Cashew Nut Shell Liquid: Composition, Uses, and Safety

Cashew nut shell liquid, usually shortened to CNSL, is a dark, viscous oil trapped inside the honeycomb-like shell of the cashew nut. It is one of the few naturally occurring sources of phenolic lipids produced at industrial scale, and its chemical makeup gives it an unusual range of properties: it can kill bacteria, harden into durable coatings, reduce methane from cattle, and poison mosquito larvae. The liquid is also a potent skin irritant, chemically related to poison ivy, which is why raw cashews are never sold in their shells. What makes CNSL interesting is that it sits at the intersection of agricultural waste product and high-value chemical feedstock, and the gap between its potential and its current regulation is wide.

What Is Actually in CNSL

The liquid is a mixture of closely related phenolic compounds, each built around a long hydrocarbon side chain attached to a phenol ring. The three main players are anacardic acid, cardanol, and cardol. In raw CNSL extracted by cold pressing or solvent wash, anacardic acid dominates, making up roughly 60–70% of the mixture, with cardol and cardanol filling out the rest. The exact proportions shift depending on how the liquid is obtained, the cashew variety, and the growing region.

Things change dramatically once heat gets involved. Anacardic acid is thermally unstable and, when heated, loses a molecule of carbon dioxide and converts into cardanol. This decarboxylation reaction is why “technical” CNSL, the version produced as a byproduct of industrial roasting during cashew processing, has a very different profile from the natural extract. Technical CNSL is cardanol-rich, with much lower anacardic acid content and reduced cardol levels compared to cold-pressed or solvent-extracted versions.¹ The decarboxylation can also be done deliberately by heating anacardic acid to a controlled temperature, which is how producers manufacture cardanol on purpose for specific industrial uses.²

This distinction between natural and technical CNSL matters for every downstream application. If you want antimicrobial activity, you want anacardic acid, which means you need the unheated version. If you want a feedstock for polymer chemistry, cardanol is the target, and the roasted byproduct works well. The thermal degradation pathway runs further, too: at still higher temperatures, cardanols and cardols themselves begin to break down.³

How CNSL Gets Out of the Shell

The traditional methods are simple: roast the nuts (which releases the liquid as a hot byproduct) or mechanically press the shells. Solvent extraction with hexane or ethanol is a third option and tends to pull out more of the liquid; ethanol extraction in one study recovered about 40% of the shell’s weight as CNSL, compared to roughly 25% with petroleum ether.⁴ Each method changes the ratio of components in the resulting liquid, because heat-based methods drive that anacardic-acid-to-cardanol conversion in real time.

A more recent approach uses supercritical carbon dioxide, which acts as a solvent at high pressure and then simply evaporates when the pressure drops, leaving behind clean CNSL with no solvent residue. Early attempts at this had low yields, but researchers found that cycling the pressure up and down during extraction pushed yields as high as 94%. The resulting liquid was a clear, light brownish-pink color with no signs of degradation or polymerization.⁵ Supercritical CO₂ extraction avoids the flammability and toxicity concerns of organic solvents, though it requires specialized high-pressure equipment that keeps costs higher.

Industrial Applications of Cardanol

Cardanol’s structure, an aromatic ring with a long aliphatic tail, makes it a surprisingly versatile building block for polymer chemistry. It behaves like a bio-based substitute for some petroleum-derived phenols, and because it comes from agricultural waste rather than crude oil, it fits neatly into efforts to develop greener industrial materials.

