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Naphthalene

Technical Fact Sheet

NPIC Technical Fact Sheets provide information that is complex and intended for individuals with a scientific background and/or familiarity with toxicology and risk assessment. This document is intended to promote informed decision-making. Please refer to the General Fact Sheet for less technical information.

Molecular Structure -
Naphthalene

Laboratory Testing: Before pesticides are registered by the U.S. EPA, they must undergo laboratory testing for short-term (acute) and long-term (chronic) health effects. Laboratory animals are purposely given high enough doses to cause toxic effects. These tests help scientists judge how these chemicals might affect humans, domestic animals, and wildlife in cases of overexposure.

Chemical Class and Type:

  • Naphthalene is a bicyclic aromatic hydrocarbon derived from coal tar or crude oil.1,2 It is an insecticide that is also used as a repellent.3 Its International Union of Pure and Applied Chemistry (IUPAC) name is naphthalene.4 The Chemical Abstracts Service (CAS) registry number is 91-20-3.3
  • Naphthalene and other polycyclic aromatic hydrocarbons (PAHs) are released from incomplete combustion processes originating in industry, domestic sources including cigarette smoke and motor vehicle exhaust, as well as natural events such as forest fires.5
  • Naphthalene was first registered as a pesticide in the United States in 1948 with the U.S. Department of Agriculture (USDA).3 A registration standard was issued by the United States Environmental Protection Agency (U.S. EPA) in 1981, and the agency reregistered naphthalene in 2008.3 See the text box on Laboratory Testing.

Physical / Chemical Properties:

  • Naphthalene is a white crystalline or colorless to brown solid. Naphthalene has a distinct aromatic odor.3,6
  • Vapor pressure1: 0.087 mmHg
  • Octanol-Water Partition Coefficient (log Kow)1: 3.29
  • Henry's constant1: 4.6 x 10-4 atm·m3/mol
  • Molecular weight1,3: 128.18 g/mol
  • Solubility (water)7: 3 mg/100 mL at room temperature
  • Soil Sorption Coefficient (Koc)3: Values from 200-1470 have been reported worldwide in a variety of soil types.

Uses:

  • Naphthalene is primarily used in the United States in the production of plastics, dyes, resins, lubricants, and fuels.4 As a pesticide, most naphthalene is used to control moths in indoor storage areas. Products are also registered to repel squirrels, bats and other animals in attics, around structures and gardens.3 Uses for individual products containing naphthalene vary widely. Always read and follow the label when applying pesticide products.
  • Signal words for products containing naphthalene may range from Caution to Danger. The signal word reflects the combined toxicity of the active ingredient and other ingredients in the product. See the pesticide label on the product and refer to the NPIC fact sheets on Signal Words and Inert or "Other" Ingredients.
  • To find a list of products containing naphthalene which are registered in your state, visit the website http://npic.orst.edu/reg/state_agencies.html and search by "active ingredient."

Mode of Action:

Target Organisms

  • Naphthalene is used for the control of clothes moths. In this application, the naphthalene vapors fill the airtight container and kill the insects. No information was found on the mode of action of naphthalene in insects.
  • Naphthalene is registered for use as a wildlife repellent in some products. In this application, it is meant to be effective via inhalation and subsequent avoidance of the odor in the treated area.3

Non-target Organisms

  • Following exposure, naphthalene is metabolically activated by the body. Different tissues produce different metabolites, leading to site-specific toxicity.8 Differences in naphthalene toxicity among species may be the result of species-specific metabolic pathways.9
  • In humans, the metabolite alpha-naphthol has been linked to the development of hemolytic anemia in some cases following ingestion or extensive dermal or inhalation exposure. Susceptibility appears to be exacerbated by a deficiency in the glucose 6-phosphate dehydrogenase enzyme, or G-6-PD. The enzyme is inherited through a sex-linked gene so that males are more likely to express the deficiency. The deficiency is also more prevalent in people of African or Mediterranean descent.10
  • Newborns appear to be more susceptible to developing hemolysis following exposure to naphthalene. This susceptibility may be due to their less-developed ability to conjugate and excrete naphthalene metabolites.1 In two studies, several young children developed hemolytic anemia following exposure to clothing, bedding and diapers that had been stored with naphthalene mothballs. Many but not all of these children appeared to have the G-6-PD enzyme deficiency.11,12 Fetuses have developed hemolytic anemia in utero following their mothers' inhalation exposure.13
  • Some metabolites deplete glutathione stores in affected tissues such as the lungs, leading to toxicity.14,15 Metabolites responsible for glutathione depletion have been identified as naphthalene oxide or 1,2-naphthoquinone and 1,4-naphthoquinone.9,16 See the Metabolism section below.

