Plastic microwave covers are widely used to prevent food splatter and help retain moisture, promoting even heating of leftovers. However, the convenience of using these covers often raises questions about their safety due to their material composition and the effects of microwave energy. The primary concern is not the microwave itself, as the plastic generally does not absorb the radio waves, but rather the heat transferred from the food to the cover. This heat can cause certain chemical components within the plastic to migrate into the food. Understanding this process and knowing how to identify safer materials is important for minimizing exposure risks.
Understanding the Risk of Chemical Migration
The safety risk associated with plastic covers centers on chemical migration, often called leaching. When food is heated in a microwave, the food’s temperature rapidly increases, transferring heat to the plastic cover. Elevated temperatures cause polymer chains within the plastic to move more freely, creating pathways that allow smaller, unbound chemical additives to escape.
Additives like plasticizers, stabilizers, and certain monomers are mixed in to give the plastic specific properties, such as flexibility or durability, but are not chemically bonded to the main material. The movement of these non-bonded compounds from the plastic to the food is known as diffusion.
Migration is significantly increased when the plastic is exposed to foods high in fat or oil. Many chemical additives, including certain phthalates and bisphenols, are lipophilic, meaning they are soluble in fats. High-fat foods act as efficient solvents, drawing these compounds out of the plastic more readily than water-based foods. Physical damage to the cover, such as scratches or warping, can also accelerate the release of these compounds.
Identifying Approved and Unapproved Plastics
Consumers can identify plastics intended for microwave use by looking for specific markings and material codes. The most reliable indicator is the “microwave safe” symbol, typically consisting of three wavy lines or a small image of a microwave oven. This symbol indicates the product has been tested to withstand microwave temperatures without melting or warping. However, a “microwave safe” label refers mainly to physical stability and does not automatically guarantee zero chemical migration.
Another set of identifiers are the Resin Identification Codes (RICs), the numbered triangle symbols found on the bottom of plastic items. While originally created for recycling, these codes offer clues about a plastic’s heat tolerance. Polypropylene, marked with the number 5 (PP), is considered the most suitable plastic for microwave covers because of its high melting point.
Plastics with codes 2 (HDPE) and 4 (LDPE) are sometimes used in microwave-safe products, but verification with the wavy line symbol is recommended. Plastics marked with codes 3 (PVC), 6 (PS, Polystyrene), and 7 (Other) should be avoided for microwaving. These materials have lower heat resistance and are more likely to leach chemicals or melt when exposed to high food temperatures.
Safe Usage and Alternatives to Plastic Covers
When using a plastic microwave cover labeled as microwave-safe, certain habits can mitigate potential risks. Ensure the cover does not touch the food, especially when heating high-fat items, to prevent localized overheating and chemical transfer. Using a dome-shaped cover, rather than a flat one, helps maintain physical distance from the meal.
Microwave covers must always be vented to allow steam to escape during heating. If the cover lacks small holes or adjustable vents, place it loosely over the dish to prevent pressure buildup. Regularly inspect the cover, and discard any plastic showing signs of wear, such as deep scratches, cracks, cloudiness, or warping.
For those who wish to avoid plastic entirely, several effective alternatives are available:
- Glass lids or heat-safe glass bowls inverted over the food are entirely inert.
- Ceramic plates can be used to cover dishes and will not release chemicals.
- Food-grade silicone lids and covers offer flexibility and high heat tolerance.
- Simple options like wax paper, parchment paper, or white paper towels placed loosely over the dish can effectively trap moisture and contain splatters.