The most commercially mature application is in coatings and resins. Epoxidized cardanol can be reacted with various hardeners to produce epoxy networks, giving manufacturers a partially bio-based alternative to conventional petroleum epoxies.⁶ Cardanol-derived curing agents called phenalkamines are already used commercially in epoxy coatings. The molecular weight of these phenalkamines can be tuned to control how fast the coating dries, how hard it gets, and how well it resists heat, giving formulators flexibility in designing coatings for different environments.⁷

Marine corrosion protection is one area where CNSL-based coatings look particularly promising. An optimized formulation reduced the corrosion rate of steel in seawater to a very low level while maintaining strong adhesion, positioning these coatings as a credible partially bio-based alternative for ships, offshore platforms, and other saltwater-exposed infrastructure.⁸

Beyond coatings, cardanol-based epoxy resins have been developed as gel polymer electrolytes for potassium-ion batteries, an emerging technology in energy storage.⁹ And CNSL-derived friction dusts are used in non-asbestos, copper-free brake pads, where modified versions of the dust improved thermal stability and reduced wear on both the pad and the rotor.¹⁰

Recyclable Plastics from Cashew Shells

One of the more forward-looking uses of cardanol is in “vitrimers,” a class of plastics that can be reshaped and recycled because their internal chemical bonds can rearrange when heated, unlike conventional thermosets that are permanently locked in place. A cardanol-based vitrimer showed measurable biodegradation in seawater, reaching a biological oxygen demand after 30 days that was comparable to microcrystalline cellulose, a well-known biodegradable material.¹¹ The cross-linked structure of the vitrimer slowed the onset of biodegradation compared to non-crosslinked cardanol epoxy, but the material still degraded within the test window. This is notable because finding plastics that are both mechanically robust and marine-biodegradable is an ongoing challenge.

Antimicrobial and Immune-Related Properties

Anacardic acid, the dominant compound in unheated CNSL, has genuine antimicrobial activity. It works against gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA), and the mechanism appears to be primarily physical rather than biochemical: the compound disrupts bacterial cell membranes through a surfactant-like action.¹² This mode of action is interesting because it is harder for bacteria to develop resistance to a compound that physically tears their membranes apart, compared to one that targets a specific enzyme.

Anacardic acid also triggers responses in human immune cells. In laboratory experiments, it stimulated neutrophils to produce reactive oxygen species and to throw out extracellular traps, which are web-like structures made of DNA and antimicrobial proteins that neutrophils use to catch and kill bacteria. The traps produced in response to anacardic acid were shown to be bactericidal.¹³ These findings are preclinical and have not been translated into approved therapies. The published literature on CNSL’s pharmaceutical potential remains largely descriptive and based on cell and animal studies rather than human trials.¹⁴

Cutting Methane from Cattle

One of the more surprising uses of CNSL is as a feed additive for dairy cows. The phenolic compounds in the liquid selectively inhibit certain microorganisms in the rumen, the large fermentation chamber in a cow’s stomach, and this shifts the fermentation pattern in ways that reduce methane production. In lab experiments, raw CNSL inhibited methane production by up to about 70% at the highest supplementation level tested, while also boosting propionate, a volatile fatty acid that the cow can use for energy instead of losing it as belched methane.¹⁵ Heated CNSL did not inhibit methane in those same experiments, reinforcing how much the biological activity depends on anacardic acid specifically.

The question is always whether lab results hold up in a real barn. A farm-scale trial found that adding CNSL to dairy cow diets did not change how much the cows ate or how much milk they produced, but it did lower the amount of methane produced per kilogram of feed consumed.¹⁶ The methane reduction on-farm was more modest than the in vitro numbers, as is typical when anything moves from a controlled flask to a living animal, but the direction held. For an industry under increasing pressure to cut greenhouse gas emissions, a feed additive derived from agricultural waste is an appealing concept.

Mosquito Control

The same phenolic compounds that disrupt bacterial membranes and alter rumen fermentation also kill mosquito larvae. Multiple studies have tested CNSL and its individual fractions against medically important mosquito species. Cardol showed the strongest larvicidal and pupicidal activity of the individual fractions, and it retained its killing power under field conditions over extended periods.¹⁷

CNSL is poorly soluble in water, which limits its usefulness in aquatic mosquito habitats. To get around this, researchers have formulated it as a nanoemulsion, shrinking the droplets to around 52 nanometers in diameter. The nanoemulsion was dramatically more potent than bulk CNSL against Anopheles culicifacies larvae, a malaria vector, requiring only about a thirteenth the concentration to kill half the larvae.¹⁸ A derivative called sodium anacardate was tested against Aedes aegypti, the dengue and Zika vector, and killed eggs, larvae, and pupae while showing no adverse effects in mice at the doses tested.¹⁹ The appeal is cost: cashew shells are waste in producing countries, so a larvicide derived from them could be cheaper and more accessible than synthetic alternatives in tropical regions where both cashews and mosquitoes are abundant.