Acute Toxicity:

Oral

  • Acute oral LD50 values in rats range from 2200 mg/kg to 2600 mg/kg.1,17,18 Based on these values, the U.S. EPA considered naphthalene to be low in toxicity via oral exposure.3 See the text boxes on Toxicity Classification and LD50/LC50.

    LD50/LC50: A common measure of acute toxicity is the lethal dose (LD50) or lethal concentration (LC50) that causes death (resulting from a single or limited exposure) in 50 percent of the treated animals. LD50 is generally expressed as the dose in milligrams (mg) of chemical per kilogram (kg) of body weight. LC50 is often expressed as mg of chemical per volume (e.g., liter (L)) of medium (i.e., air or water) the organism is exposed to. Chemicals are considered highly toxic when the LD50/LC50 is small and practically non-toxic when the value is large. However, the LD50/LC50 does not reflect any effects from long-term exposure (i.e., cancer, birth defects or reproductive toxicity) that may occur at levels below those that cause death.

  • Naphthalene appears to be more toxic to mice than rats. A study using CD-1 mice identified an acute oral LD50 of 533 mg/kg in males and 710 mg/kg in females.19 Researchers determined an LD50 of 353.6 mg/kg for female CD-1 mice based on an 8-day oral exposure.20

Dermal

  • The dermal LD50 was determined to be greater than 2000 mg/kg in rabbits.17 Naphthalene is considered low in toxicity by the U.S. EPA for dermal exposure.3
  • The U.S. EPA considered primary eye irritation to be slight to moderate and skin irritation to be moderate based on studies with rabbits. Naphthalene is considered to be low in toxicity for both exposure routes.3
  • Naphthalene was not found to be a skin sensitizer in guinea pigs.3,17
TOXICITY CLASSIFICATION - NAPHTHALENE
High Toxicity Moderate Toxicity Low Toxicity Very Low Toxicity
Acute Oral LD50 Up to and including 50 mg/kg
(≤ 50 mg/kg)		 Greater than 50 through 500 mg/kg
(>50-500 mg/kg)		 Greater than 500 through 5000 mg/kg
(>500-5000 mg/kg)		 Greater than 5000 mg/kg
(>5000 mg/kg)
Inhalation LC50 Up to and including 0.05 mg/L
(≤0.05 mg/L)		 Greater than 0.05 through 0.5 mg/L
(>0.05-0.5 mg/L)		 Greater than 0.5 through 2.0 mg/L
(>0.5-2.0 mg/L)		 Greater than 2.0 mg/L
(>2.0 mg/L)
Dermal LD50 Up to and including 200 mg/kg
(≤200 mg/kg)		 Greater than 200 through 2000 mg/kg
(>200-2000 mg/kg)		 Greater than 2000 through 5000 mg/kg
(>2000-5000 mg/kg)		 Greater than 5000 mg/kg
(>5000 mg/kg)
Primary Eye Irritation Corrosive (irreversible destruction of ocular tissue) or corneal involvement or irritation persisting for more than 21 days Corneal involvement or other eye irritation clearing in 8 - 21 days Corneal involvement or other eye irritation clearing in 7 days or less Minimal effects clearing in less than 24 hours
Primary Skin Irritation Corrosive (tissue destruction into the dermis and/or scarring) Severe irritation at 72 hours (severe erythema or edema) Moderate irritation at 72 hours (moderate erythema) Mild or slight irritation at 72 hours (no irritation or erythema)
The highlighted boxes reflect the values in the "Acute Toxicity" section of this fact sheet. Modeled after the U.S. Environmental Protection Agency, Office of Pesticide Programs, Label Review Manual, Chapter 7: Precautionary Labeling. http://www.epa.gov/oppfead1/labeling/lrm/chap-07.pdf

Inhalation

  • The inhalation LC50 for naphthalene was determined to be 0.4 mg/L or 77.7 ppm in an unspecified animal species, and it is considered moderately toxic via inhalation by the U.S. EPA.3
  • Mice exposed to 2 ppm naphthalene vapors for four hours exhibited damage to the Clara cells of the proximal airways.21 Clara cells are nonciliated bronchiolar cells containing P450 enzymes, the makeup and distribution of which are speciesdependent.22 Rats, however, were unaffected following inhalation exposure to naphthalene for four hours at concentrations up to 110 ppm.21

Signs of Toxicity - Animals

  • Three dogs fed varying amounts of naphthalene either as a single dose or over the course of five days exhibited lethargy, diarrhea, anorexia, and in one case ataxia.23 Vomiting, tremors, depression, and mydriasis, or dilation of the pupils have also been reported in dogs.24
  • Other clinical signs noted in dogs include hemolytic anemia, the presence of Heinz bodies indicating damaged hemoglobin, reduced hemoglobin content, fragmented blood cells, reticulocytosis and leukocytosis in the blood.23,24
  • Rats and rabbits exposed to naphthalene orally at doses of greater than 500 mg/kg/day developed cataracts.1,25 However, there are differences among species in the formation of cataracts, and susceptibility can vary in the same species between albino and pigmented animals.26
  • Rats and mice were found to be less sensitive to the hemolytic effects of naphthalene than dogs.1

