Proposed Priority Product: Motor Vehicle Tires Containing Zinc

DTSC proposes to list motor vehicle tires that contain zinc as a Priority Product under the Safer Consumer Products (SCP) Regulations. The rationale for this proposal is explained in the Rationale Document for Motor Vehicle Tires Containing Zinc. The SCP Regulations allow stakeholders to petition DTSC to adopt a product-chemical combination as a Priority Product. In May of 2018, the California Stormwater Quality Association (CASQA) petitioned DTSC to list motor vehicle tires with zinc-containing tire tread as a Priority Product under the SCP Regulations. Per the petition, zinc is an aquatic toxicant that, when released from tires, causes or contributes to adverse impacts to aquatic organisms and potentially imposes high costs to California stormwater agencies responsible for meeting state and federal water quality objectives.

Based on the criteria in the SCP Regulations, we have determined the petition is of sufficient comprehensiveness and quality to demonstrate the potential for exposure, adverse impacts, and adverse waste and end-of-life effects.

Zinc (typically as zinc oxide) is added to tire rubber to encourage the vulcanization—a process that strengthens rubber and allows tire manufacturers to mold it into the precise shapes found in treads and other tire components. Small pieces of tires, commonly referred to as tire-wear particles (TWP), are generated as the tire rolls along the road, particularly as vehicles brake, accelerate, and turn. TWP can be deposited on roads and surrounding areas, where they can be carried by stormwater into surrounding waterways. Additionally, TWP can be transported through the air and deposited onto hard surfaces, such as roofs, where they are also subject to transport to the aquatic environment via stormwater.

Once TWP are released into the environment, they can enter local waterways, where zinc can leach out. TWP are a significant source of zinc in the aquatic environment, and aquatic organisms in California’s streams, rivers, and lakes may be adversely impacted by exposure to zinc. Because of this, the U.S. Environmental Protection Agency (EPA) and the State Water Resources Control Board (State Water Board) have developed water quality criteria that describe the point at which the amount of zinc in a water system becomes hazardous to aquatic organisms. Many California stormwater discharges and waterways, especially those in southern California, exceed zinc water quality criteria.

Some California waterways have been classified as impaired under the provisions of section 303(d) of the Clean Water Act due to zinc contamination. In many instances, stormwater runoff from nearby urban areas, which includes TWP, represents a major source of zinc contamination in a water body. In California, municipal agencies are responsible for adhering to zinc permit requirements for stormwater discharges, to protect local waterways. Many of these agencies face challenges staying in compliance with permits due to high levels of zinc in their stormwater. Stormwater treatment can reduce zinc concentrations, but is often prohibitively expensive and, therefore, infeasible.

Public Input 

A public comment period on this proposal was open until 11:59 p.m. PDT on August 6, 2021. The petition, other supporting documents, and comments received can be viewed on CalSAFER.

On July 28, 2021, we held a two-day public workshop on chemicals in motor vehicle tires, including zinc. The workshop was held fully online, via webcast, and included presentations and time for public comment. Below are some resources related to this workshop: 

• Draft Motor Vehicle Tires Containing Zinc Rationale Document
• Zinc Workshop Agenda 
• Workshop Recording
• Workshop Presentations:
  – Michael Ernst, P.E., DTSC

– Richard Watson and Fred Krieger, California Stormwater Quality Association
– United States Tire Manufacturers Association
– Kelly Moran, PhD, San Francisco Estuary Institute
– Eric Van Genderen, PhD, and Adam Ryan, PhD, International Zinc Association
– Ken Schiff, Southern California Coastal Water Research Project
– Ali Ansarifar, PhD, Loughborough University

• Written Questions Submitted During the Workshop

We considered input from the workshop and public comment period while preparing the final Rationale Document for Motor Vehicle Tires Containing Zinc.