Naphthalene and Cancer: Mothballs and Risk

Naphthalene is a white crystalline solid with a sharp, unmistakable smell. Its CAS number is 91-20-3. It is a polycyclic aromatic hydrocarbon, or PAH — a ring-shaped carbon compound of the same broad family found in soot and smoke. It melts at about 80 °C, and it turns from solid straight to vapor at room temperature. That is why an old mothball shrinks and disappears in a closed drawer.

Most naphthalene never reaches a household. It is made from coal tar, which is roughly 10 percent naphthalene, or from petroleum. Its biggest job is as a raw material for phthalic anhydride. That in turn goes into plastic softeners, medicines and bug sprays. A 1999 breakdown of US use looks like this:

• Phthalic anhydride: 59 percent.
• Surfactants and dispersants: 21 percent.
• Insecticides: 11 percent.
• Moth repellents and other uses: 9 percent.

US imports were about 40.1 million pounds in 2017.

For families, the urgent risk from naphthalene is not a tumor years later. It is acute poisoning, usually when a small child swallows a mothball. Naphthalene destroys or changes red blood cells so they cannot carry oxygen, which is called hemolysis.

Stomach symptoms may not start until two days after contact. They include belly pain, nausea, vomiting and diarrhea, sometimes with fever. Later signs can include jaundice (yellow skin), dark or scanty urine, a fast heartbeat, shortness of breath, confusion, seizures and coma.

Some people are far more open to harm. They have a trait called G6PD deficiency, short for glucose-6-phosphate dehydrogenase deficiency. It is passed down in families. It leaves red blood cells poorly defended against this kind of stress. EPA's review notes that infants who lack the enzyme are thought to be especially sensitive.

Get help now. For a swallowed mothball, an inhaled fume exposure with symptoms, or any suspected poisoning, call 911 or the national Poison Help line at 1-800-222-1222. That line reaches a poison control center from anywhere in the United States, at any hour, at no cost. Have the person's age and weight, the product name, the time of exposure, and the amount ready if you can.

Not every mothball contains naphthalene. Many modern ones use paradichlorobenzene instead. Both are solids that turn to vapor, and both are toxic if swallowed, but they are different substances with different profiles. If a poisoning is suspected, the exact product name matters more than the word "mothball," so keep the packaging.

The federal cancer listing rests on animal data, not on people. Rats that breathed naphthalene developed nasal tumors of two rare kinds. One is olfactory epithelial neuroblastoma. That is a fast-growing tumor of the smell-sensing lining of the nose. The other is respiratory epithelial adenoma, which is also rare. Neither had been seen in the control animals for these studies. As of 2010, no nasal tumors had appeared in 1,297 male or 1,247 female control rats. In the exposed animals the neuroblastoma rate rose as the exposure level rose. Female mice that breathed naphthalene developed more benign lung tumors.

The human data are thin. Two case series exist. One reported cancer of the larynx and other sites in German workers exposed at work. The other reported colorectal cancer in a group in Africa who used a naphthalene compound as a medicine. Neither can settle the question. Both IARC and NTP call the human evidence inadequate.

IARC reviewed naphthalene in Volume 82, published in 2002. It found the human evidence inadequate. It found the animal evidence sufficient. The overall call was this: "Naphthalene is possibly carcinogenic to humans (Group 2B)."

The working group also described the likely mechanism, and it is the reason for the caution in that wording. Mice break naphthalene down quickly into reactive products inside the lung. Those products kill cells, the lung replaces them fast, and rapid cell turnover can end in tumors. Rats do not get lung tumors, which fits the same mechanism. Then comes the key measurement: the top rate of this breakdown in human lung tissue is roughly 10 to 100 times lower than in mice.

The National Toxicology Program uses two levels. It puts naphthalene at the lower one: "reasonably anticipated to be a human carcinogen." It was first listed there in 2004, in the Eleventh Report on Carcinogens. It is still there in the Fifteenth.

EPA reached a third verdict on the same data. Under its 1986 guidelines, it puts naphthalene in Group C, a possible human carcinogen. The reason given was thin human data plus limited animal evidence from breathing it.

• OSHA permissible exposure limit: 10 parts per million, or 50 mg/m3, averaged over a shift. This is the enforceable workplace rule.
• NIOSH recommended limit: 10 ppm as a shift average, with a short-term ceiling of 15 ppm.
• EPA inhalation reference concentration: 0.003 mg/m3. This is the level EPA judges a person could breathe daily for life with no likely harm. It came from nose tissue changes in a long mouse study. EPA applied an uncertainty factor of 3,000.
• EPA oral reference dose: 0.02 mg per kilogram of body weight per day.

Look at the gap between the work limit and EPA's lifetime figure. They answer different questions. One covers a trained adult for 8 hours a day, with monitoring. The other covers everyone, infants included, around the clock.

It is also a listed hazardous air pollutant under the Clean Air Act. The spill reporting amount under CERCLA is 100 pounds. Its hazardous waste codes include U165, K001 and K087.

Workplace exposure is the highest anyone gets. A national survey run from 1981 to 1983 estimated that 112,700 US workers were potentially exposed. The classic settings are coal tar distilling, wood preserving, tire plants and dye work. The controls are the usual ones. Those are ventilation, enclosed handling, and respirators where the vapor cannot be engineered away.

At home the steps are simple and mostly about air and storage. Use cedar, sealed bags or freezing for wool storage instead of mothballs. Keep any mothball product in its original container, out of reach and out of sight of children. Never scatter mothballs loose in a closet, attic, garden or crawl space. The label directions exist because the vapor is the pesticide. Air out stored clothing and bedding outdoors before use. This chemical also forms whenever things burn. So not smoking, and staying clear of idling engines, cuts this exposure along with many worse ones. Our page on tobacco and cancer covers that trade.

For the wider question of how a "possible" listing should change your behavior, see what a carcinogen is and hazard versus risk.

• NTP Report on Carcinogens: naphthalene
• IARC Monographs, volume 82
• EPA IRIS: naphthalene
• MedlinePlus: naphthalene poisoning
• OSHA table Z 1