Plastic bags are generally made from a derivative of natural gas and petroleum products. Petroleum-based products are unhealthy for humans as well as the environment in several ways:
Contact with petrochemicals can cause a variety of health problems from cancers to the creation of abnormal hormone patterns in the human body.
Natural gas is not a renewable resource; once it is used up, it is gone forever.
Plastic bags can create problems for animals. Some sea mammals get caught in the bags and aren't able to come to the surface to breathe and drown. Sometimes animals, birds, or fish ingest pieces of a plastic bag that block their digestive system and causes them to die. There are numerous ways that these bags pose a threat to animals.