Signs of Toxicity - Humans

  • Inhalation of naphthalene vapor has been associated with headaches, nausea, vomiting and dizziness. Hemolysis, the abnormal breakdown of red blood cells, may occur following ingestion or sufficient dermal exposure to either naphthalene or to naphthalene-treated fabric. People with a deficiency of the enzyme glucose-6-phosphate dehydrogenase, or G-6-PD, are at greater risk of developing hemolytic anemia. G-6-PD deficiency is an inherited trait.10 Infants may develop hemolysis even if not deficient in G-6-PD.11
  • In humans, cataracts and other ocular injury have been reported following acute and chronic occupational exposure to naphthalene.1 However, there are many documented cases of severe acute exposures that did not lead to ocular damage in people. The potential for naphthalene to cause damage to human eyes is uncertain.26
  • Children who ingested mothballs containing naphthalene exhibited diarrhea, vomiting, lethargy, lack of appetite, fever, abdominal pain, painful urination (dysuria), and dark wine-colored or discolored urine.12,23 Upon admission to the hospital, the children were diagnosed with acute hemolytic anemia and hemoglobinuria.23
  • Additional signs of toxicity in children include convulsions and coma. Infants may develop encephalopathy and kernicterus, a form of brain damage, due to the presence of increased levels of methemoglobin, hemoglobin, and bilirubin in their plasma.10,11
  • Liver and kidney damage has been noted following ingestion of naphthalene in humans, but these effects have not been consistently seen in animals.1
  • Always follow label instructions and take steps to minimize exposure. If any exposure occurs, be sure to follow the First Aid instructions on the product label carefully. For additional treatment advice, contact the Poison Control Center at 1-800- 222-1222. If you wish to report an incident to the National Pesticide Information Center, please call 1-800-858-7378.

Chronic Toxicity:

Animals

  • Researchers dosed mice with naphthalene by gavage at 27, 53, or 267 mg/kg daily for 14 days. Mice treated with 267 mg/kg weighed significantly less at the end of the experiment than the other groups. At this dose, males experienced an average decrease of 30% in thymus weight, and females lost significant spleen mass but gained lung mass.19
  • The LD50 for female CD-1 mice was determined to be 353.6 mg/kg based on an eight-day oral exposure.20
  • Mice were dosed with naphthalene by gavage at doses of 5.3, 53.0, and 133.0 mg/kg daily for 90 days. The females given the highest dose experienced decreases in brain, liver, and spleen weights but these effects were not found in the males.19
  • The chronic dietary NOAEL was identified as 100 mg/kg/day and the LOAEL identified as 200 mg/kg/day based on studies in rats.3 See the text box on NOAEL, NOEL, LOAEL, and LOEL.

    NOAEL: No Observable Adverse Effect Level

    NOEL: No Observed Effect Level

    LOAEL: Lowest Observable Adverse Effect Level

    LOEL: Lowest Observed Effect Level

  • Researchers conducted a 4-week inhalation study of naphthalene exposure in rats and identified a NOAEL of 3 ppm or 16 mg/m3 and an inhalation LOAEL of 10 ppm or 52 mg/m3. Signs of toxicity at the LOAEL were nasal lesions.27
  • A 13-week inhalation study in which rats were exposed to naphthalene only through the nose resulted in a NOAEL of 1.0 ppm or 5.2 mg/m3 and a LOAEL of 2.0 ppm or 10.0 mg/m3 because of increased nasal lesions in exposed animals.3
  • Researchers applied naphthalene to the skin of rats for 90 days. They observed atrophy of the seminiferous tubules in males and changes in the liver, kidney, thyroid, and urinary bladder in addition to non-neoplastic lesions in the cervical lymph node in the females. The NOAEL was set at 300 mg/kg/day and the LOAEL at 1000 mg/kg/day. Researchers also noted hyperkeratosis and acanthosis in the skin of exposed female rats.28

Humans

  • Children aged 1.5-36 months have developed hemolytic anemia following dermal exposure to diapers or other clothing stored with naphthalene mothballs or playing in a room used to store naphthalene mothballs.29 Infants developed hemolytic anemia and other complications following inhalation exposure to clothing, blankets, linens, or diapers stored in or around naphthalene.11 See the text box on Exposure.

    Exposure: Effects of naphthalene on human health and the environment depend on how much naphthalene is present and the length and frequency of exposure. Effects also depend on the health of a person and/or certain environmental factors.

