Effective October 1, 2023, The California Department of Toxic Substance Control (DTSC) listed motor
vehicle tires containing N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) as a “priority
product” under the Safer Consumer Products (SCP) Regulations.
This Stage 1 Alternatives Analysis (AA) report was prepared under the SCP Regulations on behalf of a
Consortium1 comprising some, but not all, manufacturers of the Priority Product for sale in California. As
conceived by Gradient and the Consortium, the initial goal of an AA is to answer the following question:
Do potentially safer, functionally acceptable, and technically feasible alternatives to the Priority Product
exist that should be given a more in-depth consideration to determine if they qualify as acceptable
alternatives?
This Stage 1 AA was based on available information and sought to determine whether there are possible
alternatives to the priority product that should be considered in greater depth to evaluate if they are suitable
alternatives to replace the priority product under regulatory guidelines (CalDTSC, 2017a). Important
elements of this work were considering the requirements (legal, regulatory, or otherwise) for the priority
product, determining the function of the chemical of concern in the priority product, determining whether
simple elimination was possible and assessing relevant factors to identify those that would suggest a
material difference exists that could affect the decision as to whether a possible alternative is a suitable
replacement for the priority product. This last element required compiling extensive information on the
potential hazards, potential performance, and chemical and physical properties of the possible alternatives.
6PPD is used in tires as an antidegradant, protecting the components of the tire from attack by ozone,
oxygen, thermal degradation, and mechanical fatigue, etc. In late 2020, it was first reported that when it
reacts with ozone, 6PPD forms a degradation product, 6PPD quinone (6PPDQ) (Tian et al. 2021); this
reaction with ozone is part of the way in which 6PPD protects tire rubber from degradation. Without 6PPD,
tires will quickly develop cracks and fractures as the rubber polymer is degraded. The antidegradant
function of 6PPD in tires is therefore essential to their safe use, and elimination of 6PPD without
replacement is not an option.
One way 6PPD and 6PPDQ may enter the environment is through tire and road wear particles (TRWP),
which are produced as the tire grips the road surface during driving. Some 6PPD and 6PPDQ on the tire
surface may also be washed off the tire by rain or vehicle washing. US EPA has also noted uncertainty
about levels of 6PPDQ exposure to the environment from tires relative to other potential sources
(Freedhoff, 2023).
Recent laboratory studies stated that 6PPDQ is acutely toxic to coho salmon, and those studies suggest that
6PPDQ exposure from stormwater runoff, under certain conditions, may result in mortality of these fish in
streams and rivers located near roadways (Tian et al. 2021). Some other salmonid species may be similarly
affected under similar circumstances, although with lower toxicological potency than coho, while other
species apparently exhibit negligible toxicological susceptibility (see for example Brinkmann et al., 2022;
Hiki and Yamamoto, 2022; Greer et al., 2023). The biological mechanisms by which the toxicity occurs,
and why it affects some species and not others, is not yet known but is the subject of active research.
Read the Q&A: Stage 1 Alternatives Analysis For 6PPD in Tires