What Is Naphthalene?

Naphthalene is an industrial organic chemical best known for its strong “mothball” odor, but its importance in drinking water is tied mainly to petroleum, coal tar, creosote, and industrial waste contamination. Chemically, it is the simplest polycyclic aromatic hydrocarbon, or PAH, consisting of two fused benzene rings. Unlike heavier PAHs such as benzo[a]pyrene, naphthalene is more volatile and more water-soluble, which makes it more mobile in groundwater and more likely to appear as a detectable dissolved contaminant near spill sites.

In commerce, naphthalene has been used in moth repellents, chemical intermediates, dyes, resins, plasticizers, solvents, surfactants, and pesticide-related manufacturing. It is also a natural constituent of coal tar and petroleum-derived materials. It can be released from diesel fuel, gasoline-range petroleum mixtures, fuel oil, wood-preserving chemicals, asphalt products, and former manufactured gas plant residues.

In drinking water, naphthalene is not usually a widespread background contaminant in the way nitrate or arsenic can be. Instead, it is most often a site-specific contaminant associated with leaking underground storage tanks, industrial disposal, contaminated sediments, refinery or fuel terminal releases, creosote treatment facilities, rail yards, old gasworks, and hazardous waste sites. A private well located downgradient of such a source can be at much higher risk than a public system drawing from a protected aquifer.

Naphthalene is considered high concern in drinking water because it can signal a broader organic contamination problem. Where naphthalene is found, related petroleum hydrocarbons, diesel range organics, benzene, toluene, ethylbenzene, xylenes, and other PAHs may also be present. Its odor can be noticeable at low levels, but smell is not a reliable safety test because concentrations that matter for health assessment require laboratory measurement.

Scientific Identity

Naphthalene has the molecular formula C₁₀H₈ and CAS number 91-20-3. It is a crystalline aromatic hydrocarbon at room temperature, with a relatively low molecular weight of about 128.17 g/mol. Its structure consists of two fused benzene rings, placing it in the low-molecular-weight PAH group. Compared with larger PAHs, naphthalene has a higher vapor pressure and greater aqueous solubility, so it behaves partly like a volatile organic compound and partly like a hydrophobic PAH.

Its water solubility is limited but environmentally meaningful, commonly reported in the tens of milligrams per liter range at room temperature. This is much higher than many heavier PAHs, which are often nearly insoluble. Naphthalene also has a moderate organic carbon partitioning tendency, meaning it can sorb to soil organic matter and sediments but can still migrate in groundwater when a continuing source is present. In petroleum plumes, it may move with dissolved-phase hydrocarbons, especially where free product, coal tar, or residual non-aqueous phase liquids remain in the subsurface.

Naphthalene can biodegrade under some aerobic and anaerobic conditions, but degradation rates vary greatly by site. Oxygen, nitrate, sulfate, iron, microbial community structure, temperature, and the presence of other petroleum constituents all affect persistence. In oxygen-depleted groundwater beneath fuel releases or tar deposits, naphthalene may persist long enough to form plumes that extend beyond the original spill area. Because it is semi-volatile, it may also contribute to vapor intrusion concerns where contaminated groundwater or soil gas lies beneath buildings.

How Naphthalene Enters Drinking Water

The most important pathway into drinking water is groundwater contamination from industrial or petroleum sources. Leaking underground storage tanks, fuel pipelines, service stations, bulk fuel terminals, refineries, rail yards, and vehicle maintenance areas can release mixtures containing naphthalene. As lighter petroleum compounds dissolve and move with groundwater, naphthalene can become part of the dissolved plume. It may be detected along with gasoline range organics, diesel range organics, benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes, and other PAHs.

Coal tar and creosote are especially important sources. Former manufactured gas plants produced tar-like residues rich in PAHs, including naphthalene. Wood treatment facilities using creosote can contaminate soil and shallow groundwater with naphthalene and related aromatic compounds. These sources may persist for decades because tarry residuals trapped in soil pores continue to dissolve slowly into groundwater.

Landfills, industrial lagoons, waste disposal pits, stormwater basins, and hazardous waste sites can also release naphthalene. In some cases, contaminated surface water may infiltrate into aquifers used by wells. Spills into rivers or reservoirs are possible, particularly where petroleum handling, shipping, or industrial drainage occurs near source waters. Public drinking water systems generally reduce this risk by monitoring source-water vulnerability and treating water, but small systems and private wells may not have routine testing for naphthalene unless a known site investigation triggers it.

