California’s Safer Consumer Products (SCP) program has played a prominent role in the state’s approach to regulating chemicals in consumer products. The SCP program was developed to evaluate and regulate specific, clearly defined chemical substances with identifiable properties and toxicological profiles. It operates by first identifying and prioritizing individual chemical substances that meet established criteria to add to a Candidate Chemicals List before determining whether to regulate product-chemical combinations.

Microplastics should not be on the Candidate Chemicals List as California proposes. They are not single chemicals or a chemical family, but a diverse collection of particles and fibers. Like vegetables, which can come in a variety of forms—asparagus, rutabaga, peas, corn, etc. “microplastics” vary widely in size, shape, polymer composition, surface characteristics, and associated substances. 

This mismatch presents a substantive challenge rather than a procedural one. Treating “microplastics” as a single chemical category is impossible to reconcile with the statutory structure of the program, implementing regulations, and the underlying science. As a result, attempting to regulate microplastics as a single category through the SCP program would stretch the program beyond its intended scope.

Microplastics Do Not Possess a Single Molecular Identity

The core, non-negotiable requirement of California’s SCP program regulations is that a regulated chemical or chemical family must possess a specific molecular identity. This is the bedrock of hazard identification and chemical risk assessment. A substance must have a known, consistent molecular structure, quantifiable properties, and predictable characteristics that allow for accurate evaluation of its hazard and potential exposure.

Microplastics completely fail this essential test. 

Microplastics are not a single substance or a family of substances with similar and predictable characteristics. They are a highly varied collection of particles originating from multiple sources, each with its own chemical makeup and potential hazard profile. This category includes tiny fibers from nylon clothing, fragments from PET plastic bottles, particles from tire wear, and countless other polymer types that go far beyond what can be listed in a blog post. And for example, from a chemical perspective, a particle of nylon bears no molecular resemblance to a fragment of PET. Treating this diverse collection as a singular chemical entity completely disregards the basic principles of chemistry and material science. So why regulate them, in a haphazard way, as one? 

The same distinction applies to plastics as a whole. Plastics represent a vast category of materials, not a single chemical. They are manufactured from numerous different chemical substances, each with a unique molecular structure and specific properties. California has never previously regulated broad material categories, like wood, as chemicals under the SCP program. Instituting this change now would break long-standing regulatory practice and significantly dilute the program’s focus. 

A Mixed Group of Things Can’t be Checked for the Same Risk.

The design of the SCP program is to identify hazardous chemicals, undertake an alternatives analysis and replace the identified hazards. Because microplastics are not a family of chemicals, much less a single chemical, they do not share common hazards, behaviors in the environment, or potential health impacts.

For regulatory officials, this creates insurmountable obstacles: Unlike a chemical with a defined molecular structure, well-characterized exposure pathways, and established dose–response relationships, there is no single hazard profile or exposure scenario that applies across all microplastics. A synthetic fiber suspended in air behaves very differently from a fragment in soil, or tire particles in stormwater runoff. 

Because potential health and environmental effects are likely to depend on this combination of physical characteristics, chemical composition, and routes and levels of exposure, the current science does not yet allow for consistent hazard characterization, comparison of alternatives, or evidence-based conclusions across the full range of materials grouped under the term “microplastics.” This does not argue for inaction, but it does underscore that the SCP framework is not the appropriate vehicle to address such a heterogenous class of materials. 

The Appropriate Policy Path Forward 

California policymakers should continue to pursue evidence-based approaches to identify the sources and fate of microplastics. However, this requires a focus on developing clear, scientific definitions and standardized analytical methods for identifying and measuring microplastics. For example, there is still no clear agreement on what size particles should count as “microplastics”, how samples should be collected and monitored over time, or which testing methods are most reliable for water, air, soil, food, and other materials. These essential scientific foundations can help accelerate the science to identify potential risks, so policymakers can make policy decisions grounded in science to most effectively mitigate the potential environmental and health impacts of microplastics.