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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).
Use of mothballs when clothing is stored out-of-season has given rise to the colloquial usage of the terms "mothballed" and "put into mothballs" to refer to anything which is put into storage or whose operation is suspended.
Older mothballs consisted primarily of naphthalene, but due to naphthalene's flammability, many modern mothball formulations instead use 1,4-dichlorobenzene, which may be somewhat less flammable. The latter chemical is also variously labeled as para-dichlorobenzene, p-dichlorobenzene, pDCB, or PDB, making it harder to identify unless all these synonyms are known to a potential purchaser. Both of these ingredients have the strong, pungent, sickly-sweet odor often associated with mothballs.
Naphthalene and 1,4-dichlorobenzene should not be mixed, as they react chemically to produce a liquid which may cause damage to items being preserved.
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.
For either of the previous insecticidal chemicals to be effective, they need to be placed with the clothing in a sealed container so the vapors can build up and kill the moths. In a sealed atmosphere like this, the vapors are not as harmful to people because they are relatively contained. The main exposures would occur when filling or opening the containers, or from wearing clothes immediately after opening (especially a problem for infants). A possible solution is to open the containers outside and let the clothes hang and air out for a day before wearing, though this practice will also expose the clothes to any moths that are flying about, risking re-infestation.
In addition to repelling or killing insects such as moths and silverfish, mothballs have been suggested for use as a stovepipe cleaner, a snake repellent, and to keep away mice or other pests. However, a major concern about the use of mothballs as a snake, mouse, or animal repellent or poison is their easy access to children, pets, and beneficial animals. Leaving them in a garden or in a living space unprotected makes it very easy for unintended victims such as children and pets to gain access to them. Mothballs are highly toxic when ingested (they have a sweet odor and taste, making this more likely), and will cause serious illness or death. In addition to this, using a large quantity of mothballs in a basement or a living space may cause serious respiratory problems in people living in the space.
Mothballs have been promoted as a squirrel repellent, and are an ingredient in some commercial repellent products. They are generally ineffective, and are no substitute for physical measures to exclude squirrels from building interiors.
Older-formula mothballs have also been used by drag racers to enhance the octane rating of fuel by dissolving the mothballs in some of the fuel and filtering out the remains with a filter paper. In the Mythbusters episode "Scuba Diver, Car Capers", it was shown to be "plausible" that adding mothballs to a car's fuel tank would increase its horsepower.
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 to be a carcinogen.
Exposure to naphthalene mothballs can cause haemolysis (anemia) in people with Glucose-6-phosphate dehydrogenase deficiency. The International Agency for Research on Cancer (IARC) classifies naphthalene as possibly carcinogenic to humans and animals  (See also Group 2B). The IARC also points out that acute exposure causes cataracts in humans, rats, rabbits, and mice. Chronic exposure to naphthalene vapors is reported to cause cataracts and retinal hemorrhage. Under California's Proposition 65, naphthalene is listed as "known to the State to cause cancer".
Toxin-free alternatives to control clothes moths include freezing, dry cleaning, washing in hot water, or thorough vacuum cleaning. Camphor is no longer recommended as a moth repellent, due to its toxicity.
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