Vapor intrusion is a related exposure pathway rather than a direct drinking water pathway. Because naphthalene can volatilize, groundwater or soil contamination beneath a building may release vapors into indoor air. This matters for water safety investigations because a contaminated well, basement sump, or subsurface plume may create both ingestion exposure and inhalation exposure.

Occurrence and Exposure

Naphthalene occurrence in drinking water is usually localized. It is most likely near petroleum release sites, former gasworks, coal tar deposits, creosote wood-preserving operations, industrial manufacturing corridors, rail yards, harbors, asphalt or tar handling areas, and waste disposal facilities. It can also be found in contaminated sediments and stormwater-impacted surface waters, but groundwater is often the more persistent drinking water concern.

People can encounter naphthalene by drinking contaminated water, using contaminated water for cooking, inhaling vapors released during showering or other household water use, or inhaling vapors from contaminated soil and groundwater beneath buildings. For water systems, the primary exposure route assessed is ingestion, but naphthalene’s volatility means indoor air should not be ignored when concentrations are elevated or when a plume is beneath a structure.

Naphthalene has a distinctive odor, often described as mothballs, tar, coal tar, or petroleum-like. Odor complaints may provide an early warning, but odor thresholds vary, mixtures can mask or intensify smells, and some contaminated waters may not have an obvious odor. A water sample that smells like petroleum, solvent, mothballs, or creosote should not be assumed safe after boiling. Boiling can drive volatile organic chemicals into indoor air and does not remove the underlying contamination.

Private well owners are at particular risk where a well is shallow, located downgradient of a known industrial property, near an old gas station, near a landfill, or close to a former manufactured gas plant. Municipal systems may also face risk when source wells intercept contaminated plumes, but larger systems often have monitoring programs, blending controls, air stripping, granular activated carbon, or alternative source management available.

Health Effects and Risk

Naphthalene is a toxic organic contaminant with health concerns centered on blood effects, liver toxicity, respiratory tract effects, developmental sensitivity, and possible cancer risk. Acute high exposures are known to cause hemolytic anemia, a condition in which red blood cells are damaged faster than the body can replace them. Symptoms may include fatigue, jaundice, dark urine, shortness of breath, and rapid heartbeat. Infants and people with glucose-6-phosphate dehydrogenase deficiency may be more vulnerable to hemolysis.

Animal studies and occupational data have also raised concern about liver effects, cataracts, respiratory tissue injury, and nasal or lung tumors following inhalation exposure. The International Agency for Research on Cancer has classified naphthalene as possibly carcinogenic to humans based on limited human evidence and sufficient or suggestive animal evidence, depending on the assessment framework used. U.S. and other national agencies may describe its carcinogenicity differently, but the overall regulatory concern is that chronic exposure should be minimized.

Drinking water risk depends on concentration, duration, body weight, age, co-exposures, and whether inhalation from household use or vapor intrusion is also occurring. A short-term detection at very low levels does not carry the same risk as long-term use of a contaminated private well. However, naphthalene is often a marker for complex petroleum or PAH contamination, so risk evaluation should consider the full contaminant mixture rather than naphthalene alone.

Because naphthalene is an organic industrial contaminant with both toxic and potential carcinogenic endpoints, vulnerable populations should be protected conservatively. Infants, pregnant people, people with blood disorders, and individuals with G6PD deficiency should avoid using water known to contain elevated naphthalene until a qualified professional has evaluated the results and an appropriate treatment or alternative supply is in place.

Testing and Monitoring

Naphthalene requires specialized laboratory analysis. It is not measured by standard home test strips, basic mineral panels, hardness tests, or typical bacteria tests. Laboratories commonly analyze naphthalene using gas chromatography/mass spectrometry, often within volatile organic compound or semivolatile organic compound methods depending on the laboratory program and regulatory purpose. For petroleum sites, it may be included in PAH panels, VOC panels, or petroleum hydrocarbon investigations.

Sampling must be done carefully because naphthalene is semi-volatile and can be affected by headspace, sample handling, and contamination from petroleum products or plastic materials. Samples are typically collected in laboratory-supplied glass containers with appropriate preservatives and minimal headspace when required. Chain-of-custody procedures are important for regulatory or real estate decisions. If the concern is a known petroleum release, the test plan should often include benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes, gasoline range organics, diesel range organics, and a PAH suite, not just naphthalene.

For private wells, one sample can confirm whether naphthalene is present, but multiple samples may be needed to understand plume behavior, seasonal changes, and treatment performance. Monitoring should include raw water before treatment and finished water after treatment. If activated carbon is installed, post-treatment monitoring is essential because carbon can eventually become exhausted and allow breakthrough.

