Jump to main content or area navigation.

Contact Us

EPA-Expo-Box (A Toolbox for Exposure Assessors)

EPA-Expo-Box icon

Other Organics

Hydrocarbons

Hydrocarbons contain only hydrogen and carbon atom in various configurations, including chains, branched chains, and rings. Most hydrocarbons fall into one of the five structural categories described in the table below.

Hydrocarbon Subclasses
Subclass Description Example Structures
n-Alkanes
Saturated, unbranched, hydrocarbon chains
Methane, CH4
CH4
n-Hexane, C6H14
C6H14
Branched alkanes
Saturated, branched, hydrocarbon chains
Isobutane, C4H10
C4H10
Neopentane, C5H12
C5H12
Unsaturated and alicyclic hydrocarbons
Unsaturated hydrocarbon chains and hydrocarbon rings
1,3-Butadiene, C4H6
C4H6
Cyclohexane, C6H12
C6H12
Alkylated benzenes
Six-carbon, aromatic rings with hydrocarbon chain substituent groups
Toluene, C7H8
C7H8
Ethylbenzene, C8H10
C8H10
Polycyclic aromatic hydrocarbons and related compounds
Multiple, six-carbon aromatic rings, sometimes in combination with other hydrocarbon ring structures
Benzo(a)pyrene, C20H12
C20H12
Acenaphthene, C12H10
C12H10

Some hydrocarbons are derived from petroleum distillates; others (e.g., pine oil and turpentine) are derived from wood. Many hydrocarbons are used as fuel components, including methane, benzene, toluene, ethylbenzene, and xylenes. One specific group of hydrocarbons currently of interest to exposure assessors are the polycyclic aromatic hydrocarbons (PAHs), which contain rings of carbon atoms with alternating single and double bonds between carbons. PAHs are found in coal tar and crude oil, and are sometimes used in medicines or the production of dyes, plastics, and pesticides (ATSDR, 1990). Commercial production of PAHs is minimal; most PAHs found in the environment are created as a byproduct of incomplete combustion of fossil fuels or wood. PAHs can be created through burning of any organic material, including smoking cigarettes and grilling meat, vegetables, or other foods. Some PAHs occur as byproducts of industrial processes such as metal production and refining. PAHs can also be released through natural processes such as volcanic eruptions or forest fires. As PAHs are accidentally produced and released rather than intentionally, they generally occur in mixtures (ATSDR, 1990).


Physicochemical Properties

In general, hydrocarbon rings without alternating single and double bonds and hydrocarbon chains are referred to as alicyclic or aliphatic hydrocarbons, respectively, and hydrocarbon rings with alternating single and double bonds are referred to as aromatic hydrocarbons. The number of double versus single bonds determines whether aliphatic hydrocarbons are "saturated" with respect to hydrogen or "unsaturated" due to the presence of double bonds. Saturated hydrocarbons are more reactive than unsaturated hydrocarbons and are more easily broken down. The bond pattern in the ringed structure of PAHs, on the other hand, results in a chemical property called aromaticity, which confers stability these compounds.

The table below provides a summary of key physicochemical factors that are likely to affect partitioning and fate of select aliphatic and aromatic hydrocarbons in the environment. For chemical-specific values, consult the resources provided in the introduction to this module.

Property Fate and Transport Implications
Aliphatic Hydrocarbons
Vapor pressure at 25°C (atm)

Vapor pressure of aliphatic hydrocarbons increases with decreasing chain length, suggesting that low-molecular-weight hydrocarbons will volatilize readily from the pure organic state, while high-molecular-weight compounds will exhibit low volatility.

Henry’s Law Constant

H increases with increasing molecular size of the hydrocarbon, however most non-aromatic hydrocarbons have a Henry’s Law Constant around 10 or greater. The high constant indicates a propensity to volatilize, implying that the compounds will readily move from water to air.