  • A 68-year-old woman developed aplastic anemia following inhalation and possibly dermal exposure to naphthalene and paradichlorobenze, which she placed into containers with stored clothing several hours a day for three weeks while working for a clothing resale business. The woman had worked in a room with poor ventilation and no air conditioning during hot weather, which contributed to high vapor concentrations of naphthalene and paradichlorobenzene.30

Endocrine Disruption:

  • Atlantic croaker fish (Micropogonias undulatus) exposed to naphthalene at either 0.5 or 1.0 ppm daily during sexual maturation demonstrated reduced rates of sexual maturity and arrested or reduced egg development. Reduced egg growth was associated with decreased gonadal steroid levels in plasma.31

Carcinogenicity:

Animals

  • Researchers exposed male and female F344 rats to naphthalene vapors at 10, 30, or 60 ppm for 6 hours a day, five days a week for two years. Respiratory epithelial adenomas and olfactory epithelial neuroblastomas occurred in the noses of both males and females. In addition, non-cancerous lesions were also noted in the lungs and noses of the animals.32
  • The National Toxicology Program (NTP) concluded that there was some evidence of carcinogenic activity of naphthalene in female B6C3F1 mice. When these mice were exposed to naphthalene vapors for two years at 10 or 30 ppm, they developed increased rates of bronchiolar and alveolar adenomas. No evidence of carcinogenic activity was found in male mice exposed to the same doses.33
  • Embryonic roundworms (Caenorhabditis elegans) were used in mechanistic studies of the carcinogenicity of naphthalene. The authors hypothesized that the 1,4-naphthoquinone metabolite of naphthalene suppressed normal cell apoptosis.34

Humans

  • The International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) concluded that there was inadequate evidence to evaluate the carcinogenicity of naphthalene to humans, but found sufficient evidence in animals to conclude that naphthalene is carcinogenic. IARC concluded their evaluation by placing naphthalene in Group 2B, possibly carcinogenic to humans.4 See the text box on Cancer.

    Cancer: Government agencies in the United States and abroad have developed programs to evaluate the potential for a chemical to cause cancer. Testing guidelines and classification systems vary. To learn more about the meaning of various cancer classification descriptors listed in this fact sheet, please visit the appropriate reference, or call NPIC.

  • Naphthalene is classified as Group C, possible human carcinogen, by the U.S. EPA. This is based on limited evidence of carcinogenicity following inhalation exposure in animals, and lack of data for humans.35

Reproductive or Teratogenic Effects:

Animals

  • The Agency for Toxic Substances and Disease Registry (ATSDR) reviewed 45 studies that examined the genotoxic potential of naphthalene. Of these studies, 10 found evidence of chromosomal aberrations, gene mutations, recombination abnormalities, or DNA fragmentation.1
  • Pregnant rabbits were fed 400 mg/kg naphthalene during days 6-18 of their gestation. No adverse effects were noted.36
  • Researchers dosed pregnant mice by gavage with 300 mg/kg/day during gestation days 7-14. The mice had reduced weight gain, reduced survival rates, and gave birth to fewer young than the controls, although the young appeared normal.20
  • The difference in results between the rabbit and rat studies above may have been due to the use of different carriers during administration, which could have altered the absorption potential of the compound.1
  • Female rats were dosed with 50, 150, or 450 mg/kg/day of naphthalene during days 6-15 of gestation. Researchers measured decreases in maternal weight gain of 31% and 53% in the mid-dose and high-dose groups, respectively, compared to the controls. No teratogenic effects were noted.1,37

Humans

  • Hemolytic anemia has been reported in infants born to mothers who ingested mothballs or inhaled high concentrations of naphthalene vapors during pregnancy.13,38
  • Naphthalene was detected in human breast milk from women in four urban areas near industrial facilities or chemical manufacturing plants.39 Naphthalene in the nursing infants was not reported and no further information was found.
  • The umbilical cord blood of pregnant women exposed to naphthalene and other volatile organic compounds including methylcyclopentane and tetrachloroethylene contained elevated levels of cytokine-producing T-cells. These data suggest that naphthalene may cross the placenta and affect the immune system of the newborn.40

Fate in the Body:

Absoprtion

  • Researchers applied 3.3 μg/cm2 naphthalene to the skin of rats either alone or in combination with sandy or clay soil. Half of the dermally applied pure naphthalene reached the bloodstream in 2.1 hours. The absorption half-life of naphthalene and clay soil was 2.8 hours, and for naphthalene and sandy soil, 4.6 hours.41 Although adsorption to soil slowed the absorption of naphthalene, the total amount eventually absorbed by the skin was unchanged.41
  • Daily use of baby oil may have increased the absorption of naphthalene through the skin of an infant who wore diapers stored in naphthalene and subsequently developed acute hemolytic anemia.42
  • Once inhaled, naphthalene is thought to enter the body by passive diffusion across the alveolar membranes. Researchers have proposed that naphthalene enters the body through intestinal membrane following oral exposure.1 Evidence suggests that absorption of naphthalene flakes or balls through the intestine can continue for several days after ingestion due to the slow dissolution of the solid form.1
  • No data were found quantifying rates of absorption of naphthalene in humans following either oral or dermal exposure.

