More than 99% of the mercury used in the manufacture of automobiles is contained in mercury switches. Switches are devices used to allow electric current to flow when closed, and when opened, they prevent current flow. Mercury switches use liquid mercury that flows into position to close the switch; the liquid mercury then conducts electricity.

Historically, automakers have used mercury in switches for convenience lighting, antilock braking systems (ABS) and active ride control systems. Mercury switches in convenience lighting make up 87% of the mercury switches used in automobiles. They are used for hood and trunk lighting. When the hood or trunk reaches a certain opening angle, the mercury flows to close the switch and the light goes on. Likewise, when the hood or trunk reaches a certain closing angle, the mercury flows to open the switch and the light goes off. In anti-lock braking systems, when a mercury switch detects deceleration rates, it turns off the 4-wheel drive and turns it back on once the deceleration event has passed. The ABS application of mercury switches comprises approximately 12% of the use of mercury switches in automobiles. Mercury switches in active ride control systems comprise approximately 1% of the mercury switches used in automobiles. These switches adjust suspension during cornering events.

Although automakers have begun to phase out the use of mercury switches in automobiles, U.S. automakers still used an estimated 4 million mercury switches in 2000. An estimated 215 million mercury switches (and perhaps as many as 250 million) are contained in vehicles currently on the road; this totals some 172 to 200 metric tons of mercury. In addition, foreign and domestic automakers are introducing new automotive applications of mercury including high intensity discharge (HID) headlamps and background lighting in automotive displays.

Sources: http://www.cleancarcampaign.org; "Toxics In Vehicles: Mercury" report by Ecology Center, Great Lakes United, and UTenn Center for Clean Products and Clean Technologies.