Solubility in water (mg/L)

As the number of carbons in the chain increase (i.e., molecular weight increases), solubility tends to decrease; branching also decreases solubility. Aliphatic hydrocarbons are less soluble than aromatic hydrocarbons of similar molecular weight. As hydrocarbons commonly occur in mixtures, solubility can vary greatly depending on the composition of the released hydrocarbon fraction.

Octanol-Water Partition Coefficient (log value)

The hydrophobicity of aliphatic hydrocarbons generally increases with increasing chain length; larger hydrocarbons have higher sorption potential and are more likely to bioaccumulate.

Octanol-Air Partition Coefficient (log value)

Like most other properties, Koa also increases with increased molecular volume, implying that the high-molecular weight aliphatic hydrocarbons (longer chain length) are less likely to partition to air, and more likely to remain sorbed to soil, sediment, or biota than the shorter-chain hydrocarbons.

Summary: Lighter aliphatic hydrocarbons—with smaller chain lengths—have higher volatility and water solubility, and are less likely to sorb to particles or sediment than larger, longer-chain hydrocarbons. The behavior of a hydrocarbon mixture will therefore vary greatly depending on the mixture composition, and will change over time as the lighter constituents will more readily volatilize and migrate through soils, while heavier constituents will settle into soils, sorb to organic matter, and persist at the site of release.
Aromatic Hydrocarbons
Vapor pressure at 25°C (atm)

Vapor pressure is inversely related to PAH size. PAHs with more aromatic rings have higher vapor pressures and are more likely to sorb to particulate matter in air than smaller PAHs.

Henry’s Law Constant

Henry’s Law Constants decrease with increasing molecular weight of PAHs, suggesting that low-molecular weight PAHs volatize readily from water to air whereas higher molecular weight PAHs will remain in water.

Solubility in water (mg/L)

PAHs have low water solubility, meaning they are primarily found sorbed to particles suspended in the water column or settled to sediment.

Octanol-Water Partition Coefficient (log value)

High Kow values signify an affinity to move from water to lipid, suggesting potential for bioaccumulation and bioconcentration in aquatic systems.

Octanol-Air Partition Coefficient (log value)

High log Koa values imply that PAHs in the atmosphere will sorb to particulates

Summary:While air release of PAHs is most likely, residence time in air is limited. PAHs are expected to partition to soil or sediment.

Top of Page


Routes

Exposure to hydrocarbons occurs most commonly from unintentional ingestion or inhalation. Exposure from dermal contact can also occur. The Routes Tool Set of EPA-Expo-Box provides additional information and resources organized by route.

Route Potential Sources of Hydrocarbon Exposure
Inhalation
  • Inhalation of ambient air, particularly in urban environments with asphalt-paved roads and heavy vehicle traffic, can result in exposure to PAHs. Hydrocarbons can volatilize out of the asphalt material, are released in vehicle exhaust, and are a component of tire wear and brake lining particles.
  • Environmental tobacco smoke is a large source of PAHs. Smokers or people who live or work in close proximity to smokers may be exposed to PAHs through inhalation.
Ingestion

Ingestion of hydrocarbons occurs as the compounds are a common contaminant in processed foods, particularly foods that are grilled or smoked. PAHs can also occur in non-processed foods such as fruits and vegetables that are grown in environments with contaminated soil or air.

  • Some PAHs are bioaccumulative, which can result in higher contamination levels in some fish, meats, and milk products (both cow’s milk and human breast milk).
  • PAHs have been detected in some drinking water sources.
Dermal Contact

Dermal exposure can occur through contact with contaminated soil or water, or anthropogenic surfaces such as asphalt roads or coal tar surfaces.

  • Exposure is common through use of multiple consumer products (e.g., gasoline and other fuels, furniture polish, household cleansers, and propellants).

Top of Page


Media

Hydrocarbons are common contaminants in multiple environmental media, including air, water, and soil. The Media Tool Set of EPA-Expo-Box provides additional information and resources organized by media.

