Carcinogens are materials that cause cancer. This is a chronic toxic effect. Information about cancer-causing agents has been obtained by studying populations exposed to chemicals for long periods, usually in the workplace. Data have also been obtained through tests using laboratory animals. This does not mean that a material will actually cause cancer in humans, but it is a good indication. Example of known carcinogens include benzene, asbestos, arsenic compounds, vinyl chloride, and mustard gas.