Image provided by Leon Contreras on Unsplash
TL;DR: While most harmful air pollution is caused by human activity, natural sources like wildfires, sandstorms, sea spray, volcanoes, vegetation, decomposition, lightning, and radon gas also release pollutants that affect air quality. These emissions can pose health risks and contribute to climate change while others are not so harmful. Monitoring both natural and human-made pollution aids in understanding and addressing their impacts.
Most harmful air pollution is generated by anthropogenic sources. In fact, human mobile sources, including cars, are responsible for more than half of the air pollution in the United States. Yet, many natural sources generate air pollutants such as particulate matter, ozone, methane, volatile organic compounds, and more, impacting air quality in both great and small ways.
Particulate Matter
Particulate matter (PM) refers to solid or liquid compounds suspended in the air. PM varies in size and can potentially damage lung tissue, causing significant adverse health effects. Particulate matter air pollution is typically categorized into two groups: PM2.5 and PM10. The former refers to any PM with a diameter of 2.5 micrometers or smaller, while the latter refers to PM with a diameter of 10 micrometers or smaller.
Particulate matter is primarily emitted by combustion sources, such as vehicles, diesel engines, and industrial facilities. However, a variety of natural sources generate particulate matter as well.
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Particulate matter: Sand and dust storms
In dry regions lacking much vegetation, such as deserts, high winds can lift sand and dust particles into the air, causing storms of particulate matter. The composition of this particulate matter varies based on the location of the storm. These storms usually consist of both PM10 and PM2.5, although there is more of the former.
Although sand and dust storms are primarily natural occurrences, about 25% of dust emissions originate from human activities such as deforestation and unsustainable land management practices. Moreover, climate change is making these storms more frequent.
Sand and dust storms directly impact 151 different countries. They affect over 300 million people daily and can lead to both respiratory and cardiovascular diseases. These storms cause significant economic losses, disproportionately affecting impoverished populations.
Dust can travel thousands of kilometers on high winds, necessitating global cooperation to prevent and mitigate the impacts of these storms. Strategies include environmental monitoring, structural stabilization, and sustainable land management among others.
Particulate matter: Sea salt aerosols
Sea spray aerosols consist of inorganic sea salt and organic matter. Sea salt mainly consists of sodium chloride (NaCl), with traces of magnesium (Mg) and sulfate (SO42-). Wind stress at the ocean’s surface emits these sea salt aerosols. Sea spray particles vary in size, including both PM2.5 and PM10.
When local sea waters are polluted, often by anthropogenic sources, such as sewage, plastics, and detergents, sea salt aerosols can have adverse effects on human health. Sea spray containing certain harmful algal blooms can similarly have negative effects. Yet, unpolluted sea spray aerosols may have mild or even beneficial health effects.
Wildfires
Unlike sea salt aerosols, wildfires produce a wide variety of harmful air pollutants. Not only do wildfires emit both harmful PM2.5 and PM10 (in the form of ash), but they also produce black carbon, carbon monoxide, nitrogen oxides, ozone, carbon dioxide, and volatile organic compounds (VOCs).
Wildfires are one of the largest sources of black carbon, aka soot, which is very harmful to human health and the environment. Black carbon can lead to lung and heart diseases, and premature death. It reduces sunlight, impacts plant ecosystems, and absorbs solar heat, contributing to global warming at a rate up to 1,500 times greater than that of CO2.
Wildfire air pollutants can cause a range of adverse health effects, including difficulty breathing, increased risk of asthma, heart failure, and premature death. Although wildfires can be a natural phenomenon, largely human-driven global warming has exacerbated their frequency and intensity.
Volcanoes
When erupting, volcanoes emit gases into the atmosphere including hydrogen sulfide, radon, sulfuric acid, hydrogen, carbon monoxide, hydrogen chloride, hydrogen fluoride, and helium. Volcanoes may also produce particulate matter in the form of ash. However, the most abundant gases released by erupting volcanoes are carbon dioxide (CO2) and sulfur dioxide (SO2). Of these two, sulfur dioxide poses the most immediate risk to public health.