Media Sources of Hydrocarbons
Air
  • Anthropogenic releases of hydrocarbons to air include residential burning of wood and open burning of organic matter (e.g., leaves, yard clippings), waste incineration, vehicle exhaust, cigarette smoke, and production of coal tar, coke, and asphalt.
  • Accidental releases of petroleum hydrocarbons occur through oil and gas spills, both large scale (such as the U.S. Gulf Oil spill in 2010) and small scale (drips from fuel pumps at automobile fill-up stations).
  • Natural releases of hydrocarbons to air include forest fires and volcanic eruptions.
Water
  • Stormwater runoff in urban areas can contain hydrocarbons due to vehicle exhaust, asphalt paving, or rubber tire wearings.
  • Runoff in industrial areas with wood treatment plants, petroleum refineries, or municipal waste treatment centers may contain hydrocarbons.
Soil

The main source of hydrocarbons in soils is atmospheric deposition.

  • Contamination of soils along roadways occurs through deposition of emissions from vehicle exhaust, as well as emissions from the wearing of rubber tires and the asphalt.
  • Soils may also become contaminated with hydrocarbons following activities such as disposal of sludge from wastewater treatment plants, landfilling, or application of compost or certain pesticides. Soils at or near sites where goal, gas, or wood was burned may also be contaminated.
Consumer Products

Hydrocarbons are found in many consumer products (e.g., gasoline and other fuels, furniture polish, household cleansers, and propellants).

Top of Page


Exposed Populations

Hydrocarbons, especially PAHs, are ubiquitous in the environment and the general population can be exposed in both indoor and outdoor environments. Certain populations are at risk for higher exposures than the general population:

  • Cigarette smokers or people who live or work in close proximity to smokers may be exposed to PAHs through inhalation of smoke.
  • As PAHs are a common contaminant of vehicle exhaust, urban populations may be more highly exposed than rural populations; however, rural populations may have high exposures to diesel fuels used in farm equipment or trucks, which might be less efficient at combustion due to older age than some vehicles in urban areas.
  • Certain occupations may result in high exposure to PAHs, including but not limited to: mechanics, construction workers involved with pavement, roads, or roofs, utility workers exposed to creosote, coal miners or those in the petroleum refinery business, steel foundry and aluminum workers.

See the Lifestages and Populations Tool Set of EPA-Expo-Box for resources related to particular population groups and lifestages.

Top of Page


Tools

Top of Page

Measure of a substance’s volatility, or its propensity to partition to the vapor (gaseous) phase from its condensed phase (i.e., solid or liquid). This can be used to predict whether inhalation or other exposure routes are more relevant.

Related to Vapor Pressure. Reflects chemical partitioning between the aqueous, dissolved phase and the gaseous phase.

Measure of a substance’s partitioning between dissolved and insoluble phases. Depends on the solute (e.g., water, alcohol) and other substances dissolved in the solute.

Measure of a substance’s partitioning between dissolved and insoluble phases. Depends on the solute (e.g., water, alcohol) and other substances dissolved in the solute.

Ratio of a chemical that has reached equilibrium in adjacent fractions of octanol and water. This ratio is used frequently to estimate how an organic chemical will partition in the environment (e.g., between dissolved and sorbed fractions in surface water) as well as how it will behave in with respect to human tissues. A compound with a high octanol-water partition coefficient is more likely to bioaccumulate in human tissues, especially fatty tissues.

Ratio of a chemical that has reached equilibrium in adjacent fractions of octanol and air. This ratio is used frequently to estimate how an organic chemical will partition in the environment (e.g., between gaseous and particulate fractions in the atmosphere, between soil organic matter and air) as well as how it will behave with respect to human respiratory tissues. A compound with a high octanol-air partition coefficient is more likely to bioaccumulate in human respiratory tissues, particularly is the log octanol-water partition coefficient moderate.

Jump to main content.