Plant Phytoncides: Nature's Antibacterial Compounds and Their Health Benefits
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Phytoncides are biologically active substances produced by plants that inhibit the growth of or kill bacteria, fungi, and multicellular organisms such as flies, worms, and insects. Functioning as a natural immune factor for plants, phytoncides protect their host against disease and form part of the plant's defense system. The term itself comes from the Greek phyton ("plant") and the Latin caedere ("to kill"), literally meaning "killed by plants."
Phytoncides are substances produced by plants
What are plant phytoncides: definition and origin of the term
Phytoncides are volatile and non-volatile secondary metabolites that plants release to defend themselves against microbes, insects, and herbivores. They are not a single chemical but a broad group of compounds of varied chemical nature, united by one functional property: they suppress or destroy competing organisms. Every higher and lower plant produces some form of phytoncide, making these compounds one of the most widespread chemical defense mechanisms in the plant kingdom.
The defensive role of phytoncides explains why a wounded or diseased plant releases far larger quantities of them. When tissue is damaged, the plant floods the injury site with antimicrobial volatiles to seal it against infection. This same defensive chemistry is what humans encounter as the characteristic scent of a pine forest, a crushed garlic clove, or freshly cut grass.
The discovery of phytoncides: the research of B. P. Tokin
Phytoncides were discovered in 1928 by the Soviet biologist Boris Petrovich Tokin (B. P. Tokin), who worked at Moscow State University. Tokin noticed that single-celled organisms (infusoria) placed next to a paste of crushed onion all died within a short time. From this observation he concluded that plants emit substances capable of killing microorganisms at a distance, and he coined the term "phytoncides" to describe them.
Boris P. Tokin presented his discovery at an international congress of cytologists in Amsterdam, bringing the concept to a wider scientific audience. His work established that phytoncides occur in both higher and lower plants and can exist in gaseous or dissolved form. Tokin's research laid the foundation for decades of study into how plant-derived antimicrobials might be applied against infectious disease.
The chemical nature of phytoncides
Phytoncides are chemically diverse because they belong to several families of secondary metabolites rather than a single class of molecule. The most studied phytoncides are volatile isoprenoids — terpenes and terpenoids — but plants also produce sulphur-containing compounds, phenolics, and alkaloids that perform the same defensive function.
Secondary metabolites and the composition of volatile substances
The volatile fraction of phytoncides consists largely of volatile organic compounds (VOCs) that evaporate readily into the surrounding air. In forest atmospheres these VOCs are dominated by monoterpenes, and their concentration varies with season, temperature, humidity, and time of day — warm summer afternoons in dense conifer stands produce the highest levels. These airborne molecules are what a person breathes in during a walk through the woods.
Onion and garlic illustrate the sulphur-based branch of phytoncide chemistry. Garlic owes its antimicrobial power to allicin and diallyl disulfide, compounds formed when the bulb is crushed, while onion releases related lachrymatory sulphur volatiles. These differ chemically from the conifer terpenes yet serve the same defensive purpose.
Alpha-pinene and other active compounds
Alpha-pinene, beta-pinene, and limonene are among the most abundant and biologically active phytoncides emitted by trees. Alpha-pinene in particular has drawn research interest for its anticancer activity in laboratory studies, alongside its well-documented antimicrobial effects. These monoterpenes give coniferous forests their resinous fragrance and are the principal components measured when scientists quantify forest VOC exposure.
Hinoki cypress (Chamaecyparis obtusa) is a notable source of therapeutic phytoncides, with stem oil rich in compounds that influence human immune function. Other plants contribute distinct active molecules: Sophora flavescens yields sophoraflavanone G, a phytoncide with marked antibacterial properties, demonstrating how widely the chemistry of plant defense ranges across species.
