What Is PAHs?

PAHs, or polycyclic aromatic hydrocarbons, are a large family of organic chemicals made of two or more fused aromatic rings. They are not a single compound with one formula or one CAS number; instead, drinking water investigations usually measure a list of individual PAHs such as naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, indeno[1,2,3-cd]pyrene, and benzo[ghi]perylene.

PAHs are produced whenever organic material is incompletely burned. They are also naturally present in crude oil, coal, coal tar, asphalt, creosote, and many petroleum-derived residues. In drinking water, the most important sources are typically industrial releases, historic manufactured gas plant sites, wood-treatment facilities, petroleum spills, coking operations, refinery areas, contaminated stormwater, and waste disposal locations.

The drinking water concern is not only taste, odor, or visible petroleum contamination. Several PAHs are toxic, mutagenic, or carcinogenic, and benzo[a]pyrene is widely used as a marker for carcinogenic PAH risk. Because PAHs are hydrophobic, they often attach to soil, sediment, pipe scale, and organic particles. Lower-molecular-weight PAHs such as naphthalene are more water-soluble and more mobile in groundwater than heavier PAHs, while heavier PAHs tend to persist in contaminated sediments and source materials.

Scientific Identity

PAHs are semivolatile organic compounds with fused benzene-like rings and no heteroatoms in their basic structure. Their water behavior varies strongly by molecular size. Two- and three-ring PAHs, including naphthalene, acenaphthene, fluorene, phenanthrene, and anthracene, are more volatile and more soluble than four- to six-ring PAHs. Larger PAHs such as benzo[a]pyrene and benzo[ghi]perylene have very low water solubility, high organic-carbon partitioning, and strong affinity for sediments, soils, activated carbon, and organic matter.

Environmental laboratories often report PAHs as individual compounds rather than “total PAHs” because toxicity, mobility, treatment performance, and regulatory status differ by compound. A water sample containing mostly naphthalene behaves differently from one containing trace benzo[a]pyrene associated with fine suspended particles. Alkylated PAHs, common in petroleum and diesel releases, may also be important at contaminated sites, although standard drinking water panels may focus on priority pollutant PAHs rather than all petroleum-related PAH homologs.

PAHs are chemically stable compared with many readily biodegradable organic chemicals. They can degrade through sunlight-driven reactions in surface water, microbial degradation in soils and aquifers, and oxidative treatment processes, but degradation can be slow under oxygen-limited groundwater conditions. Their persistence is one reason old coal tar deposits, creosote contamination, and petroleum source zones can continue to affect groundwater long after the original industrial use has ended.

How PAHs Enters Drinking Water

PAHs enter drinking water sources primarily through contaminated land and water pathways. Historic manufactured gas plant sites are a well-known source because coal tar residuals contain high concentrations of PAHs. Dense, tar-like nonaqueous phase liquids can remain in subsurface soils and slowly release PAHs to groundwater for decades. Wood-preserving sites that used creosote can also generate PAH plumes, especially where treatment cylinders, drip pads, storage tanks, or disposal areas leaked into soil.

Petroleum releases are another important pathway. Diesel fuel, heating oil, lubricating oil, crude oil, and some fuel mixtures contain PAHs. Gasoline contains fewer heavy PAHs than diesel or coal tar, but fuel releases may still include naphthalene and other lighter aromatic compounds. Stormwater from roads, parking lots, airports, industrial yards, and rail corridors can carry PAHs from vehicle exhaust particles, tire and asphalt wear, soot, spilled fuel, and oily residues into rivers, reservoirs, and shallow groundwater recharge zones.

Industrial air deposition can also contribute. Emissions from coking facilities, metal smelters, waste incineration, power generation, asphalt production, and large fires can deposit PAH-containing particles onto watersheds. Once deposited, PAHs may bind to sediment and organic matter, then be transported during runoff events. Surface water intakes downstream of urban and industrial areas may therefore see PAH pulses after storms, spills, dredging, or wildfire ash runoff.

For private wells, risk is highest when a well is near a known contaminated site, old gasworks, rail yard, petroleum storage area, landfill, wood-treatment facility, industrial lagoon, or underground storage tank release. Poor well construction, shallow wells, fractured bedrock, and permeable sandy aquifers can increase vulnerability. PAHs may also be present with other petroleum hydrocarbons, volatile organic compounds, phenols, and metals, so a PAH detection often triggers broader source investigation.

