Lead wheel weights are used worldwide to balance vehicle tires. An estimated 64 million kg/year (70,000 ton/year) of lead is used worldwide in the manufacture of wheel weights. Automobile and light truck wheel weights vary in size and weight, ranging between 5-150 mm (0.2-6 in) in length and 7-113 grams (0.25-4oz) in weight. A typical vehicle contains between 200 and 250 grams of lead in wheel weights. This amounts to 10-12.5% of vehicle lead use, excluding the lead-acid battery. Recent studies have shown that lead deposition from lead wheel weights is a significant and previously not quantified source of lead being releases to the environment. The majority of wheel weights currently in use are clip-on types that are attached at the edge (horn) of a wheel's rim; however some new aluminum rims require adhesive weights due to their shape.
An average vehicle contains ten wheel weights (two on each of the four wheels and two more on the spare). Although some effort is made to collect and recycle these weights at the end of a vehicle's life, most of them are overlooked and often end up in the environment or as contaminants in the metal recycling process. A disturbingly large number fall off onto the road during vehicle use. In October of 2000, Dr. Robert A. Root (Environmental Health Perspectives, Volume 108, Number 10. October 2000) published a study documenting the rates at which these weights fall off their host vehicles and are gradually abraded into lead dust. His study was the first to examine this phenomenon, and it established that lead wheel weights are, in his words, "a major source of lead exposure that heretofore has not been recognized." The Ecology Center surveyed a one-mile stretch of urban roadway in Ann Arbor in 2002 and recorded very similar lead deposition rates.
In Michigan, the 8.5 million registered vehicles are serviced with nearly 500 metric tons of lead each year for new tires and repairs, and the entire fleet contains nearly 1,700 metric tons. The six Great Lakes states used approximately 2,500 metric tons of lead each year for lead wheel weights. Using lead wheel weight failure rates from existing research we estimate that as much as 60 metric tons of lead are deposited on streets in Michigan and 275 metric tons are deposited on the streets of Great Lakes states.
|State||Weights Loss (urban only)||Lead Lost Per Year (metric tons)||Lead Lost During Tires Life, average 3.5 years (metric tons)||Total Lead Used (based on average tire life of 3.5 years)||Percent Mass Lost|
Dr. Root estimates that an average of 11.8 kg/km (40 lb/mi) of lead is deposited each year along the 2.4-km (1.5-mi) length of street in Albuquerque. Urban lead deposition, which he estimates at 1.5 million kg/year (3.3 million lb/year), poses a significant lead poisoning threat to poor and minority populations that are already overexposed to lead burdens. Dr. Root estimates that wheel weights fall off on major Albuquerque thoroughfares at a rate of 3,730 kg/year (8,200 lb/year).
Dr. Root's findings indicate that this lead is rapidly abraded into fine dust particles, which are susceptible to atmospheric corrosion, and are expected to turn into lead oxides, hydroxides, and bicarbonates under ambient environmental conditions. These conversions make lead more soluble, and increase the risk that lead will contaminate surface, groundwater, and drinking water supplies.
Studies conducted by the Wisconsin Department of Natural Resources in Madison, Wisconsin, have shown that approximately 40% of the runoff from residential areas and 70% of the runoff from commercial areas had lead levels "high enough to kill aquatic life." Concentrations of lead in Madison's runoff ranged from 3-160 µg/L. "The primary source of many metals in urban runoff is vehicle traffic," the authors write. "Concentrations of zinc, cadmium, chromium and lead appear to be directly correlated with the volume of traffic on streets that drain into a storm sewer system. Streets and parking lots are the primary sources of lead in urban (runoff)."
Although a variety of alternatives have been considered, the most viable options appear to be Zinc or ZAMA (an alloy of zinc, aluminum, and copper) or steel weights. Significant portions of European and Asian new cars have been converted to zinc or steel weights (respectively) already. (see vehicle survey)