Skin Irritation and the Poison Ivy Connection

If you have ever had a poison ivy rash, you already have a rough idea of what raw CNSL can do to skin. The phenolic compounds in cashew shell liquid are structurally related to urushiol, the allergen in poison ivy and poison oak. This is not a coincidence: cashew trees belong to the same plant family, Anacardiaceae, as poison ivy. In clinical testing, every patient who reacted to cashew extract also reacted to poison ivy urushiol, and the three who were not sensitive to poison ivy had no reaction to cashew extract either.²⁰ The cross-reactivity is strong enough that people with severe poison ivy sensitivity should be cautious around unprocessed cashew shells.

This is the reason cashews are always sold shelled and either roasted or steamed. The processing destroys or removes the irritant compounds from the edible kernel. But the shells themselves, and the liquid inside them, remain hazardous to handle without protection.

Occupational Health in Cashew Processing

Workers who shell, peel, and roast cashews in processing facilities face real health risks from CNSL exposure. A study of cashew processing workers in the Kollam district of Kerala found that roughly half reported dermatological problems, with shelling and peeling workers at highest risk.²¹ Chronic lung disease was present in about a fifth of workers surveyed.

A separate study of cashew workers in Cuddalore district found that musculoskeletal pain was the most common complaint overall, but the chemical hazards were task-specific. Blackish staining of the skin, caused by CNSL contact, affected about 30% of workers. Burns were concentrated among roasting and shelling workers. Eye irritation was highest among peeling workers, and respiratory problems were predominantly seen in those working roasting operations, where heated CNSL produces fumes.²² Much of the world’s cashew processing happens in countries where enforcement of workplace safety standards is uneven, which means these problems persist at scale.

Aquatic Toxicity and Environmental Limits

The same biological potency that makes CNSL useful as a larvicide makes it dangerous when it reaches waterways as industrial effluent. Ecotoxicological testing on Artemia, a standard test organism for aquatic toxicity, found that cardol and cardanol were both highly toxic at very low concentrations. Cardol was the more acutely dangerous of the two, with a lethal concentration for half the test organisms of just 0.56 milligrams per liter after 24 hours. Cardanol reached similar toxicity after a longer exposure window.²³

This matters because cashew processing generates large volumes of CNSL as waste, and disposal of that liquid is an ecological concern in cashew-producing regions. The dual identity of CNSL, simultaneously a valuable raw material and a toxic waste stream, is one of the strongest arguments for developing more industrial uses: every kilogram of CNSL turned into coatings, brake pads, or feed additives is a kilogram that does not need to be disposed of.

The Food Safety Question

Given its antimicrobial properties, CNSL has been explored as a natural food preservative. Extracted CNSL applied to beef at concentrations of 1–2% by weight showed preservative effects. But the researchers themselves flagged significant safety unknowns. Cresols identified in their CNSL extract are recognized as toxic at high levels and are regulated in occupational and environmental contexts. Cardanol and anacardic acid, while naturally occurring, do not yet have established acceptable daily intakes as food additives from major regulatory agencies. The study concluded that CNSL should be considered a promising candidate preservative, but that practical application would require comprehensive toxicological evaluation and regulatory approval before it could be used in food intended for consumers.²⁴

This gap between demonstrated efficacy and established safety thresholds runs through much of CNSL research. The compound clearly does many biologically interesting things. Whether those things can be done safely in applications where humans are directly exposed, whether through food, pharmaceuticals, or consumer products, remains a question that regulators have not yet answered in detail. Industrial applications like coatings and brake pads sidestep the problem, since the CNSL is chemically locked into a polymer matrix and no longer biologically active in its free form. But any use involving direct human or environmental exposure will need more rigorous safety data than currently exists.