Where vapor intrusion is possible, water testing may need to be paired with soil gas, sub-slab vapor, indoor air, or basement air evaluation. A water sample alone may not characterize total exposure if the contaminant source is in shallow groundwater beneath the building.

Treatment Methods

Activated carbon is generally the best practical treatment for naphthalene in drinking water because naphthalene adsorbs strongly to high-quality carbon compared with many highly water-soluble contaminants. Granular activated carbon and carbon block systems can remove naphthalene by trapping molecules on the internal pore surfaces of the carbon. Performance depends on carbon type, bed depth, flow rate, water temperature, organic matter, competing contaminants, and influent concentration.

Activated carbon works best when contamination is relatively stable, flow rates are controlled, and the system is sized for the actual concentration and water use. A point-of-use carbon system may be adequate when the goal is only to treat drinking and cooking water and contaminant levels are low. A point-of-entry system is more appropriate when naphthalene levels are elevated, when odor is present throughout the home, when shower inhalation exposure is a concern, or when other petroleum VOCs are present.

Activated carbon can fail if it is undersized, overloaded by high dissolved organic carbon, exposed to a mixture of competing petroleum hydrocarbons, operated at excessive flow, or used beyond its adsorption capacity. Breakthrough may occur before taste or odor returns. For this reason, serious naphthalene contamination should be managed with lead-lag carbon vessels, sampling ports between vessels, scheduled laboratory testing, and replacement based on results rather than calendar time alone.

Reverse osmosis can be useful at a single tap, particularly in combination with carbon, but it is not usually chosen as the sole treatment for a whole home. Advanced oxidation and air stripping are more common in municipal, remediation, or industrial-scale systems. For private wells impacted by a confirmed plume, treatment should be selected by a qualified water treatment professional or environmental engineer using laboratory data for the full contaminant mixture.

Regulations and Guidelines

Regulatory treatment of naphthalene varies by country, state, province, and local environmental program. In the United States, naphthalene has not generally been regulated with a nationwide federal Maximum Contaminant Level under the Safe Drinking Water Act in the same way as benzene or trichloroethylene. However, U.S. EPA health advisories, risk assessments, and cleanup screening levels may be used by agencies when evaluating contaminated groundwater, hazardous waste sites, or public water supply impacts. Exact advisory values and cleanup targets can change over time and may differ depending on exposure assumptions.

The World Health Organization has evaluated many organic chemicals in drinking water, but not every PAH has a formal guideline value in every edition or jurisdictional adoption. Some international drinking water standards focus on selected carcinogenic PAHs, especially benzo[a]pyrene or sums of heavier PAHs, and may not include naphthalene in the regulated PAH total. This means a water supply can meet a PAH regulatory parameter while still needing site-specific evaluation for naphthalene if a petroleum or coal tar source is known.

Many states, provinces, and countries use their own groundwater quality criteria, drinking water advisory levels, notification levels, health-based guidance values, or remediation standards for naphthalene. These values may differ because they use different toxicological endpoints, cancer classifications, body weights, water intake assumptions, treatment feasibility factors, or risk management policies. Local environmental agencies may also set site-specific cleanup goals for groundwater plumes affecting wells.

For consumers, the most important regulatory point is that absence of a universal enforceable drinking water limit does not mean naphthalene is safe at any concentration. A detection should be interpreted against the most relevant local health-based guidance and in the context of other petroleum or PAH contaminants. Public water customers should request the utility’s current monitoring data if naphthalene is associated with a known source-water issue. Private well owners should consult their local health department, environmental agency, or certified laboratory for interpretation.

Related Contaminants

Frequently Asked Questions

Does naphthalene in water smell like mothballs?

Often yes. Naphthalene has a distinctive mothball, tar, or coal-tar odor. However, odor is not a reliable measure of safety. Some people detect it at very low levels, while other petroleum chemicals can mask it. Laboratory testing is required to confirm concentration.

Is naphthalene the same as PAHs?

Naphthalene is one specific PAH. It is the simplest two-ring PAH and is more volatile and more water-soluble than many heavier PAHs. Its behavior in groundwater can be different from larger PAHs such as benzo[a]pyrene, which tend to bind more strongly to sediments.

Can I remove naphthalene by boiling water?

No. Boiling is not recommended for naphthalene contamination. It may volatilize some chemical into indoor air and does not provide controlled removal. Use an appropriate certified or professionally designed treatment system, or use an alternate safe water source until the problem is resolved.

Should I use a whole-house filter or an under-sink filter?

An under-sink activated carbon or reverse osmosis system may be suitable for low-level contamination limited to drinking and cooking water. A whole-house activated carbon system is more appropriate when concentrations are higher, odor is present throughout the home, shower inhalation is a concern