Biodegradable Cosmetic Ingredients — What It Really Means and Why Natural ≠ Biodegradable

"Biodegradable formula" — this phrase appears on more and more cosmetics, suggesting the product is safe for the environment. The catch is that biodegradability isn't binary — it's a spectrum, and the details matter enormously. An ingredient can be technically biodegradable yet break down so slowly that it causes ecological harm in the meantime.

What biodegradability is — the scientific definition

Biodegradability is the ability of a substance to be broken down by microorganisms (bacteria, fungi) into simple chemical compounds: carbon dioxide, water and biomass. It sounds straightforward, but the devil is in the details — above all in the rate and conditions of that breakdown.

OECD 301 tests — the gold standard

The Organisation for Economic Co-operation and Development (OECD) developed a series of biodegradability tests designated as OECD 301 (A–F). These are standardised laboratory methods that measure how quickly a substance breaks down under aerobic conditions:

• OECD 301B (CO₂ Evolution Test) — measures the amount of CO₂ released as the substance is broken down by microorganisms from the activated sludge of a wastewater treatment plant.
• OECD 301D (Closed Bottle Test) — measures oxygen consumption in a closed bottle. The simplest and most commonly used.
• OECD 301F (Manometric Respirometry) — measures the pressure drop caused by oxygen consumption.

A substance is classified as "readily biodegradable" when, within 28 days, it breaks down by at least 60% (CO₂) or 70% (DOC — dissolved organic carbon). On top of that, this threshold must be reached within a 10-day "window" after breakdown begins.

Readily vs poorly biodegradable

This distinction is crucial and often ignored in marketing:

• Readily biodegradable — breaks down ≥60% within 28 days under standard conditions. Examples: glycerin, citric acid, most sugar-based surfactants (Coco-Glucoside).
• Inherently biodegradable — does break down, but more slowly than 28 days. May require the microorganisms to adapt. Examples: some ethoxylated surfactants, EDTA.
• Persistent / non-degradable — does not break down to any significant degree under environmental conditions. Examples: cyclic silicones D4/D5, microplastic particles.

D4/D5 silicones — PBT substances

Cyclomethicone (D4, Cyclotetrasiloxane) and cyclopentasiloxane (D5, Cyclopentasiloxane) are among the most widely used silicones in cosmetics — from foundations to hair serums. They give products a silky texture and help with spreadability.

The problem is that D4 meets the criteria of a PBT (Persistent, Bioaccumulative, Toxic) substance — it is persistent in the environment, accumulates in living organisms and shows toxicity. D5 is classified as vPvB (very Persistent, very Bioaccumulative).

Since 2020, the European Chemicals Agency (ECHA) has restricted the use of D4 and D5 in rinse-off cosmetics to concentrations below 0.1%. This means a shampoo with a high level of D4/D5 is non-compliant with EU regulations — but leave-on cosmetics may still contain them.

Check whether your cosmetics contain D4/D5 — head to the PurScore ingredient database and search for "Cyclomethicone" or "Cyclopentasiloxane".

Microplastics — Regulation 2023/2055

In October 2023, the European Commission adopted Regulation (EU) 2023/2055 restricting the intentional addition of microplastics to products, including cosmetics. It is a landmark regulation that:

• Defines microplastics as synthetic polymers in solid form, smaller than 5 mm, insoluble in water and non-degradable.
• Bans the use of microplastic particles (polyethylene, polypropylene) as abrasives in rinse-off cosmetics (e.g. scrubs with "beads") — from October 2023.
• Introduces a gradual phase-out of microplastics from other cosmetic categories by 2035.
• Requires manufacturers to report the amount of microplastics in their products.

In practice, this means a scrub with polyethylene beads should already have disappeared from the European market. But take note — soluble synthetic polymers (e.g. Carbomer, Acrylates Copolymer in dissolved form) are not covered by this regulation, although their environmental impact is the subject of ongoing research.

You'll find more on how to spot microplastics in cosmetic formulas in the guide Microplastics in Cosmetics — How to Recognise Them.

Natural ≠ biodegradable — why they aren't the same thing

This is one of the most common mistakes in thinking about eco-cosmetics. Naturalness and biodegradability are two different properties:

• Natural and biodegradable: vegetable glycerin, lavender essential oil, beeswax — these break down easily.
• Natural but POORLY biodegradable: lanolin (wax from sheep's wool) breaks down very slowly. Natural resins (e.g. shellac) can persist in the environment for months.
• Synthetic and READILY biodegradable: Coco-Glucoside (a synthetic surfactant made from sugar and coconut oil) breaks down faster than many "natural" ingredients.
• Synthetic and non-degradable: D4/D5 silicones, microplastics.

The takeaway: judge biodegradability based on OECD test data, not on where the ingredient comes from. A "100% natural formula" label says nothing about the impact on the aquatic environment after the product is rinsed away.

How to check the biodegradability of your cosmetics

As a consumer your tools are limited, but you can:

1. Check the ingredients in the PurScore database — many entries include information on biodegradability and environmental classification.
2. Look for certifications — ECOCERT, COSMOS and Nordic Swan require at least 95% readily biodegradable ingredients in rinse-off products.
3. Avoid the known "red flags": Cyclomethicone, Cyclopentasiloxane, Polyethylene, Polypropylene, Nylon-12 — these ingredients are persistent in the environment.
4. Ask the manufacturers — EU regulations require manufacturers to hold biodegradability data. They are obliged to make it available.

You can read more about the differences between natural and synthetic ingredients in the guide Natural vs Synthetic Ingredients.