What is Naphthalene?
Naphthalene is a white solid. It evaporates easily and has a strong smell. Fossil fuels, such as petroleum and coal, naturally contain naphthalene. Burning tobacco or wood also produces naphthalene. 1-Methylnaphthalene is a naphthalene-related compound which is also called alpha methylnaphthalene. It is a clear liquid. Another naphthalene-related compound, 2-methylnaphthalene, is also called beta methylnaphthalene. It is a solid like naphthalene.
Uses of Naphthalene
The major products made from naphthalene are moth repellents, in the form of mothballs or crystals, and toilet deodorant blocks. It is also used for making dyes, resins, leather tanning agents, and the insecticide, carbaryl. 1-Methylnaphthalene and 2-methylnaphthalene are used to make other chemicals such as dyes, resins, and, for 2-methylnaphthalene, vitamin K. Along with naphthalene, they are present in cigarette smoke, wood smoke, tar, and asphalt, and at some hazardous waste sites.
Structural diagram: National Institutes of Health
Naphthalene in the Environment
Naphthalene enters the environment from industrial uses, from its use as a moth repellent, from the burning of wood or tobacco, and from accidental spills. Naphthalene at hazardous waste sites and landfills can dissolve in water. Naphthalene can become weakly attached to soil or pass through the soil into underground water.
Most of the naphthalene entering the environment is from the burning of woods and fossil fuels in the home. The second greatest release of naphthalene is through the use of moth repellents. Only about 10% of the naphthalene is from coal production and distillation, and less than 1% is attributable to naphthalene production losses. Cigarette smoking also releases small amounts of naphthalene.
Naphthalene evaporates easily. That is why you can smell mothballs. In the air, the moisture and sunlight make it break down, often within 1 day. The naphthalene can change to 1-naphthol or 2-naphthol. These chemicals have some of the toxic properties of naphthalene. Some naphthalene will dissolve in water in rivers, lakes, or wells. Naphthalene in water is destroyed by bacteria or evaporates into the air. Most of the naphthalene will be gone from rivers or lakes within 2 weeks. Naphthalene breaks down faster in water containing other pollutants, such as petroleum products.
Naphthalene binds weakly to soils and sediment. It easily passes through sandy soils to reach underground water. In soil, some microorganisms break down naphthalene. When near the surface of the soil, it will evaporate into air. Healthy soil will allow the growth of microorganisms which break down most of the naphthalene in 1 to 3 months. If the soil has few microorganisms, it will take about twice as long.
Naphthalene does not accumulate in the flesh of animals and fish that you might eat. If dairy cows are exposed to naphthalene, some naphthalene will be in their milk; if laying hens are exposed, some naphthalene will be in their eggs. Naphthalene and the methylnaphthalenes have been found in very small amounts in some samples of fish and shellfish from polluted waters.
Exposure to Naphthalene
You can be exposed to naphthalene in your home through the use of mothballs or by breathing air that contains tobacco smoke.
You can also be exposed to naphthalene if you work in an industry such as coal-tar production, wood preserving, tanning, or ink and dye production.
Health Effects of Naphthalene
The NIOSH Pocket Guide to Chemical Hazards  lists the following symptoms related to naphthalene exposure:
Nausea, vomiting, abdominal pain
Hematuria, renal shutdown
Optical neuritis, corneal damage
When checked on 12 December 2015, US EPA's IRIS Database stated that while naphthalene is a possible human carcinogen, its carcinogenic potential cannot be assessed.  When checked on 7 April 2015, the International Agency for Research on Cancer had determined that naphthalene is possibly carcinogenic to humans. 
 NIOSH Pocket Guide to Chemical Hazards, published by the National Institute for Occupational Safety and Health