Properties of phytoncides
Phytoncides possess bactericidal properties, meaning the ability to kill microbes, and this is the property that first drew Tokin's attention to crushed onion. Beyond killing bacteria, they act against fungi, protozoa, and insects, which is why they are used in the fight against certain contagious diseases. Phytoncides occur in both higher and lower plants and may be present in gaseous or dissolved form.
Bactericidal properties
Many common plants demonstrate strong antibacterial action. Fir needles, the young shoots of pine, the bark of young spruce, wormwood, burnet, oregano, radish, horseradish, and hyssop all carry bactericidal phytoncides. So too do the juices of grapes, wild strawberries, strawberries, raspberries, blackberries, lingonberries, and cranberries.
Garlic and onion possess anti-diphtheria and anti-typhoid properties, a reflection of their potent sulphur-based phytoncides.
Antimicrobial and insecticidal mechanism of action
Phytoncides act against microbes and insects by disrupting cell membranes, interfering with respiration, and denaturing the proteins of target organisms. Their volatile components can reach and damage microbes at a distance, while contact with the concentrated compounds destroys insects, mites, and larvae outright. This dual antimicrobial and insecticidal capacity is precisely why a wounded plant releases a surge of phytoncides — to neutralize invaders before they establish infection.
The defensive reach of phytoncides extends to drug-resistant pathogens. Some plant phytoncides have shown activity against antibiotic-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus, which is one reason researchers continue to mine plant defenses for new antimicrobial leads.
Volatile and non-volatile phytoncides
Phytoncides divide into two functional types: volatile phytoncides that act at a distance through the air, and non-volatile (nonexcretory) phytoncides that take effect when the fruits or tissues of the plant are consumed. Volatile phytoncides without any odour have the same action as fragrant ones — the antimicrobial effect does not depend on scent. When plants are wounded or fall ill, they release especially large quantities of these compounds.
Plants as sources of phytoncides
Phytoncide-rich plants span trees, vegetables, and berries, with conifers among the most prolific producers of airborne compounds. Large amounts of phytoncides are found in the fruits of ash, bird cherry, cedar, sea buckthorn, and many other plants.
Phytoncides of coniferous trees (fir, pine, spruce, cedar)
Coniferous trees are the richest natural emitters of volatile phytoncides, releasing terpenes such as alpha-pinene and limonene into the surrounding air. Fir, pine, spruce, and cedar saturate forest atmospheres with these compounds, which is why coniferous woodland air carries such strong antimicrobial and restorative properties. The resinous scent of a pine or fir forest is the direct sensory evidence of phytoncide concentration in the air.
Garlic, onion, and other phytoncidal vegetables
Garlic and onion are the best-known phytoncidal vegetables, owing their power to sulphur compounds like allicin and diallyl disulfide. Radish, horseradish, and similar pungent vegetables join them, releasing volatiles sharp enough to kill microbes on contact. These culinary plants have been used in traditional medicine across many cultures precisely because their phytoncides act against bacteria responsible for serious infectious diseases.
Berries and fruits with bactericidal properties
A wide range of berries and fruits carry bactericidal phytoncides in their juices and pulp. Chokeberry, barberry, lingonberry, grape, pomegranate, blackberry, viburnum, cranberry, lemon, red rowan, blackcurrant, and bilberry all demonstrate antibacterial activity. These compounds contribute both to the plant's own preservation and to the well-documented antimicrobial value of the fruits when eaten.
Phytoncides and human health
Phytoncides influence human health by entering the body through breathing and stimulating measurable changes in the immune and nervous systems. The aromatic volatiles a person inhales in a forest are absorbed through the respiratory tract, where they trigger responses ranging from enhanced immune cell activity to reduced stress hormone levels. This connection between forest air and physiology is the foundation of a growing field of medical research.
Aromatic volatile substances and their effect on the body
Aromatic volatile substances from plants act on the human body both pharmacologically, through their absorbed chemistry, and psychologically, through the sense of smell. Inhaled tree-derived VOCs interact with the nervous system to shift the body toward a calmer, more restorative state, while their antimicrobial nature also reduces the microbial load of the air being breathed. This dual influence — on mood and on biology — is what makes forest air feel distinctly refreshing.
