Mothballs are small balls of chemical pesticide and deodorant, sometimes used when storing clothing and other articles susceptible to damage from mold or moth larvae (especially clothes moths like Tineola bisselliella).
Older mothballs consisted primarily of naphthalene, but due to naphthalene's flammability, many modern mothball formulations instead use 1,4-dichlorobenzene. The latter formulation may be somewhat less flammable, although both chemicals have the same NFPA 704 rating for flammability. The latter chemical is also variously labeled as para-dichlorobenzene, p-dichlorobenzene, pDCB, or PDB, making it harder to identify unless all these acronyms are known to a potential purchaser. Both of these formulations have the strong, pungent, sickly-sweet odor often associated with mothballs. Both naphthalene and 1,4-dichlorobenzene undergo sublimation, meaning that they evaporate from a solid state directly into a gas; this gas is toxic to moths and moth larvae.
Naphthalene and 1,4-dichlorobenzene should not be used together because the mixture may cause damage to items being preserved.
Due to the health risks of 1,4-dichlorobenzene, and flammability of naphthalene, other substances like camphor are sometimes used.
Mothballs are stored in air-tight bags made of a non-reactive plastic such as polyethylene or polypropylene (other plastics may cause a reaction). The clothing protected by their presence must also be enclosed in one of these plastics and must be sealed within it. Otherwise the vapors will tend to escape and will lose the effectiveness. Manufacturer's instructions regularly warn against using mothballs for any purpose other than those specified by the packaging, as such uses are not only harmful and noxious, they are also frequently considered illegal.
Although occasionally used as snake repellent, mothballs are not effective for this purpose. Use as a rodent, squirrel, and bat repellent is also largely ineffective, is illegal in many areas, and tends to cause more annoyance to humans than to the target pest. Mothballs, however, continue to be advertised as squirrel repellent and are an ingredient in some commercial vermin and snake repellent products. Evidence of their effectiveness as a squirrel repellent is lacking.
The US Department of Health and Human Services (DHHS) has determined that 1,4-dichlorobenzene "may reasonably be anticipated to be a carcinogen". This has been indicated by animal studies, although a full-scale human study has not been done. The National Toxicology Program (NTP), the International Agency for Research on Cancer (IARC) and the state of California consider 1,4-dichlorobenzene a carcinogen.
Exposure to naphthalene mothballs can cause acute hemolysis (anemia) in people with glucose-6-phosphate dehydrogenase deficiency. IARC classifies naphthalene as possibly carcinogenic to humans and other animals (see also Group 2B). IARC points out that acute exposure causes cataracts in humans, rats, rabbits, and mice. Chronic exposure to naphthalene vapors is reported to also cause cataracts and retinal hemorrhage. Under California's Proposition 65, naphthalene is listed as "known to the State to cause cancer".
As discussed in more detail at Tineola bisselliella, alternatives to mothballs to control clothes moths include dry cleaning, freezing, thorough vacuuming, and washing in hot water. Camphor is also used as a moth repellent, particularly in China. Unlike naphthalene and dichlorobenzene, camphor has medicinal applications and is not regarded as a carcinogen, though it is toxic in large doses.
Pheromone traps are also an effective tool used when attempting to protect valuable clothing.
In popular culture
The term "mothball" also has a metaphorical usage: it means "to stop work on an idea, plan, or job, but leaving it in such a way that you can start on it again at some point in the future".
- Urinal deodorizer block – some types contain similar chemicals to mothballs
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The dictionary definition of mothball at Wiktionary