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. 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 synonyms 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.
In addition to repelling or killing insects such as moths and silverfish, mothballs can be used as a repellent to keep bats from establishing themselves in attics. Mothballs may repel snakes or mice. Mothballs have the potential to be a hazard to pets, livestock or children that may come into contact with them. Recent issues with imported and unregulated mothballs from China have also become a huge concern as there is no way to know what chemicals were used during the process. However, mothball labels instruct proper use in a sealed container. 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.
Some have claimed mothballs are ineffective as snake repellents or as rodent repellents. Mothballs, however, continue to be advertised as a squirrel repellent and are an ingredient in some commercial repellent products. Some websites claim they are ineffective as a deterrent to prevent squirrels from nesting in building interiors.
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#Pest control, alternatives to mothballs to control clothes moths include dry cleaning, freezing, thorough vacuuming, or 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.
- Urinal deodorizer block — some types contain similar chemicals to mothballs
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