A Mylar balloon is a type of balloon made from nylon with a metallic coating. They are also known as foil balloons and are not biodegradable, as they are made from synthetic materials. Mylar balloons are not porous, so air does not leak out of them easily, as long as they are properly sealed. They are often printed and can be covered in a thin, shiny metal such as aluminum. These reflective balloons are also called “foil balloons” because of their metal coating, and they are commonly sold at grocery stores. Mylar balloons typically last longer than latex balloons because by using a plastic polyester material, an airtight and nonporous environment is created so that the helium is unable to escape. Latex balloons are more flexible and thinner once inflated, but Mylar keeps its shape longer and can be formed into countless shapes.