Plastic sample bags are designed to store and transport liquid, semi-solid, or solid samples for scientific, environmental, food processing, pharmaceutical, or medical applications. They are commonly made of plastic such as low-density polyethylene, and come in a variety of dimensions, capacities, and thicknesses. Sample bags may have a zip top or fold-over closure to provide a tight seal. Clear sampling bags provide visibility of the sample, while amber bags protect samples from ultraviolet rays. A sample bag may have a label area or color coding for identification. Some may be imprinted with the universal biohazard symbol. To prevent contamination of samples, sterile sample bags are available. Applications include tissue culture, water or soil sampling, storage of sterile fluids, and transport of samples or waste. Gas sample bags are designed to sample gas and vapor for volatile organic compounds, sulfur vapor, and light and permanent gases.