E-cigarettes create vapor made of fine and ultrafine particles of particulate matter,[1] which have been found to contain propylene glycol, glycerin, nicotine, flavors, tiny amounts of toxicants,[1] carcinogens,[2] heavy metals, and metal nanoparticles, and other substances.[1] Its exact composition varies across and within manufacturers, and depends on the contents of the liquid, the physical and electrical design of the device, and user behavior, among other factors.[notes 2][3] E-cigarette vapor potentially contains harmful chemicals not found in tobacco smoke.[4] E-cigarette vapor contains fewer toxic chemicals,[1] and lower concentrations of potential toxic chemicals than cigarette smoke.[97] The vapor is probably much less harmful to users and bystanders than cigarette smoke,[96] although concern exists that the exhaled vapor may be inhaled by non-users, particularly indoors.[6]