Occurrence and Exposure

PAHs are usually not found at elevated levels in properly protected municipal drinking water supplies, but they are significant at contaminated sites and in some source waters affected by industrial runoff or spills. Their occurrence is strongly location-specific. A rural well far from fuel storage and industrial land may have no detectable PAHs, while a shallow well near a former manufactured gas plant or creosote site may require detailed testing and treatment.

Exposure from drinking water can occur by ingestion, food preparation, and, for the more volatile lower-molecular-weight PAHs, inhalation during showering or other indoor water use. However, many heavier carcinogenic PAHs are not highly volatile, so ingestion of contaminated water and ingestion of particle-bound residues are usually more relevant than inhalation. Skin contact may contribute in highly contaminated water, particularly with oily films or petroleum odors, but the main public health concern is long-term exposure to carcinogenic and mutagenic PAHs.

PAHs in tap water may be accompanied by sensory clues, but absence of odor does not prove safety. Naphthalene can produce a mothball-like odor at sufficient concentrations. Petroleum mixtures may cause oily, fuel-like, tar-like, or asphalt-like smells. Heavy PAHs such as benzo[a]pyrene can be present at health-relevant trace levels without obvious taste or odor. Laboratory testing is therefore essential when a site history suggests coal tar, petroleum, creosote, or industrial contamination.

Health Effects and Risk

The health risk from PAHs depends on which compounds are present, their concentrations, exposure duration, and whether exposure occurs as a dissolved chemical, particle-bound chemical, or petroleum mixture. Benzo[a]pyrene is one of the most important drinking water indicators because it is carcinogenic and has been extensively studied. Several PAHs can be metabolized in the body to reactive intermediates that bind DNA, forming adducts that may contribute to mutation and cancer development.

Long-term exposure to carcinogenic PAHs is associated with increased cancer concern, especially when mixtures contain benzo[a]pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, dibenz[a,h]anthracene, or indeno[1,2,3-cd]pyrene. Risk assessment often uses toxicity equivalency approaches that compare the potency of individual PAHs to benzo[a]pyrene, although regulatory programs vary in how they apply these methods.

Non-cancer concerns may include effects on the liver, immune system, blood, skin, and developing fetus, depending on the PAH mixture and dose. Occupational and environmental studies of PAH mixtures are complicated because smoke, soot, petroleum residues, and industrial emissions contain many co-contaminants. In drinking water, the practical concern is that PAHs are markers of a broader contamination source that may also include benzene, toluene, ethylbenzene, xylenes, phenols, petroleum hydrocarbons, metals, or other semivolatile organic compounds.

Infants, pregnant people, and individuals with long-term dependence on a contaminated private well may face higher concern because exposure can be continuous. Any confirmed detection of regulated carcinogenic PAHs in drinking water should be evaluated with a qualified water professional, health agency, or environmental laboratory, especially if levels exceed applicable standards or if oily residues are visible.

Testing and Monitoring

PAHs require specialized laboratory analysis. Common analytical approaches include gas chromatography-mass spectrometry for semivolatile organic compounds and high-performance liquid chromatography with fluorescence detection for selected PAHs. In the United States, laboratories may use EPA drinking water or wastewater/solid waste methods adapted to the project, such as methods for semivolatile organic compounds and PAH-specific analysis. The correct method should match the reporting limits needed for drinking water decisions, not merely general site screening.

Sampling technique matters. PAHs can adsorb to plastic, sediment, and organic particles, so laboratories typically specify amber glass bottles, proper preservatives or cooling, and strict holding times. The sampling plan should clarify whether the result represents total recoverable PAHs, dissolved PAHs, or unfiltered water. For drinking water exposure, unfiltered first-draw and flushed samples may answer different questions: a flushed sample is often better for source water conditions, while first-draw sampling may reveal contamination from plumbing scale or stagnant water in a system.

A complete investigation often includes both individual PAHs and related petroleum indicators. If the suspected source is diesel, heating oil, or coal tar, testing may include diesel range organics, gasoline range organics, volatile organic compounds, phenols, and metals. If the source is a contaminated aquifer, repeat testing over time is important because PAH levels can change with groundwater elevation, pumping patterns, storms, remediation activity, or migration of a plume.

Treatment Methods

Activated carbon is the leading treatment for PAHs in drinking water because PAHs are hydrophobic organic molecules that adsorb strongly to carbon surfaces. Granular activated carbon can be used in point-of-entry systems for whole-house treatment or in point-of-use systems for drinking and cooking water. Carbon block filters may also be effective when properly certified and sized for organic chemical reduction, although performance depends on contact time, carbon mass, flow rate, influent concentration, and competing natural organic matter.

Activated carbon works best when PAH concentrations are low to moderate, water is relatively free of suspended solids and oil droplets, and the system is maintained before breakthrough. It may fail early if the water contains high dissolved organic carbon, petroleum sheen, iron fouling, biological growth, sediment, or a complex mixture of solvents and fuel hydrocarbons that compete for adsorption sites. Because PAHs can break through without taste or odor, carbon systems used for known contamination should be monitored with scheduled sampling rather than replaced only when water tastes different.

Point-of-use activated carbon is often appropriate when PAHs are present at low levels and the primary exposure route is drinking and cooking. Point-of-entry treatment may be appropriate for private wells with confirmed contamination at multiple taps, for higher concentrations, for homes where water is used extensively for food preparation, or where lighter PAHs such as naphthalene create odor or potential inhalation concerns. In severe petroleum or coal tar contamination, treatment should be paired with source control, alternate water, or well replacement rather than relying solely on a household filter.

Regulations and Guidelines

PAH regulation is usually compound-specific rather than based on one universal “total PAHs” drinking water limit. In the United States, the EPA has a federal drinking water maximum contaminant level for benzo[a]pyrene, with a health goal of zero because it is treated as a carcinogenic contaminant. Other PAHs may be addressed through site cleanup standards, state groundwater criteria, hazardous waste programs, spill response requirements, or risk-based remediation goals rather than a single national tap water standard.

Internationally, limits vary by jurisdiction. The World Health Organization has provided health-based guidance for benzo[a]pyrene in drinking water, and many countries use benzo[a]pyrene as a key PAH indicator. The European Union and some national regulations include parametric values for benzo[a]pyrene and for a defined sum of selected PAHs, but the specific compounds included and numerical values should be checked against the current local law. Local environmental agencies may impose stricter requirements near contaminated sites, source-water protection areas, or industrial releases.

For private wells, regulatory protection is often limited because many jurisdictions do not require routine PAH testing for individual household wells. If a private well is near a contaminated site, landfill, fuel release, or industrial facility, homeowners may need to request PAH analysis directly from an accredited laboratory or through a local health department. Results should be compared with the most protective applicable drinking water standards, groundwater screening levels, and health-based advisory levels for the specific PAHs detected.

Related Contaminants

Frequently Asked Questions

Are PAHs one chemical or a group of chemicals?

PAHs are a group of chemicals, not one substance. A laboratory report should list individual PAHs such as naphthalene, phenanthrene, anthracene, pyrene, and benzo[a]pyrene. This matters because lighter PAHs are more mobile in groundwater, while heavier PAHs are often more particle-bound and may carry greater carcinogenic concern.

Can I smell PAHs in drinking water?

Sometimes, but not reliably. Naphthalene may produce a mothball-like odor, and petroleum mixtures may smell oily, tarry, or fuel-like. However, carcinogenic PAHs can be present at trace concentrations without a noticeable odor, so smell cannot be used as a safety test.

Is activated carbon enough for a contaminated well?

Activated carbon is often the best household treatment for PAHs, but it must be properly sized and monitored. For a confirmed plume or petroleum source, a treatment professional should evaluate whether point-of-use or point-of-entry treatment is needed, whether sediment pretreatment is required, and how often post-treatment samples should be collected.

Do PAHs mean there is petroleum contamination?

PAHs can indicate petroleum contamination, but they can also come from coal tar, creosote, soot, asphalt, industrial combustion, wildfire ash, or contaminated sediments. The pattern of individual PAHs helps investigators identify whether the source is fuel oil, coal tar, creosote, urban runoff, or combustion deposition.

Should I use the water while waiting for PAH test results?

If there is a strong petroleum or tar odor, visible sheen, known nearby spill, or notice from an environmental agency, use an alternate drinking and cooking water source until testing confirms safety. For bathing and other uses, the decision depends on concentrations, the PAHs present, and whether volatile