There are environmental factors that govern the selection of brake pad materials. For example, the bill SSB 6557 adopted in Washington State in 2010 limits the amount of copper that is allowed to be used in friction materials, to be eventually phased out to trace amounts, due to the negative impact of high copper levels on aquatic life. For its substitution, different material combinations have been developed, though no direct replacement is available yet. Other materials, such as compounds made with antimony, are being studied.