Distribution

  • Tissue distribution of naphthalene was examined following dermal exposure in rats. After 48 hours, 0.56 ± 0.14% (mean ± SE) of the dose was found in the treated skin, 0.02 ± 0.01% in the ileum, 0.01 ± 0.00% in the duodenum, and 0.01 ± 0.00% in the kidney. Most of the dose was excreted.41
  • Naphthalene metabolites formed in the liver may reach the lungs via the bloodstream.43
  • Researchers dosed chickens, swine, and dairy cows via oral intubation for 1 day in an acute study and for 31 days in a chronic exposure study to evaluate the distribution of naphthalene in tissues.44 The kidneys of the chronically exposed chickens contained the greatest residual naphthalene and metabolites (2.4 ± 0.22% [mean ± SE] of the original dose), followed by lungs, spleen, liver, and heart.44 Naphthalene residues in the acutely exposed chickens were greatest in the kidneys, fat, lungs, and the liver.44 The kidneys contained 42.9 ± 1.05% of the dose 24 hours after exposure. Kidney concentrations dropped to 8 ± 1.07% in 72 hours.44
  • Chickens orally dosed with naphthalene laid eggs with detectable levels of the naphthalene in both the albumin and yolk. Eggs contained the chemical following acute and chronic exposures.44
  • Swine fat contained the greatest residues in the acute exposure case, but the heart, spleen and liver contained the greatest residues following chronic exposure. The fat of acutely exposed swine contained 3.48 ± 2.16% (mean ± SE) of the total dose after 24 hours and 2.18 ± 1.16% after 72 hours. In the chronically exposed swine, maximum tissue concentrations contained less than 0.2% of the dose.44
  • Researchers examined the liver, kidneys, lungs, heart, spleen, and fat of dairy cows given oral doses of naphthalene. Tissue concentrations were all less than 1% following either chronic or acute exposure.44 Residues of naphthalene were found in the cows' milk.44
  • Naphthalene was detected in six of eight samples of human breast milk taken from women residing in urban areas. Concentrations were not reported.39

Metabolism

  • Naphthalene is metabolically activated by a number of cytochrome P450 enzymes.5 These metabolites are subsequently detoxified by glutathione.9 They may also be metabolized by P450 enzymes and epoxide hydrolase prior to excretion.1
  • The metabolite naphthalene oxide appears to be largely responsible for the toxicity of naphthalene in lung tissue, although other metabolites may also play a role.9
  • Reactive metabolites of naphthalene include 1,2-naphthoquinone, 1,4-naphthoquinone, 1,2-dihydroxy-3,4-epoxy-1,2,3,4- tetrahydronaphthalene, and 1,2-naphthalene oxide. These metabolites may be involved in the toxicity of naphthalene.1
  • Researchers repeatedly exposed mice to naphthalene via inhalation and noted reduced susceptibility of lung tissue to necrosis compared with damage from single exposures.9,45 Researchers concluded that this resistance to injury may be due to increased levels of glutathione synthesis in the lung tissue.9,46
  • Researchers have identified 30 metabolites of naphthalene in the urine of mammals. In rodents, the most common urinary metabolites were 1-naphthol, 2-naphthol, 1-naphthyl glucuronide, 1-naphthyl sulfate, 1,2-dihydro-1,2-dihydroxynaphthalene, 1,2-dihydroxynaphthalene, and 1,2-dihydro-2-hydroxy-1-napahthylglucuronide.5
  • The metabolite 1,2-naphthoquinone reacted with the ocular tissues in rabbits dosed with naphthalene via gavage. The aqueous humor and ciliary body of the eye were affected, and the lenses were damaged from the reaction of 1,2-naphthoquinone with enzyme and structural proteins, as well as coenzymes.47

