Mothballs are small, white pellets used to protect stored clothing and materials from pests like moths and their larvae. They are designed to slowly disappear over time, leading many to believe they are simply “evaporating.” However, the process by which a mothball shrinks and vanishes is a specific physical change that does not involve a liquid state. The disappearance of a mothball is a direct transition from a solid to a gas, setting it apart from how liquids dissipate.
The Process of Sublimation
The physical change that causes mothballs to vanish is known as sublimation. Sublimation is the process where a substance converts directly from a solid into a gas or vapor, bypassing the liquid phase entirely. This is unlike evaporation, which requires a substance to first melt into a liquid before turning into a gas. For example, ice melts into water before it can evaporate into steam.
The molecules within the mothball, even at room temperature, possess enough energy to break free from the solid structure and release into the surrounding air as a gas. This process is similar to dry ice (solid carbon dioxide), which turns straight into a visible vapor without melting. This continuous release of vapor gives mothballs their characteristic, pungent odor. This gas is the active ingredient used to repel or kill pests.
Chemical Composition of Mothballs
The ability of a mothball to sublime at standard room temperature is entirely due to its specialized chemical composition. Mothballs are typically made of either naphthalene or paradichlorobenzene (PDB). Both are crystalline organic solids that possess a high vapor pressure. This pressure allows them to release gas molecules into the air without needing to reach their melting point.
Older mothball formulations primarily used naphthalene, but modern products frequently contain PDB. The choice of chemical directly influences the rate at which the solid converts to a gas and the level of toxicity in the resulting vapor. Both chemicals are classified as pesticides by the U.S. Environmental Protection Agency (EPA) because the gas they release is toxic to insects. This high vapor pressure allows the chemical to function as a fumigant, protecting fabrics in a sealed storage container.
Safety Concerns Regarding the Vapors
The pungent odor signals that the chemical pesticide is being inhaled, raising safety concerns for humans and pets. Since the active ingredients are designed to turn into a toxic gas, exposure to high concentrations can irritate the eyes and lungs, causing headaches, dizziness, and nausea. Naphthalene is considered the more toxic compound and is associated with hemolytic anemia (destruction of red blood cells).
Children and pets are particularly vulnerable, both from inhaling the heavier-than-air vapors and from accidental ingestion. A single mothball, if swallowed by a small child, can cause serious poisoning. Due to the toxicity of the released gas, mothballs should only be used in tightly sealed containers as directed by the label. Improper use, such as scattering them in open areas, allows concentrated pesticide vapors to contaminate living spaces and pose a health risk.