Naphthalene and Cancer: Mothballs and Risk

This page is for households that use or find mothballs, and for people who handle naphthalene at work. It covers poisoning first, then what is known about cancer.

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 has an odor like mothballs.

Most naphthalene never reaches a household. IARC says it is made from coal tar and petroleum, and used mainly as a raw material for phthalic anhydride, naphthalene sulfonates and dyes. Use as a moth repellent is smaller.

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 2 days after contact. They include belly pain, nausea, vomiting and diarrhea. Later signs can include jaundice (yellow skin), confusion and seizures.

Some people are far more easily harmed. They have a trait called G6PD deficiency, short for glucose-6-phosphate dehydrogenase deficiency. People with G6PD deficiency are more vulnerable to the effects of naphthalene.

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, weight and condition, the product name, the time of exposure, and the amount ready if you can. Bring the container to the emergency room if possible.

If a poisoning is suspected, the exact product name matters. Keep the packaging and tell poison control what it says.

The cancer classifications rest 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 tumor of the smell-sensing lining of the nose. The other is respiratory epithelial adenoma, which is also rare. IARC describes both as rare in untreated rats. Female mice that breathed naphthalene developed more benign lung tumors.

The human data are thin. IARC had only two case series, and said no inference about cancer could be drawn from them. It calls 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 (NTP) lists naphthalene in its Report on Carcinogens as "reasonably anticipated to be a human carcinogen," the lower of its two listing levels.

• OSHA permissible exposure limit: 10 parts per million, or 50 mg/m3. This is the legally enforceable workplace limit, though NTP notes such limits may not reflect newer studies.
• NIOSH recommended limit: 10 ppm as a shift average, with a short-term limit of 15 ppm.

These are workplace limits for adults on the job. They are not safe-level promises for children or for round-the-clock home exposure.

People who handle naphthalene at work are covered by the limits above.

At home, exposure comes from mothball vapor and from smoke. Avoiding mothball fumes and not smoking lower it. Our page on tobacco and cancer covers smoking in more depth. For a child who may have swallowed a mothball, use the poison steps above.

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: naphthalene (summary)
• MedlinePlus: naphthalene poisoning
• CDC NIOSH Pocket Guide: naphthalene