Foam cups, often generically referred to as Styrofoam, are formally known as Expanded Polystyrene (EPS). This plastic foam is widely used for disposable food service items due to its low cost, light weight, and excellent thermal insulation properties. A common concern for consumers is whether these cups pose a health risk when holding hot beverages. This article examines the material science and regulatory status of EPS to provide an objective look at the science behind the safety claims.
The Chemical Composition of Polystyrene
The foam cup is constructed from polystyrene, a type of plastic that is a long-chain polymer. This polymer is built from thousands of smaller, repeating units called monomers, specifically the aromatic hydrocarbon known as styrene. The solid, stable polystyrene polymer is fundamentally different from the liquid styrene monomer used in its creation. The manufacturing process involves taking tiny beads of polystyrene and expanding them, usually with steam, which transforms them into the lightweight, air-filled structure of EPS. The final foam product is approximately 95% air, which accounts for its insulating quality.
The primary concern regarding the material’s safety is the presence of residual, unreacted styrene monomer. Although the manufacturing process is designed to convert almost all the styrene monomer into the stable polystyrene polymer, trace amounts of the monomer remain trapped within the solid matrix. This residual styrene is capable of migrating from the cup material into the liquid contents.
How Temperature Affects Chemical Migration
The transfer of chemical components from the cup into the beverage, a process known as migration or leaching, is governed by the principles of physics and chemical kinetics. High temperatures dramatically increase the kinetic energy within both the plastic polymer and the liquid. This elevated energy causes the polymer chains to vibrate more intensely, which loosens the molecular bonds trapping the residual styrene monomer. The higher energy allows the styrene molecules to more easily break free from the polymer matrix and diffuse into the surrounding liquid. Studies have consistently shown that the migration of styrene increases considerably as the temperature of the contents rises. For hot beverages typically served between 160 and 185 degrees Fahrenheit, this effect is pronounced, leading to higher concentrations of the monomer in the drink compared to if the cup held a cold liquid.
Furthermore, the chemical nature of the beverage itself can influence the rate of leaching. Styrene is a non-polar molecule, which means it tends to dissolve more readily in non-polar substances like fats and oils. Therefore, beverages with a higher fat content, such as hot milk or certain creamy soups, can exhibit a greater migration of styrene than plain water or black coffee. Similarly, the presence of acids, such as those found in tea with lemon, or alcohol can slightly enhance the migration rates.
Official Health Classifications of Styrene
The U.S. Food and Drug Administration (FDA) is responsible for determining the safety of materials that come into contact with food. The FDA has approved polystyrene for use in disposable beverage cups based on extensive testing that evaluates the maximum amount of residual styrene that can migrate out of the material under intended use conditions. The FDA concludes that the extremely small, trace amounts of styrene migrating from polystyrene cups do not pose a health risk to the general public. Scientific data indicates the estimated daily exposure to styrene from these food-contact materials is vastly lower than the established safety limits. The acceptable daily intake for styrene is calculated to be thousands of times higher than the actual daily intake from polystyrene products.
In contrast, other scientific organizations classify the styrene monomer based on its potential to cause harm at higher exposure levels. The International Agency for Research on Cancer (IARC), an arm of the World Health Organization, has classified styrene as a Group 2A substance, meaning it is “probably carcinogenic to humans.” This determination is based primarily on evidence from animal studies and limited human studies involving workers who experience high-level, chronic occupational exposure. This classification reflects a hazard potential, not the actual risk posed by the minute consumer exposure from a beverage cup.
Consumer Guidance for Reducing Exposure
For individuals seeking to minimize their exposure to styrene from disposable containers, several practical measures can be easily adopted. The most direct recommendation is to select alternative materials for hot beverages whenever possible. Choosing ceramic mugs, glass vessels, or stainless steel tumblers completely eliminates the possibility of chemical migration from polystyrene cups.
If using foam cups cannot be avoided, consumers should:
- Limit the amount of time the hot liquid remains in contact with the material, as migration increases over time.
- Strictly avoid microwaving any polystyrene containers, as the high heat generated can cause the plastic to deform and dramatically accelerate the release of chemicals.
- Ask for non-polystyrene options when ordering food to-go or bring a reusable container.
Products made from polystyrene are usually identifiable by the number 6 within the recycling symbol.