Natural sources of air pollution include volcanoes, wildfires, desert dust, ocean spray, vegetation emissions, lightning, and radioactive gases seeping from soil. These sources are so significant that an MIT study found over 50 percent of the world’s population would still be exposed to fine particulate matter levels exceeding World Health Organization guidelines even if every human-made emission were eliminated. Dust, sea salt, and organic compounds from vegetation alone account for that baseline exposure.
Volcanoes
Volcanic eruptions and ongoing degassing release a potent mix of gases and ash into the atmosphere. The most measurable of these is sulfur dioxide: volcanoes collectively emit 20 to 25 million tons of it every year, according to satellite-based estimates compiled by Michigan Technological University. Sulfur dioxide reacts in the atmosphere to form sulfate aerosols, tiny particles that scatter sunlight and can degrade air quality hundreds of miles downwind.
Carbon dioxide is the other major volcanic gas, though it’s harder to measure from space. During large eruptions, volcanic ash clouds inject particles high into the stratosphere, where they can linger for months and temporarily cool regional or even global temperatures. Smaller, continuously degassing volcanoes contribute a steadier stream of pollutants at lower altitudes, affecting nearby communities year-round.
Geothermal Vents
Even without an eruption, the Earth vents gases through geothermal features like hot springs, fumaroles, and geysers. The main emissions are hydrogen sulfide and carbon dioxide. In Rotorua, New Zealand, a geothermal hotspot where homes sit directly above active ground, indoor vents have been measured emitting up to roughly 200 parts per million of hydrogen sulfide and 15 percent carbon dioxide. Those concentrations are high enough to pose an acute respiratory hazard, particularly for children playing at floor level near the source. The rotten-egg smell of hydrogen sulfide is detectable at very low levels, but at high concentrations the gas actually deadens the sense of smell, making it more dangerous than it seems.
Wildfires and Biomass Burning
Wildfire smoke is one of the most complex natural air pollutants. A single fire releases carbon dioxide, carbon monoxide, methane, nitrogen oxides, ammonia, formaldehyde, benzene, and fine particulate matter (PM2.5), the tiny particles most harmful to lungs. After carbon dioxide and carbon monoxide, PM2.5 represents the next largest share of wildfire emissions by weight.
The chemistry of the smoke depends on what’s burning and how. High-temperature flames produce more highly oxidized gases like carbon dioxide and nitrogen oxides, along with eleite carbon soot. As flames die down and fuel smolders, incomplete combustion takes over, releasing carbon monoxide, methane, methanol, and organic acids like formic and acetic acid. Temperate forests produce roughly 11.7 grams of PM2.5 per kilogram of fuel burned, while tropical savannas produce about 4.4 grams per kilogram, reflecting differences in vegetation density and moisture content.
Wildfire smoke can travel thousands of miles. During intense fire seasons in western North America or Australia, PM2.5 levels in cities far from any active fire regularly spike above health thresholds, turning skies hazy and triggering air quality alerts.
Desert Dust
Wind-blown mineral dust from arid and semi-arid regions is one of the largest natural contributors to global particulate matter. The Sahara Desert alone sends massive plumes across the Atlantic each year, reaching the Caribbean, South America, and the southeastern United States. Saharan dust is composed primarily of silicates (about 64 percent by volume), with smaller fractions of sulfates (14 percent), quartz (6 percent), calcium-rich particles (5 percent), and iron-containing minerals like hematite (1 percent).
This dust affects air quality, visibility, and climate. The iron in it can fertilize ocean ecosystems when it settles on the water’s surface, but for people breathing it in, dust storms raise PM10 and PM2.5 levels sharply. The exact climate effect of desert dust, whether it warms or cools the atmosphere, remains difficult to pin down because the dust composition and concentration vary so much from storm to storm.
Vegetation Emissions
Trees and other plants are surprisingly prolific polluters. They release volatile organic compounds, primarily isoprene and terpenes, as a byproduct of photosynthesis and as a defense mechanism. These compounds form inside leaf cells and are released into the air, where they react with sunlight and other gases to create ground-level ozone and secondary particulate matter.
Heat is the biggest trigger. As temperatures climb, plants ramp up isoprene production significantly. Drought, physical damage from wind, and insect feeding also boost emissions. Recent research has found that even plant species previously thought to be non-emitters actually release isoprene when their leaves are damaged. Globally, vegetation emissions of volatile organic compounds rival or exceed those from human industry, and on hot summer days in heavily forested regions, they are the dominant driver of haze formation.
Ocean Spray and Marine Gases
The ocean produces air pollution through two main pathways: sea spray aerosols and gas emissions from marine organisms. Breaking waves fling tiny droplets of seawater into the air, carrying salt, organic matter, and halogen compounds like chloride and bromide. These particles contribute to the global particulate matter burden, particularly in coastal and island environments.
The second pathway involves a sulfur compound called dimethyl sulfide, produced by phytoplankton. When this gas escapes into the atmosphere, it oxidizes into sulfur-containing acids that are low enough in volatility to form new particles. These particles grow and eventually serve as cloud condensation nuclei, the seeds around which cloud droplets form. In pristine marine environments, this process is a major influence on cloud cover and, by extension, how much sunlight reaches the surface. Halogen compounds from sea salt aerosols also participate in the chemical breakdown of dimethyl sulfide, adding bromine and chlorine gases to the marine atmosphere.
Lightning
Lightning strikes generate extreme heat, up to 30,000 Kelvin in the bolt channel, which is hot enough to force nitrogen and oxygen in the surrounding air to combine into nitrogen oxides. These gases play a central role in atmospheric chemistry: they help form ground-level ozone and contribute to acid rain. Early estimates from the American Meteorological Society suggested lightning could produce 30 to 40 megatons of nitrogen oxide per year, potentially accounting for as much as 50 percent of the total atmospheric supply. More recent analyses have revised the figure downward, but lightning remains a significant natural source, especially in tropical regions where thunderstorms are most frequent.
Biological Particles
The air carries a constant load of biological material: pollen, fungal spores, bacteria, and fragments of plant and animal tissue. Fungal spores are among the most abundant. Cladosporium, Aspergillus, and Alternaria are common airborne genera that release allergenic compounds capable of triggering asthma and respiratory allergies. In tropical environments, spores from mushroom-type fungi (basidiospores) and sac fungi (ascospores) appear to be even more potent allergens. A study of asthma and allergy patients in San Juan, Puerto Rico, found that over 90 percent showed immune reactivity to airborne basidiospores and ascospores.
Outdoor mold concentrations have been positively linked to increases in asthma-related healthcare visits. Pollen, while less of a year-round presence, causes seasonal spikes in airborne biological particles that are especially pronounced in spring and early fall depending on the plant species involved.
Radon
Radon is a colorless, odorless radioactive gas that seeps out of soil and rock as uranium and thorium naturally decay underground. It enters buildings through cracks in foundations, gaps around pipes, and other openings, and can accumulate to hazardous levels indoors. The EPA recommends taking action to reduce radon in any home where levels reach 4 picocuries per liter (pCi/L) or higher, and suggests considering remediation at levels between 2 and 4 pCi/L. Radon is the second leading cause of lung cancer after smoking, and unlike most other natural air pollutants, its primary health impact occurs inside buildings rather than outdoors.