From Methane to Ground-Level Ozone: A Profile of Petrochemical Emissions
Common toxic gas pollutants at oil and gas production sites include methane (CH4), hydrogen sulfide (H2S), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and volatile organic compounds (VOCs), such as benzene, ethylbenzene, hexane, toluene, and xylene.
Methane (CH4)
CH4 is a hydrocarbon, the primary component of natural gas. It is much more efficient at trapping heat than CO2 but has a shorter atmospheric lifetime (about 12 years) (UNEP). In fossil fuel production, methane is released through flaring and venting.
Methane is responsible for a third of current global warming and roughly half of the observed rise in ground-level ozone (O3). As it breaks down, it depletes the atmosphere’s ‘detergent’ (hydroxyl radicals), reducing the Earth’s natural ability to filter out other pollutants.
While CH4 itself is non-toxic, the resulting ground-level ozone triggers a range of respiratory issues, including breathing problems, reduced lung function, asthma, and chronic lung diseases (CCAC). It is estimated that reducing methane emissions could prevent 1 million premature deaths each year (UNEP).
Children and teenagers are at heightened risk from ground-level ozone (O3) because their lungs are still developing and they inhale a higher “dose” of air per pound of body weight than adults. Biologically, ozone acts like a “sunburn” on the respiratory tract. This exposure not only triggers immediate asthma attacks and increases susceptibility to infections but, through repeated exposure, can lead to permanent reductions in lung function and the potential development of chronic asthma that persists throughout adulthood (EPA).
Sulfur Dioxide (SO2)
SOx are a group of highly reactive gases, with SO2 being the most prevalent in the lower atmosphere. It is produced from the burning of fossil fuels (coal and oil) and the smelting of mineral ores that contain sulfur. It is heavy, colorless, and possesses a pungent, irritating odor. It dissolves easily in water and eventually forms sulfuric acid (H2SO4), the primary component of acid rain (WHO).
In addition to acid rain, SO2 causes foliar damage, which inhibits plant growth. When it settles in soil, it leaches aluminum, which is toxic to plants and flows into streams, harming fish by disrupting their salt-water balance (EPA). It converts to sulfate particles in the air, contributing to haze in parks and cities, and altering the amount of sunlight reaching plants.
SO2 targets the upper respiratory tract in humans. Even brief exposure (i.e., 10 minutes) to high concentrations can cause a significant drop in lung function. It also contributes to the formation of PM2.5 (fine particulate matter), which can penetrate deep into the lungs and enter the bloodstream (EPA, ALA). Studies have shown that long-term exposure increases mortality rates, especially among those with cardiac and lung diseases. A combination of particulates and SO2 is significantly more toxic because it reaches deeper into the lungs, whereas either pollutant alone would not.
Children living near industrial hubs show higher rates of decreased peak flow rates (the speed of air leaving the lungs). Long-term exposure is correlated with the development of chronic cough and increased phlegm production (UNICEF).
Nitrogen Dioxide (NO2)
NOx is a byproduct of high-heat combustion in which nitrogen and oxygen in the air react with each other. NO2 is particularly significant as it is a strong oxidant and a precursor to several secondary pollutants (EPA).
NOx is a primary driver of photochemical smog. Through a complex cycle involving sunlight, it breaks down to form ground-level ozone (O3) (EPA). It also leads to “nitrogen saturation’ in forests, which makes trees more susceptible to frost and pests. NO2 settles into coastal waters and lakes, fueling massive algal blooms. When the algae die and decompose, they strip the water of oxygen, creating “Dead Zones” where fish and other marine life cannot survive.
NO2 contributes to a reddish-brown tint in the air, affecting visibility.
NO2 is linked to increased airway responsiveness in humans. This means the lungs react more violently to allergens such as pollen or dust. Chronic exposure is associated with an increased risk of cardiovascular disease and premature mortality (WHO).
Consistent research has established a direct link between maternal exposure to NO2 and adverse birth outcomes, including premature birth, low birth weight, and stunted fetal development due to the pollutant’s ability to impair placental function.
Emerging international evidence suggests early-life exposure to NO2 may affect cognitive development and is a significant factor in the prevalence of childhood asthma in urban areas (WHO).
Hydrogen Sulfide (H2S)
H2S, also known as “Sewer Gas,” is found in crude petroleum, natural gas, refinery gas, and oil well emissions. Natural gas can be comprised of up to 28% H2S. At very low concentrations, it has a distinct rotten egg smell, but at high concentrations (above 100 ppm), it deadens the sense of smell entirely, making it a silent killer (NCBI). Fossil fuels with a high concentration of H2S are termed “sour”.
H2S is a highly toxic, flammable, and corrosive gas that poses significant risks to infrastructure and aquatic ecosystems, particularly when it interacts with moisture or in oxygen-depleted (anaerobic) conditions, destroying roots and leading to rapid coastal and riverbank erosion (NCBI). H2S is oxidized in the atmosphere to eventually form sulfuric acid (H2SO4), the main compound of acid rain. H2S is highly soluble in water and can accumulate in equipment, potentially leading to catastrophic consequences for employees.
H2S is highly lethal; even brief exposure to high concentrations can cause immediate collapse and death. Survivors of high-dose exposure often suffer from permanent neurological symptoms, including memory loss and impaired motor function. It behaves similarly to cyanide. It binds to iron in the mitochondrial cytochrome enzymes, effectively suffocating the body at a cellular level (NCBI).
Long-term exposure to low levels of H2S can cause chronic bronchitis, skin discoloration (sulfhemoglobinemia), persistent respiratory symptoms such as cough and chest tightness, obstructive and restrictive pulmonary disease, and asthma (NCBI).