Tree essential oils and their therapeutic effect
Essential oils distilled from trees concentrate the same phytoncides found in forest air and produce comparable therapeutic effects. Hinoki cypress stem oil, eucalyptus, pine, and tea tree oils are widely used for their antimicrobial and calming properties. Experiments have shown that simply diffusing hinoki cypress oil in a room can raise the activity of immune cells in the people breathing it, mirroring the effect of a forest visit.
Influence on immune function and anti-cancer protein production
Phytoncides enhance immune function chiefly by boosting natural killer (NK) cells, the lymphocytes that destroy virus-infected and tumour cells. Exposure to phytoncides increases both the number and activity of NK cells and raises the intracellular levels of the anti-cancer proteins they use to kill targets — perforin, granulysin, and granzyme A/B. Research by Dr. Qing Li in Japan measured these biomarkers in human subjects and found that forest exposure produced significant, lasting increases in NK cell activity.
The clinical methodology behind these findings involved measuring NK cell activity, lymphocyte percentages, and anti-cancer protein levels in blood samples taken before and after phytoncide exposure. Studies showed that a single forest trip elevated NK activity, and that the boost could persist for more than a week, while repeated trips suggested long-term immune benefits. These results have positioned phytoncides as candidates for future immunotherapy applications.
Reducing stress: adrenaline and noradrenaline
Phytoncide exposure lowers the body's stress hormones, including adrenaline, noradrenaline, and cortisol. By stimulating the parasympathetic nervous system — the "rest and digest" branch — forest air shifts the body out of a state of stress arousal, reducing the stress hormones that, in chronic excess, suppress immune function. This drop in adrenaline and noradrenaline is one of the mechanisms linking time among trees to both calmer mood and stronger immunity.
Forest bathing (shinrin-yoku) and the restorative effect of the forest
Forest bathing, known in Japan as shinrin-yoku, is the practice of spending unhurried, mindful time in a forest to absorb its atmosphere through all the senses. The term was coined in Japan in the 1980s, and the practice has since become the focus of rigorous scientific research into health outcomes — much of it led by Dr. Qing Li. Studies link forest bathing to lower blood pressure, reduced stress, improved mood, better sleep quality, and the enhanced NK cell activity described above.
Forest bathing versus direct forest exposure
Forest bathing differs from ordinary forest exposure in its intentional, sensory focus rather than in mere physical presence among trees. Shinrin-yoku is not exercise or hiking toward a destination; it is slow, mindful immersion — pausing, breathing deliberately, and noticing sights, sounds, and scents. This unhurried quality is what allows the body to settle into a parasympathetic state and absorb the maximum benefit from the phytoncides in the air.
Sterilisation of the forest atmosphere and environmental effects
The volatile phytoncides released by trees partially sterilise the forest atmosphere, lowering the airborne microbial count beneath a dense canopy. This natural air purification is what underlies the long-recognised health-promoting properties of green spaces. Environmental conditions shape the effect: phytoncide levels in forest air rise in warm, humid, windless weather and in stands of conifers, and fall in cold or open conditions.
The health-promoting properties of green spaces
The health-promoting properties of green plantings are explained precisely by the action of phytoncides. Beyond forests, urban parks, gardens, and even indoor plants release defensive volatiles that improve the surrounding air. The conservation work of organisations such as the Adirondack Council to protect large forested wilderness areas helps preserve exactly the kind of phytoncide-rich environments that deliver these benefits.
Applications of phytoncides
Phytoncides are applied across medicine, aromatherapy, agriculture, and food preservation, all of which exploit their antimicrobial and insecticidal action. Because the compounds occur naturally and target a broad range of pathogens and pests, they offer alternatives to synthetic chemicals in several fields.