Excretion

  • Workers were exposed to low levels of naphthalene, naphthols, and methylated naphthalenes during naphthalene distillation processes at a coke plant. Their urine contained the metabolites 1-naphthol, 2-naphthol, and 1,4-naphthoquinone.48
  • Excretion rates of 1-napthol ranged from 0.19 to 0.31 mg/h in factory workers, with maximum excretion occurring 2-3 hours following cessation of the workday exposure. The biological half-life of the metabolite 1-naphthol was estimated at 4 hours. All of the workers in this study were smokers.49
  • Researchers applied radio-labeled naphthalene to the skin of rats as pure compound, adsorbed to sandy soil, and adsorbed to clay soil. Excretion was measured in the feces, urine, and expired air for 48 hours after dosing. Rats treated with pure naphthalene excreted 70.3 ± 4.5% (mean and SE) in their urine, 3.7 ± 0.7% in their feces, and 13.6 ± 3.4% from their lungs. Rats exposed to treated sandy soil excreted 84.7 ± 3.8% of the naphthalene in their urine, 2.4 ± 0.1% in their feces, and 0.9 ± 0.1% from their lungs. Rats exposed to treated clay soil excreted 86.7 ± 0.4% of the radio-labeled dose in their urine, 2.2 ± 0.2% in their feces, and 5.9 ± 2.5% in expired air.41
  • Rats exposed dermally to naphthalene excreted less than 0.5% of the parent compound in their urine. They also excreted the urinary metabolites 2,7-diydroxynaphthalene and 1,2-dihydroxynaphthalene. The metabolites 1,2-naphthoquinone, 1-naphthol, and 2-naphthol were also present in lower concentrations.41
  • Naphthalene was excreted in the milk of a dairy cow within 8 hours of a single oral exposure of 30.69 mg. Other cows were orally exposed to 5.12 mg naphthalene per day for 31 days.44 Upon cessation of exposure, the concentration of naphthalene in the milk dropped rapidly until it was no longer detected on day 33, 2 days after dosing ceased.44
  • Chickens exposed to naphthalene via gavage in a single acute exposure or 31 consecutive daily exposures eliminated 75- 80% of the dose within 48 hours after exposure ceased.44

Medical Tests and Monitoring:

  • Biomarkers of exposure to naphthalene have been reported in the scientific literature.5 Scientists have used high-pressure liquid chromatography and gas chromatography to detect and quantify naphthalene metabolites in urine of laboratory animals and humans.5 This method of testing for exposure to naphthalene has not been well-studied in humans, and its clinical significance is currently unknown.
  • Smoking has been considered a major source of urinary metabolites of naphthalene.5,50 In addition, levels of urinary naphthols may be affected by genetic polymorphisms in enzyme expression.50 Exposure to the insecticide carbaryl may also lead to the formation of 1-naphthol, the metabolite used in workplace exposure monitoring.9
  • Methods also exist for detecting naphthalene and its metabolites in blood, urine, feces, breast milk, and body fat but these tests are not routinely done in a doctor's office.1

Environmental Fate:

Soil

  • Naphthalene may be lost from soil via evaporation, volatilization, and biodegradation. The relative importance of each pathway will vary depending on soil depth and the presence and composition of soil biota, including bacteria, fungi, cyanobacteria, and algae.51,52
  • Soil and sediment bind naphthalene to a moderate extent depending on soil type. Naphthalene will move rapidly through sandy soil. However, increasing organic carbon content will increase naphthalene's sorption to soil.51
  • Naphthalene will evaporate from the soil surface, but this process will decrease with increasing soil depth.51

    The "half-life" is the time required for half of the compound to break down in the environment.

    1 half-life = 50% remaining
    2 half-lives = 25% remaining
    3 half-lives = 12% remaining
    4 half-lives = 6% remaining
    5 half-lives = 3% remaining

    Half-lives can vary widely based on environmental factors. The amount of chemical remaining after a half-life will always depend on the amount of the chemical originally applied. It should be noted that some chemicals may degrade into compounds of toxicological significance.

  • The effective half-lives for volatilization from soil at 1 cm and 10 cm depths were 1.1 days and 14.0 days, respectively. These values were determined through modeling a system that assumed no evaporation.53 See the text box on Half-life.
  • Biodegradation will remove naphthalene from soil, with an estimated half-life of more than 80 days. If the soil is contaminated with other polycyclic aromatic hydrocarbons (PAHs), biodegradation may be much more rapid, with a half-life of a few hours.51
  • Naphthalene is broken down by soil bacteria to naphthalene diol, salicylic acid, and catechol. Some bacteria may utilize naphthalene as their sole carbon source.54 Bacterial oxidation pathways include five metabolites of naphthalene along the degradation pathway: cis-1,2-dihydroxy-1,2-dihydronaphthalene, 1,2-dihydroxynaphthalene, cis-o-hydroxybenzalpyruvic acid, salicylic acid, and catechol, which was subsequently subject to ring cleavage.55
  • Fungal degradation of naphthalene produces naphthalene-1,2-oxide via cytochrome P-450 oxidation. The oxide can be subsequently hydrolyzed to trans-1,2-dihydroxy-1,2-dihydronaphthalene, or alternatively conjugate with glucuronide or sulfate to break down first to 1-naphthol and 2-naphthol and subsequently to 4-hydroxy-1-tetralone.55 Other reported metabolites include naphthalene trans-1,2-dihydrodiol, 1,2-naphthoquinone, and 1,4-naphthoquinone.52

