Cars can build up static charge by driving on dry road surfaces. However many tire compounds contain enough carbon black to provide an electrical ground which prevents charge build-up. Newer "high mileage" tires use more silica and can increase the buildup of static. A driver who does not discharge static by contacting a conductive part of the car will carry it to the insulated handle of the nozzle and the static potential will eventually be discharged when this purposely-grounded arrangement is put into contact with the metallic filler neck of the vehicle. Ordinarily, vapor concentrations in the area of this filling operation are below the lower explosive limit (LEL) of the product being dispensed, so the static discharge causes no problem. The problem with ungrounded gas cans results from a combination of vehicular static charge, the potential between the container and the vehicle, and the loose fit between the grounded nozzle and the gas can. This last condition causes a rich vapor concentration in the ullage (the unfilled volume) of the gas can, and a discharge from the can to the grounded hanging hardware (the nozzle, hose, swivels and break-a-ways) can thus occur at a most inopportune point. The Petroleum Equipment Institute has recorded incidents of static-related ignition at refueling sites since early 2000.