Particular substances, known as carcinogens, have been linked to specific types of cancer. Common examples of non-radioactive carcinogens are inhaled asbestos, certain dioxins, and tobacco smoke. Although the public generally associates carcinogenicity with synthetic chemicals, it is equally likely to arise in both natural and synthetic substances.[14] It is estimated that approximately 20,000 cancer deaths and 40,000 new cases of cancer each year in the U.S. are attributable to occupation.[15] Every year, at least 200,000 people die worldwide from cancer related to their workplace.[16] Millions of workers run the risk of developing cancers such as lung cancer and mesothelioma from inhaling asbestos fibers and tobacco smoke, or leukemia from exposure to benzene at their workplaces.[16] Cancer related to one's occupation is believed to represent between 2–20% of all cases.[17] Most cancer deaths caused by occupational risk factors occur in the developed world.[16] Job stress does not appear to be a significant factor at least in lung, colorectal, breast and prostate cancers.[18]