the entirety of Earth’s atmosphere every three days and has allowed us to better understand the global distribution of carbon monoxide.
Terra’s MOPITT sensor has allowed us to observe changes in carbon monoxide levels over time. In 2021, a study was published, observing a downward trend in the global average of carbon monoxide since the year 2000. As of 2021, there has been a drop of about 15% in the average levels of ambient CO air pollution.
This decline in CO air pollution is speculated to result from cleaner-burning technologies being adopted, stricter air quality standards, and fire reduction in certain parts of the world. However, the rate of CO air pollution decline has slowed in recent years.
Carbon monoxide air pollution is more common in the Northern Hemisphere than the Southern Hemisphere. This is due to the fact that the Northern Hemisphere contains more landmasses, people, and fires. Additionally, the amount of CO air pollution in the atmosphere changes seasonally. Carbon monoxide pollution tends to increase in the winter and early spring when there is less sunlight. Less sunlight subsequently results in less hydroxyl radical, a molecule that converts and removes CO from the atmosphere.
Winter also has the highest degree of hospital admissions from CO poisoning. Colder weather facilitates the increased use of gas, electricity, and heating appliances. Moreover, colder temperatures encourage more people to remain indoors, where CO can build up to extremely unhealthy levels.
Use cases for measuring ambient carbon monoxide air pollution
Monitoring ambient carbon monoxide levels can be particularly useful during wildfires. Wildfires release large amounts of CO into the atmosphere. During the 2020 California wildfires, for instance, NASA’s Atmospheric Infrared Sounder (AIRS) observed CO concentrations ten times higher than typical amounts at high altitudes.
Since CO persists for a long time, measuring it is useful for tracking large sources of pollution from wildfires that extend beyond CO gas. Monitoring carbon monoxide from wildfires also helps determine wildfire’s real-time effect on air quality safety, helping protect both firefighters and the general public from its dangerous effects.
Measuring ambient CO air pollution is also valuable for determining the cardiovascular health risk from common urban air pollution, as well as identifying and confirming traffic and urban air pollution hotspots.
Clarity’s take on carbon monoxide measurement
Clarity offers a carbon monoxide air pollution sensor as part of our Multi-Gas Module, which also measures ozone and nitrogen oxides. The Multi-Gas Module is solar-powered, as well as UV and weather-resistant. This module attaches to our flagship Node-S air quality sensor, which monitors particulate matter (PM2.5) and nitrogen dioxide (NO2) air pollution.
If you’re interested in measuring carbon monoxide with the Multi-Gas Module, feel free to reach out to learn more from our team!