Water

  • Naphthalene will be lost from water by volatilization, sorption, photolysis, and biodegradation.51 The relative contributions of these processes will depend in part on the water's characteristics, including depth, flow rate, and contamination level.51
  • Wastewater from secondary treatment plants was discharged into spreading basins for groundwater recharge. Naphthalene concentrations in the wastewater declined 68-94% during movement across the settling basin.56
  • Researchers modeling volatilization of naphthalene from water concluded that evaporation is affected more by water movement than by air movement above the water's surface.57 Maximum volatilization would be expected to occur under conditions of warm temperatures, shallow waters, and wind. Surface films, aerosol formation, waves, water depth, and current speed will affect volatilization rates.57
  • The presence of algae in the water at concentrations of 1-10 mg chlorophyll a per liter of water increased the photodegradation of naphthalene.58
  • Cyanobacteria and microalgae metabolize naphthalene.52 The primary metabolite was found to be 1-naphthol, with 4-hydroxy- 1-tetralone and cis-naphthalene dihydrodiol as lesser metabolites.59,60

Air

  • Hydroxyl radicals react with naphthalene in the atmosphere at a rate of 2.17 x 10-11 cm3/molecules sec.61 Based on this rate, the half-life of naphthalene in air is less than one day.1 Nitrate may also react with naphthalene in the atmosphere.51
  • Napthalene degraded to form 1-naphthol, 2-naphthol, 1-nitronaphthalene, and 2-nitronaphthalene in the presence of hydroxyl radicals and nitric oxides. Researchers noted that other unidentified metabolites also were present.62

Plants

  • Naphthalene may be deposited on foliage as a result of volatilization from the soil. In addition, naphthalene may be transferred from the plant's roots to the shoots during transpiration.63
  • Naphthalene adsorbed onto the roots of fescue and alfalfa placed in an aqueous solution containing 18.4 mg/L dissolved naphthalene. The amount adsorbed was affected by the lipid contents of the roots and plant age.64
  • The marsh grass Spartina alterniflora contained more residues of naphthalene in its roots than in the new shoot growth when grown in pots containing contaminated soil.65 Concentrations in soil were four orders of magnitude greater than those found in shoots.65 Roots contained up to 43 μg/g and leaves contained up to 0.2 μg/g naphthalene.65
  • PAH residues in cabbage, carrots, leeks, lettuce and endive grown in an industrial area in northern Greece were most similar to the mixture in air, suggesting that gaseous deposition is a major accumulation pathway.66

Indoor

  • Naphthalene has been detected in indoor air. It is a product of combustion when organic materials are burned. Tobacco smoking, cooking, and moth repellents may all be sources of naphthalene indoors.1
  • Clothing stored with naphthalene mothballs may adsorb the naphthalene. The clothing itself then may act as a source of naphthalene in indoor air.67
  • Indoor air concentrations measured in 24 low-income houses in Durham, NC ranged from 334 to 9700 ng/m3 naphthalene, with a mean concentration of 2190 ng/m3 (SD=1870 ng/m3).68 House dust contained a mean concentration of 0.33 μg/g naphthalene (SD=0.85 μg/g).68 Samples were collected from rural and urban areas, and included both smokers' and nonsmokers' households.68

Food Residue

  • No tolerances were found for naphthalene in food.
  • Cabbage, carrots, leeks, lettuce and endives were grown in an industrial area in northern Greece, then washed and peeled. The vegetables contained naphthalene at concentrations ranging from 0.37 μg/kg (cabbage) to 63 μg/kg (endive) dry weight.66
  • Researchers measured naphthalene residues in samples of the food consumed by 24 low-income households in North Carolina. They found 0.01-18.7 μg/kg naphthalene in the adults' food (mean= 3.75 μg/kg) and 0.10-54.9 μg/kg naphthalene in the food of the children (mean= 4.08 μg/kg).68

Ecotoxicity Studies:

Birds

  • The acute oral LD50 in bobwhite quail (Colinus virginianus) was 2690 mg/kg, and the LC50 was estimated as greater than 5620 mg/kg. In both cases, naphthalene was considered by the U.S. EPA as practically nontoxic by ingestion.3