Medicine and the fight against infectious diseases
In medicine, phytoncides are used against contagious diseases thanks to their bactericidal reach. Garlic and onion phytoncides act against the agents of diphtheria and typhoid, while preparations from conifer needles serve as antiseptics. The growing crisis of antibiotic resistance has renewed medical interest in phytoncides, since some — including sophoraflavanone G from Sophora flavescens — show activity against MRSA and vancomycin-resistant Enterococcus, supporting ongoing research into new therapeutic applications.
Aromatherapy and folk medicine
Aromatherapy and folk medicine harness phytoncides through essential oils and plant preparations valued across many traditional healing systems. Diffused tree oils such as eucalyptus, pine, hinoki cypress, and tea tree are used to ease breathing, disinfect indoor air, and promote relaxation. These uses predate modern science but align closely with the documented antimicrobial and mood-balancing effects of the compounds involved.
Agriculture: protecting crops from pests
In agriculture, phytoncides serve as natural protectors of cultivated plants against pests and disease. Because wounded and infected plants release large quantities of phytoncides, these compounds — and plants rich in them — are used to suppress the pests of crops. Plants also use VOCs to signal to one another: when attacked by spider mites such as Tetranychus urticae, some plants emit volatiles that warn neighbours and even attract the mites' predators, a form of inter-species chemical defense that agriculture can exploit.
Preserving food during storage and transport
Phytoncides help preserve food during storage and shipping by suppressing the microbes that cause spoilage. The antibacterial volatiles of plants such as garlic, horseradish, and certain berries slow the growth of bacteria and fungi on stored produce. This natural preservative action offers a way to extend shelf life and protect food in transit without relying solely on synthetic additives.
Interaction of plants through phytoncides
Plants communicate and compete with one another through the phytoncides they release into the air and soil. The volatile substances of some plants act on other plants, helping or harming them, which is the basis of inter-species chemical signalling. Lima beans attacked by mites, for example, emit VOCs that prime nearby plants to mount their own defenses — evidence that phytoncides form a genuine chemical language among vegetation.
Which flowers should not be combined in bouquets
Certain flowers must not be combined in a bouquet because the volatile phytoncides of one species harm the other. Lilac and lily of the valley are a classic example — placed together, the volatiles of one cause the other to wilt prematurely. This everyday phenomenon is a direct demonstration of how phytoncides allow one plant to affect another at a distance.
How to use phytoncides for rest and recovery
The simplest way to use phytoncides for recovery is to spend regular, unhurried time among trees and to bring nature into daily routine. You do not need a wilderness — a city park, a tree-lined street, or even indoor plants will deliver some of the benefit. The goal is consistent exposure to plant volatiles combined with the calm, mindful state that lets the body respond to them.
- Walk slowly and breathe deliberately. Treat a forest walk as shinrin-yoku rather than exercise: pause often, take slow breaths, and let the scent of conifers fill the air you inhale.
- Choose coniferous woodland where possible. Fir, pine, spruce, and cedar stands emit the highest concentrations of volatile phytoncides, especially on warm, still days.
- Engage all the senses. Notice sounds, light, and texture as well as scent — this mindful focus is what shifts the nervous system into a restorative, parasympathetic state.
- Build restorative moments into a busy schedule. Short, frequent visits to green space help, and walking meetings outdoors are a practical workplace wellness strategy that combines movement, fresh air, and phytoncide exposure.
- Bring plants indoors. Indoor plants and diffused tree essential oils are a realistic alternative when access to a forest is limited, releasing defensive volatiles into the home or office.
Incorporating nature into daily life is increasingly treated as a measurable wellbeing strategy. Workplace wellbeing specialists such as Leanne Spencer advocate walking meetings and regular outdoor breaks, and tools like the Cadence Wellbeing Scorecard help track such habits — a structured way to ensure the restorative effect of phytoncides becomes a routine rather than a rare escape.
