The short answer
Naphthalene is the chemical in mothballs, also formed by burning. It is classified as possibly carcinogenic based on animal studies. Ventilating stored items and avoiding mothball fumes reduce exposure.
Naphthalene is classified as a possible human carcinogen (IARC Group 2B).
People are mainly exposed by breathing fumes from mothballs, smoke, or exhaust.
It is most strongly linked to a possible link based mainly on animal studies.
A carcinogen classification describes hazard — whether something can cause cancer — not your personal risk at a given exposure.
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The full explanation.
The simple version
Naphthalene is the strong-smelling chemical in old-fashioned mothballs. It also forms when things burn, so it is in tobacco smoke and vehicle exhaust. It is classified as possibly carcinogenic, based mainly on animal studies.
What naphthalene is
Naphthalene is a white solid that easily turns to vapor, best known from mothballs and some deodorizer blocks. It also forms during combustion. IARC classifies it as possibly carcinogenic (Group 2B), based largely on studies in animals.
How people are exposed
Common ways people come into contact with it:
- Breathing fumes from mothballs and deodorizer blocks
- Tobacco smoke and vehicle exhaust
- Some workplaces that produce or use it
The cancer connection
Naphthalene's classification is based mainly on respiratory tumors in animal studies; human cancer evidence is limited.
The International Agency for Research on Cancer (IARC), the cancer arm of the World Health Organization, places naphthalene in Group 2B, possibly carcinogenic to humans — the weakest of the "maybe" categories, often based mainly on animal studies. In the United States, the National Toxicology Program's Report on Carcinogens lists it as reasonably anticipated to be a human carcinogen.
Hazard is not the same as risk
It helps to separate two ideas that are easy to mix up: hazard and risk. When an agency lists naphthalene as a carcinogen, it is making a statement about hazard — whether the substance is capable of causing cancer under some conditions. It is not, by itself, a statement about your personal risk, which depends on how much you are exposed to, for how long, and other factors. Two substances in the same group can carry very different real-world risks. The label answers "can it cause cancer?" — not "how likely is it to cause cancer for me?"
How to lower your exposure
- Use mothball alternatives like cedar, and store treated items in ventilated areas
- Avoid breathing mothball fumes and keep them away from children
- Do not smoke, and reduce exposure to exhaust
If you are looking at your overall cancer risk, small, steady steps add up. See our overview of cancer prevention and what raises cancer risk to put any single exposure in context.
The bottom line
Naphthalene is a possible human carcinogen (IARC Group 2B). The most important thing you can do is understand where exposure comes from and take reasonable steps to reduce it, without losing sleep over a single label. Focus your energy on the biggest, most controllable risks in your own life.
Words to know
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Common questions
▸Does naphthalene cause cancer?
Possibly. Naphthalene is classified as a possible human carcinogen, usually based mainly on animal studies. This is a signal for more research, not a confirmed human cause of cancer.
▸How are people exposed to naphthalene?
Most exposure happens by breathing fumes from mothballs, smoke, or exhaust.
▸Which cancers are linked to naphthalene?
It is most strongly linked to a possible link based mainly on animal studies.
▸How can I reduce my exposure to naphthalene?
The main steps are ventilating stored items and avoiding mothball fumes.
▸Does a carcinogen label mean I will get cancer?
No. A classification is about hazard — whether naphthalene can cause cancer under some conditions — not a prediction that any one exposed person will develop cancer. Your actual risk depends on the amount and length of exposure and other factors.
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