Sulfur dioxide can lead to irritation of the throat, eyes, and respiratory tract, as well as difficulty breathing. People with certain underlying conditions, such as asthma or heart disease, tend to be more sensitive to these effects.
In some cases, volcanic gases can be odorless, invisible, and very dangerous, potentially even lethal when inhaled at high concentrations. However, local authorities usually have systems in place to monitor and alert people living in at-risk areas.
Carbon dioxide (CO2) is a greenhouse gas that contributes to global warming. Volcanoes release significant amounts of CO2 into the atmosphere both through eruptions and underground magma. However, through the burning of fossil fuels and other activities, humans currently emit 60 or more times more carbon dioxide than volcanoes.
Vegetation
In addition to removing air pollutants such as carbon dioxide (CO2), vegetation can also emit volatile organic compounds (VOCs). These compounds have high vapor pressure and low water solubility. VOCs originate from many household products and are often more prevalent indoors than outdoors.
Plants release VOCs for a variety of reasons, such as attracting pollinators and responding to environmental conditions. During an herbivore attack, some plants emit VOCs to attract predators, causing the herbivores to scatter. Certain plant VOCs can even inhibit bacterial growth. Plants often release VOCs into the air to send chemical signals to other plants of varying species.
More than 1700 VOCs have been identified in different species from both flowering plants and non-flowering plants. While VOCs are primarily released from flowers, other parts of plants such as fruits, leaves, stems, and even roots can also release these compounds.
While VOCs from anthropogenic sources can cause various health and environmental issues, VOCs produced by plants are less harmful and can even have positive effects on human health.
Decomposition of Organic Matter
Decomposition, the breaking down of organic matter, is a complex process with multiple steps that can vary under different conditions. Many different organisms such as bacteria, molds, fungi, and worms all contribute to decomposition. Humans have replicated and accelerated this process, composting organic matter to be used in agriculture. However, both natural and man-made decomposition occur through one of two processes: anaerobic or aerobic.
Anaerobic decomposition occurs without oxygen present. In nature, this happens under specific conditions such as deep within marshes or mud. This type of decomposition produces ammonia, methane, nitrous oxide, and carbon dioxide.
Aerobic decomposition, on the other hand, is more common in nature. Aerobic decomposition occurs in the presence of oxygen. This process produces carbon dioxide and ammonia. The aerobic decomposition of animals and humans produces a variety of volatile organic compounds responsible for causing an unpleasant odor.
Lightning
Through extreme temperatures, lightning produces nitrogen oxides that contribute to ground-level ozone. In 2011, the US EPA analyzed lightning-related air pollution in the Mountain West states, where there tends to be a high degree of lightning and a low level of anthropogenic emissions. They found that lightning produced roughly 30% of the monthly NOx output in these states.
When certain NOx gases react with sunlight, they produce ozone. Ground-level ozone negatively affects human health and contributes to climate change. The relationship between lightning and air pollution may work in reverse as well: air pollution in the form of aerosols may increase the rate of lightning strikes.
Lightning’s effects on air quality might not be entirely negative. Lightning produces the hydroxyl (OH) and hydroperoxyl radicals (HO2), which help clean the air of certain greenhouse gases, such as methane. Although this research seems promising, more information is needed, and it is unclear whether these beneficial effects outweigh the negative air pollution caused by lightning.
Radon Gas
Uranium is a naturally occurring radioactive element found in rock and nearly all soils. When uranium decays, it undergoes a series of transformations, eventually forming radon gas.
Radon gas cannot be seen, smelled, or tasted. It is a radioactive gas that can cause serious health problems, such as lung cancer.
Radon gas travels through air pockets in soil and then into the air above ground. It can even enter buildings through cracks in their foundation. When indoors, radon gas can build up to dangerous levels. Fortunately, radon testing, radon reduction systems, and radon-resistant construction features all help combat and reduce radon gas exposure.
Taking Action
Natural sources of air pollution are varied, and this list is by no means exhaustive. Yet, regardless of the source, air pollution can be studied, understood, and addressed by investing in air monitoring equipment.