Fish and Aquatic Life

  • The 96-hour LC50 for rainbow trout (Onchorynchus mykiss) exposed to naphthalene was 2.0 mg/L. The NOAEC was 0.86 mg/L. The 96-hour LC50 in bluegill sunfish (Lepomis macrochirus) was 3.2 mg/L and the NOAEC was 1.4 mg/L. Researchers estimated the 96-hour LC50 in fathead minnows (Pimephales promelas) to be 6.6 mg/L. Naphthalene is moderately toxic to these three species of fish.3 Embryo-larval toxicity in fathead minnows was noted at concentrations of 0.85 mg/L, and the NOAEC was 0.62 mg/L.69
  • The 40-day NOAEC for coho salmon (Onchoryncus kisutch) exposed to naphthalene was 0.37 mg/L and the 40-day LOAEC was 0.67 mg/L. No ecotoxicity category was assigned.3 Chronic exposure resulted in reductions in feeding, growth, and survival rates.69
  • Rainbow trout (Salmo gairdneri) exposed to 9.2 ppb naphthalene for 72 hours followed by 72 hours of exposure to clean water contained increasing concentrations of naphthalene through the exposure period. Fish skin contained 2103 ppb naphthalene and 440 ppb of metabolites at 72 hours of exposure. Depuration for 72 hours reduced naphthalene levels to 223 ppb and metabolites to 275 ppb in the skin.70 Epidermal mucus was suggested to be an excretion pathway for some contaminants.70
  • Atlantic croaker fish (Micropogonias undulatus) were exposed to naphthalene at concentrations of 0.5 or 1.0 ppm for up to eight weeks, beginning two weeks before sexual maturity. Fewer exposed fish reached sexual maturity, and many fish that matured exhibited reduced ovarian growth and delayed or arrested oocyte development.31

    EC50: The median effective concentration (EC50) may be reported for sublethal or ambiguously lethal effects. This measure is used in tests involving species such as aquatic invertebrates where death may be difficult to determine. This term is also used if sublethal events are being monitored.

    Newman, M.C.; Unger, M.A. Fundamentals of Ecotoxicology; CRC Press, LLC.: Boca Raton, FL, 2003; p 178.

  • Reports regarding the toxicity of naphthalene to water fleas (Daphnia magna) are variable. The 48-hour EC50 was 1.6 mg/L, the NOAEC for mortality as an endpoint was 0.48 mg/L, and the NOAEC for sublethal effects was greater than 8.8 mg/L. Naphthalene is moderately toxic to water fleas according to the U.S. EPA.3 See the text box on EC50.
  • Pacific oysters (Crassostrea gigas) exposed to naphthalene had a 96-h EC50 of 199 mg/L, which placed naphthalene in the U.S. EPA's practically nontoxic category for this species. In contrast, the 96-h LC50 of grass shrimp (Palaemonetes pugio) was 2.35 mg/L, which is considered moderately toxic.3
  • Naphthalene is considered slightly toxic to the green algae (Chlorella vulgaris) by the U.S. EPA. The 48-hour EC50 was 33 mg/L.3

Terrestrial Invertebrates

  • No data were found on the acute toxicity of naphthalene to the honey bee (Apis mellifera).
  • Soil invertebrates (Folsomia candida, or springtail) were exposed to naphthalene in chronic tests. The NOAEC was 88 μmol/kg soil and the LOAEC was 409 μmol/kg soil based on effects on reproduction and survival.3
  • Earthworms (Enchytaeus crypticus) were also studied following chronic exposure to naphthalene in soil. The NOAEC was 220 μmol/kg soil and the LOAEC was 2045 μmol/kg soil.3
  • Pseudomonas putida bacteria were added to soil that had been mixed with naphthalene 68 days earlier, and soil that had just been mixed with naphthalene. The bacteria metabolized the freshly treated soil much more rapidly, suggesting that aging of naphthalene in the soil matrix made it unavailable for biodegradation.71 In both cases, most of the degradation occurred in the first 3 days.71
  • In the same study, earthworms (Eisenia foetida) incorporated 2.3% of radioactive naphthalene from freshly treated soil into their bodies, but only 1.4% of radioactive naphthalene that had been added to soil 68 days earlier.71

Regulatory Guidelines:

  • The U.S. EPA set the chronic reference dose or cRfD for naphthalene at 0.1 mg/kg/day and acute reference dose or aRfD at 0.4 mg/kg/day for naphthalene based on studies of exposure in rats.3 See the text box on Reference Dose (RfD).

    Reference Dose (RfD): The RfD is an estimate of the quantity of chemical that a person could be exposed to every day for the rest of their life with no appreciable risk of adverse health effects. The reference dose is typically measured in milligrams (mg) of chemical per kilogram (kg) of body weight per day.

    U.S. Environmental Protection Agency, Technology Transfer Network, Air Toxics Health Effects Glossary, 2009. http://www.epa.gov/ttnatw01/hlthef/hapglossaryrev.html#RfD

  • The U.S. EPA classifies naphthalene as Group C, possible human carcinogen.35 The IARC classification is Group 2B, possibly carcinogenic to humans.4 See the text box on Cancer.
  • The Recommended Exposure Limit (REL) for a time-weighted average (TWA) is 10 ppm or 50 mg/m3, and the Short Term Exposure Limit (STEL) is 15 ppm or 75 mg/m3.72
  • The Permissible Exposure Limit (PEL) is 10 ppm or 50 mg/m3 for naphthalene.72
Date Reviewed: November 2010

Please cite as: Gervais, J.; Luukinen, B.; Buhl, K.; Stone, D. 2010. Naphthalene Technical Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/naphtech.pdf.

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