Mothballs are small, solid pieces of pesticide designed to protect stored items from fabric-eating pests, particularly clothes moth larvae. These blocks are characterized by a distinct, pungent odor that permeates any enclosed space. This strong smell results from the active chemical ingredient slowly turning into a gas, which is the mechanism used to kill or repel insects. Understanding this chemistry reveals why these products are both effective pest control tools and a source of health concerns.
The Two Primary Chemical Compositions
A mothball consists almost entirely of one of two potent chemical compounds. Historically, the primary ingredient was Naphthalene, an aromatic hydrocarbon derived from coal tar. This compound is a white crystalline solid with a particularly strong odor. Naphthalene-based mothballs are highly flammable, which has led to a decline in their use in many modern formulations.
The more common active ingredient in contemporary mothballs is Paradichlorobenzene, also known as 1,4-Dichlorobenzene or PDB. PDB is also a white crystalline solid and is generally considered less toxic and less flammable than Naphthalene, though it is still regulated as a pesticide. A single mothball product is typically made of one of these two compounds.
The Mechanism of Vapor Action
The effectiveness of a mothball relies on a unique physical process called sublimation. Sublimation is the transition where a substance moves directly from a solid state into a gaseous state, bypassing the liquid phase entirely. Both Naphthalene and Paradichlorobenzene have weak intermolecular forces that allow them to sublimate easily at room temperature. This slow, continuous process releases the chemical’s toxic vapor into the surrounding environment.
The resulting fumes act as the insecticide, killing the larvae and eggs or aggressively repelling the adult moths. For the mothball to work properly, the toxic vapor must be highly concentrated. Manufacturer instructions require them to be used in tightly sealed containers. If they are simply placed in an open closet, the vapors dissipate too quickly to be an effective pesticide.
Health and Safety Implications
The same toxic vapor that kills insects poses risks to the health of humans and pets. Since the aroma of mothballs is the chemical itself, smelling them means inhaling the active pesticide. Chronic exposure to these vapors, especially in poorly ventilated indoor areas, can lead to immediate symptoms like headaches, nausea, dizziness, and irritation of the eyes and throat.
Ingestion presents a much more severe and acute danger, particularly if the mothballs are mistaken for candy by children or consumed by pets. Naphthalene is especially concerning because it can cause hemolytic anemia, a condition where red blood cells are destroyed faster than they can be made. This risk is significantly heightened in individuals with a genetic condition called glucose-6-phosphate dehydrogenase (G6PD) deficiency. Paradichlorobenzene exposure is linked to potential damage to the liver, kidneys, and central nervous system.
Safer Alternatives for Garment Storage
Given the toxicity concerns, many people look to non-chemical solutions to protect their stored garments. Physical barriers and meticulous cleanliness are often the most reliable defense against fabric pests. Thoroughly cleaning clothing before storage and placing items in airtight containers or vacuum-sealed bags prevents pests from accessing the fibers in the first place.
For a natural deterrent, several plant-based materials can be used as alternatives to chemical mothballs. Eastern red cedar wood contains oils that naturally repel moths and other insects, and products like cedar blocks or chips are widely available. Similarly, sachets filled with dried herbs such as lavender, rosemary, or cloves emit fragrances that are pleasant to people but act as effective, non-toxic deterrents to pests. These natural options require periodic refreshing, such as sanding cedar or replacing sachets, to maintain their potency.