Due to their higher metabolic rate and oxygen demand, children succumb more quickly to the cellular suffocation caused by H2S. Furthermore, since H2S is denser than air, a child standing in a low-lying area may breathe a lethal dose (NCBI).
Carbon Monoxide (CO)
CO is a colorless, odorless, highly flammable, and tasteless gas that is slightly less dense than air. It is a product of incomplete combustion of carbon-based fuels (gas, oil, coal, wood) and commonly known as the “silent killer’ because it is undetectable by human senses (WHO).
CO contributes to the formation of ground-level ozone (O3) through chemical reactions with other pollutants in the presence of sunlight. This secondary ozone is a major component of smog and can damage vegetation and ecosystems (EPA).
CO is a highly toxic asphyxiant, which prevents the blood from carrying oxygen to vital organs, leading to headaches, dizziness, and at high levels, permanent brain damage or death. Long-term exposure to CO leads to flu-like symptoms, such as headache, fatigue, nausea, or vomiting. Long-term exposure can also lead to numbness, unexplained vision problems, sleep disturbances, impaired memory, and concentration.
Children are at a significantly higher risk from exposure to CO because of their higher metabolic rate and smaller body mass, which causes CO levels in their blood to rise faster than in adults. Exposure among children can lead to permanent damage to the brain and heart, and cause nervous system and psychiatric symptoms.
Volatile Organic Compounds (VOCs)
Benzene, Ethylbenzene, Hexane, Toluene, and Xylene
VOCs are organic chemicals that have a high vapor pressure at room temperature, causing them to evaporate easily into the air. They are often characterized by strong odors and found in emissions during fossil fuel production; they are also used and produced in the manufacture of paints, pharmaceuticals, and refrigerants (EPA).
VOCs are a primary ingredient in the formation of ground-level ozone (O3) and secondary organic aerosols. When they react with nitrogen oxides (NOx) in the presence of sunlight, they create photochemical smog, which reduces visibility and damages plant tissues by inhibiting photosynthesis.
In humans, exposure can cause immediate irritation of the eyes, nose, and throat, as well as headaches and loss of coordination. Long-term exposure can be much more severe. Benzene, for instance, is a known human carcinogen linked to leukemia, while other VOCs can cause damage to the liver, kidneys, and central nervous system. Ethylbenzene is a possible human carcinogen and can be released into the air or found in wells near underground fuel storage tanks.
Children are more susceptible because their metabolic systems are less efficient at detoxifying these chemicals. Since many VOCs (i.e., benzene) are heavier than air, they settle closer to the ground where children play. Chronic exposure in early life is linked to an increased risk of childhood asthma and potentially impaired cognitive development.
Hexane is both a naturally occurring and anthropogenic hydrocarbon that is refined from crude oil. Its primary use is as a volatile solvent with major uses in printing, textile manufacturing, and the extraction of vegetable oils from crops such as soybeans (NIH).
Human exposure to hexane occurs mainly through inhalation. Adverse neurological, respiratory, developmental, and reproductive effects are the most important health concerns related to exposure to hexane. Muscle atrophy and decreased body weight are also common. Higher rates of self-reported respiratory symptoms have been observed among workers exposed to hexane, whereas reduced lung function has been reported among children residing near sources of hexane emissions (NIH).
The general population is exposed to very low levels of n-hexane at all times, while those living in urban centers may be exposed to slightly higher levels due to emissions of n-hexane, which are associated with motor fuel. There is currently little information on the carcinogenic potential of hexane (NIH).
In the environment, hexane reacts with nitrogen oxides in the presence of sunlight to form ground level ozone and degrades air quality. When getting into water, hexane disrupts aquatic life, particularly microinvertebrates and juvenile fish. Hexane affects bees and other pollinators, impacting the agricultural sector. Hexane accumulates in poorly drained soils, altering soil microbiota, reducing fertility, and hindering seed germination (Researchgate).
Exposure to toluene can cause nausea and headaches and can affect the nervous system. Symptoms usually stop when exposure ends. Xylene is irritating to the skin, eyes, and respiratory tract. It can cause systemic toxicity by ingestion or inhalation. The most common route of exposure is via inhalation.
Ground-Level Ozone (O3)
Unlike the protective ozone layer in the stratosphere, ground-level, or tropospheric, ozone is a secondary pollutant formed by reactions between nitrogen oxides (NOx), methane, CO, and VOCs in the presence of sunlight. It is a pale-blue, highly reactive gas with a sharp, irritating odor.
O3 damages trees and other plants by affecting them at the cellular level. High ozone levels negatively affect plant growth, photosynthesis, vitality, flowering, water balance, and plant defense. Chronic exposure leads to visible leaf injury, reduced biomass, and significant decreases in crop yields for wheat, soy, and rice. The negative effects ripple through entire ecosystems, damaging biodiversity, harming insects, wildlife, and even soil (Yale).
In humans, O3 oxidizes cells lining the respiratory tract, often described as a “sunburn” of the lungs. Short-term exposure causes coughing, throat irritation, and chest pain, while making the lungs more susceptible to infection (EPA). It is a major driver of premature mortality—responsible for hundreds of thousands of deaths annually—and millions of asthma-related emergency room visits.
Children are at a higher risk because their lungs are still developing and they have to breathe more air relative to body weight. Repeated exposure during childhood is a likely contributor to the initial development of asthma. Ozone is the third most important greenhouse gas in addressing climate change. It also has a secondary “doubled” climate impact: by damaging vegetation and reducing photosynthesis, it impairs the ability of forests and plants to act as carbon sinks, leaving more CO2 in the atmosphere.