What “biodegradable” really means on a soap or detergent

“Biodegradable” sounds like a clear promise: use it, rinse it away, nature takes care of the rest. But almost every organic substance breaks down eventually, given enough time. The word only means something when there’s a test behind it, with a clock and a pass mark. Here’s what that test looks like, what it says about soaps and detergents, and what Canadian law expects of the claim.

Why “biodegradable” needs a definition

Biodegradation is the breakdown of a substance by living things, mostly bacteria, into simpler molecules. Take it far enough and you end up with carbon dioxide, water and minerals. Scientists call that complete breakdown “ultimate biodegradation”. A weaker version, “primary biodegradation”, just means the original molecule has changed into something else, which may or may not be harmless.

The problem is time. A cotton T-shirt is biodegradable. So is a fallen tree, over decades. If a label says “biodegradable” with no timeframe and no test, it hasn’t told you much. The useful question is not “does it break down?” but “how fast, how completely, and under what conditions?”

The standard test: OECD 301

The most widely used benchmark is a set of lab tests published by the Organisation for Economic Co-operation and Development, called the OECD 301 series (adopted in 1992). It’s the reference regulators and manufacturers reach for when they say a chemical is “readily biodegradable”.

The guideline describes six methods, labelled 301 A through F. They differ in what they measure: some track how much dissolved organic carbon disappears from the water, others measure the carbon dioxide produced or the oxygen consumed as microbes eat the substance. All of them work the same basic way: dissolve the test chemical in water with minerals, add microbes (usually from a sewage treatment plant’s activated sludge), keep it aerated and in the dark, and watch what happens.

The 28-day clock and the pass mark

The test normally runs for 28 days. To be called readily biodegradable, the substance has to reach a pass level: 70 percent removal of dissolved organic carbon, or 60 percent of the theoretical carbon dioxide or oxygen demand for the methods that measure breathing. The 60 percent figure is lower because some of the carbon gets built into new bacterial cells rather than released as gas.

There’s a second condition called the 10-day window. Once breakdown reaches 10 percent, the substance has to hit the pass level within the next 10 days, all inside the 28. A chemical that gets there only after day 28 is explicitly not classed as readily biodegradable.

What the test is and isn’t

The OECD guideline itself says the methods are stringent, and that a low result doesn’t necessarily mean a substance won’t biodegrade in the environment, only that more work is needed to show it. Real rivers and treatment plants have more microbes, more time and different conditions than a lab flask. The test is deliberately tough so that a pass means something.

How the surfactants in your detergent actually break down

Surfactants are the cleaning molecules in soap and detergent. They have a water-loving end and an oil-loving end, which is how they lift grease off fabric or skin and carry it away in the rinse.

A review of surfactant biodegradation published in a peer-reviewed biochemistry journal describes what happens after that. Detergent surfactants flow into sewage treatment plants, where they’re partly broken down by oxygen-using bacteria and partly adsorbed onto sewage sludge, the solid material that settles out. Some of that sludge is later applied to land.

The review found that the most common anionic surfactants (the negatively charged type used in most laundry detergents and many body washes) pose little risk in oxygen-rich soil, with safety margins the authors put at more than a hundredfold. Two caveats came with that:

• Some breakdown products matter more than the parent. An older family of nonionic surfactants, alkylphenol ethoxylates, degrades into substances with oestrogen-like activity, which is one reason they have been phased out of household products in many places.
• Oxygen matters. Biodegradation is faster and more complete with oxygen. In anaerobic conditions, such as deep sludge or sediment, breakdown is slower and less well understood, and the review noted data gaps for cationic (positively charged) surfactants, the type used in fabric softeners.

So “the surfactants in modern detergents biodegrade” is broadly true, but with the same qualifier as everything else here: in a treatment plant, with oxygen, over days to weeks.

What Canadian law asks of the claim

Canada doesn’t define “biodegradable” in a regulation. What it does is treat the word as an environmental claim, and environmental claims fall under the Competition Act, enforced by the Competition Bureau.

The Bureau’s guidance, “Environmental claims and the Competition Act”, was published on June 5, 2025. Its first principle is that claims must be truthful in both their literal meaning and the general impression they create. Its second is that claims about a product’s environmental benefits should be adequately and properly tested, before the claim is made, using tests suited to the specific claim. The guidance notes that things like long consumer experience have been found by courts not to count as testing.

The Bureau also warns against vagueness. Its example is “eco-friendly”: a word that can suggest the product is good for the environment across its whole life cycle, when the evidence may only cover one part. “Biodegradable” with no timeframe, no conditions and no test carries the same risk. A specific claim, such as “surfactants readily biodegradable per OECD 301”, is the kind of thing the Bureau’s principles favour.

One recent change: the Bureau’s greenwashing page notes that in March 2026, Parliament removed a separate requirement, added in 2024, that claims about a business’s overall environmental activities be backed by an “internationally recognized methodology”. The testing requirement for product claims is what still applies to a bottle of detergent.

Reading the label at home

A few practical translations:

• “Biodegradable” alone: a claim with no clock. Not false, but not informative.
• “Readily biodegradable” or “biodegradable per OECD 301”: the manufacturer is pointing at a standard test with a 28-day pass mark. That’s a real claim you can hold them to.
• “Biodegradable surfactants”: says something about the cleaning agents, nothing about fragrance, preservatives or dyes.
• “Plant-based”: says where the carbon came from, not how fast it breaks down. Plant-derived and petroleum-derived surfactants can both pass or fail the test.

None of this changes how you should use the product. Biodegradation takes days and needs microbes and oxygen, so wash water still belongs in the drain, not in a stream, whatever the label says. If you’re comparing products, you can browse laundry products and read the claims with the 28-day rule in mind.

The short version: “biodegradable” is a promise about what happens after the rinse, and it’s only as strong as the test behind it. Look for a timeframe and a standard, expect the claim to cover the surfactants rather than the whole formula, and treat words without either as decoration.