A plastic bag, polybag, or pouch is a type of container made of thin, flexible, plastic film, nonwoven fabric, or plastic textile. Plastic bags are used for containing and transporting goods such as foods, produce, powders, ice, magazines, chemicals, and waste. It is a common form of packaging.
Several design options and features are available. Some bags have gussets to allow a higher volume of contents, some have the ability to stand up on a shelf or a refrigerator, and some have easy-opening or reclosable options. Handles are cut into or added into some.
Bags can be made with a variety of plastics films. Polyethylene (LDPE, LLDPE, etc.) is the most common. Other forms, including laminates and co-extrusions can be used when the physical properties are needed.
Plastic bags usually use less material than comparable to boxes, cartons, or jars, thus are often considered as "reduced or minimized packaging".
Depending on the construction, plastic bags can be suited for plastic recycling. They can be incinerated in appropriate facilities for waste-to-energy conversion. They are stable and benign in sanitary landfills. If disposed of improperly, however, plastic bags can create unsightly litter and harm some types of wildlife.
Bags are also made with carrying handles, hanging holes, tape attachments, or security features. Some bags have provisions for easy and controlled opening. Reclosable features, including press-to-seal zipper strips, are common for kitchen bags bought empty and for some foods. Some bags are sealed for tamper-evident capability, including some where the press-to-reseal feature becomes accessible only when a perforated outer seal is torn away.
Boil-in-bags are often used for sealed frozen foods, sometimes complete entrees. The bags are usually tough heat-sealed nylon or polyester to withstand the temperatures of boiling water. Some bags are porous or perforated to allow the hot water to contact the food: rice, noodles, etc.
Plastic bags are used for many medical purposes. The non-porous quality of plastic film means that they are useful for isolating infectious body fluids; other porous bags made of nonwoven plastics can be sterilized by gas and maintain this sterility. Bags can be made under regulated sterile manufacturing conditions, so they can be used when infection is a health risk. They are lightweight and flexible, so they can be carried by or laid next to patients without making the patient as uncomfortable as a heavy glass bottle would be. They are less expensive than re-usable options, such as glass bottles. Moderate quality evidence from a 2018 systematic review showed that plastic wraps or bags prevented hypothermia compared to routine care, especially in extremely preterm infants.
Flexible intermediate bulk containers are large industrial containers, usually used for powders or flowables.
Open bags with carrying handles are used in large numbers. Stores often provide them as a convenience to shoppers. Some stores charge a nominal fee for a bag. Heavy-duty reusable shopping bags are often considered environmentally better than single-use paper or plastic shopping bags. Because of environmental and litter problems, some locations are working toward a phase-out of lightweight plastic bags.
American and European patent applications relating to the production of plastic shopping bags can be found dating back to the early 1950s, but these refer to composite constructions with handles fixed to the bag in a secondary manufacturing process. The modern lightweight shopping bag is the invention of Swedish engineer Sten Gustaf Thulin. In the early 1960s, Thulin developed a method of forming a simple one-piece bag by folding, welding and die-cutting a flat tube of plastic for the packaging company Celloplast of Norrköping, Sweden. Thulin's design produced a simple, strong bag with a high load-carrying capacity, and was patented worldwide by Celloplast in 1965.
From the mid-1980s onwards, plastic bags became common for carrying daily groceries from the store to vehicles and homes throughout the developed world. As plastic bags increasingly replaced paper bags, and as other plastic materials and products replaced glass, metal, stone, timber and other materials, a packaging materials war erupted, with plastic shopping bags at the center of highly publicized disputes.
In 1992, Sonoco Products Company of Hartsville, SC patented the "self-opening polyethylene bag stack". The main innovation of this redesign is that the removal of a bag from the rack opens the next bag in the stack.
The number of plastic bags used and discarded worldwide has been estimated to be on the order of 1 trillion annually. The use of plastic bags differs dramatically across countries. While the average consumer in China uses only 2 or 3 plastic bags a year, the numbers are much higher in most other countries: Denmark: 4, Ireland: 20, Germany: 65, USA: > 300, Poland, Hungary, Slovakia: more than 400.
A large number of cities and counties have banned the use of plastic bags by grocery stores or introduced a minimum charge. In September 2014, California became the first state to pass a law banning their use. Local manufacturers of plastic bags, under the legislation, would receive financial support to assist them make more durable multi-use bags, that would be sold by grocery stores rather than given away, as were the plastic bags. In India, government has banned use of plastic bags below 50 microns.
Non-compostable plastic bags can take up to 1000 years to decompose. Plastic bags are not capable of biodegradation but rather they photodegrade, a process by which the plastic bags are broken down into smaller toxic parts. In the 2000s, many stores and companies began to use different types of biodegradable bags to comply with perceived environmental benefits.
Plastic shopping bags escape the garbage collection process, and are also disposed of improperly and can end up in streams, which then lead them to end up in the open ocean. To mitigate marine plastic pollution from single-use shopping bags, many jurisdictions around the world have implemented bans or fees on the use of plastic bags. They can also affect urban environments. Waterways and drains can be clogged by plastic bags and have been linked to severe flooding. An estimated 300 million plastic bags end up in the Atlantic Ocean alone, due to escaping the garbage. The way in which the bags float in open water can resemble a jellyfish, posing significant dangers to marine mammals and Leatherback sea turtles, when they are eaten by mistake and enter the animals' digestive tracts. After ingestion, the plastic material can lead to premature death. Once death occurs and the animal body decomposes, the plastic reenters the environment, posing more potential problems. 
Marine animals are not the only animals affected by improper plastic bag disposal. Penguins can get trapped and suffocate because of the bags. Plastic bags can also be mistaken for materials used in a bird's nest, which could lead to accidental ingestion of the plastic. Additionally, sea birds, when hunting, sense for dimethyl sulfide (DMS) which is produced by algae. Plastic bags are a breeding ground for algae, so the sea birds mistakenly eat the bag rather than the fish that typically ingests algae. (National Geographic)
Even though the bags are plastic, most recycling plants do not recycle them. The type of plastic in plastic bags makes it difficult to be recycled into new material. Additionally, the material frequently causes the equipment used at recycling plants to jam, thus having to pause the recycle machinery and slow down daily operations.
Thin conformable plastic bags, especially dry cleaning bags, have the potential for causing suffocation. About 25 children in the United States suffocate each year due to plastic bags, almost nine out of ten of whom are under the age of one. This has led to voluntary warning labels on some bags which pose a hazard to small children.
Plastic bags are used for diverse applications: