Burning wood produces carbon dioxide, water vapor, carbon monoxide, thousands of volatile organic compounds, fine soot and tar particles, and a mineral-rich ash. The exact mix shifts dramatically depending on the fire’s temperature, its oxygen supply, and the moisture and species of the wood itself, which is why the smoke from a roaring bonfire looks and smells nothing like the haze drifting off a dying campfire.
The Main Gases
The biggest products by mass are carbon dioxide and water vapor. Wood is roughly half carbon, with the remainder a mix of oxygen and hydrogen locked inside cellulose, hemicellulose, and lignin, plus trace minerals. When fire supplies enough heat and oxygen, those carbon atoms combine with oxygen to form CO2, and the hydrogen atoms combine with oxygen to form water vapor. The oxygen already built into wood’s molecular structure contributes to these reactions, which is part of why wood contains roughly half the energy of fossil fuels like coal or oil per kilogram.1Biomass and Bioenergy. Review Wood would burn
Carbon monoxide is the third major gas. It forms whenever combustion is incomplete, meaning some carbon atoms grab only one oxygen atom instead of two. In a well-ventilated, hot fire, most CO gets converted to CO2 before leaving the flame zone. In a poorly ventilated or low-temperature fire, CO levels climb steeply, which is why running a wood stove with restricted airflow in an unvented room can be lethal. Smaller amounts of methane, nitrogen oxides, and other light gases also escape, but CO2, water vapor, and CO account for the bulk of the gaseous output.
Volatile Organic Compounds
As wood heats up, rising temperatures break apart the large molecules in cellulose, hemicellulose, and lignin before full ignition even begins. The fragments that escape as gases are collectively called volatile organic compounds, and their number and concentration jump sharply once temperatures reach around 250 °C.2PubMed Central. Volatile Organic Compounds Arising from Wood Polymers on Thermal Loading of Spruce Wood The type of treatment applied to wood also changes the VOC profile, but even clean, untreated lumber gives off a complex cocktail of organic chemicals at these temperatures.
Among the most watched VOCs are polycyclic aromatic hydrocarbons, or PAHs, ring-shaped carbon molecules many of which are known carcinogens. During wood combustion, the proportion of heavier, more complex PAHs increases as temperature rises. This is because chemical synthesis reactions in the flame zone build bigger ring structures at higher temperatures, a pattern distinct from what happens in coal fires.3Atmospheric Environment. Different formation mechanisms of PAH during wood and coal combustion under different temperatures
One compound deserves special mention: levoglucosan. It forms when cellulose breaks down and is so specific to burning plant material that atmospheric scientists use it as a chemical fingerprint to track wood smoke in air quality studies.4PubMed Central. Levoglucosan and Its Isomers as Markers and Biomarkers of Exposure to Wood Burning When air-quality researchers detect levoglucosan in particulate samples, they can estimate how much of the local pollution comes from biomass burning rather than from vehicle exhaust or factory emissions.5PubMed. Validation of a HILIC/ESI-MS/MS method for the wood burning marker levoglucosan and its isomers in airborne particulate matter
Smoke Particles and Soot
The visible part of wood smoke is airborne particulate matter: a mix of black carbon (soot), condensed tars, and organic droplets. The size, shape, and chemistry of these particles vary enormously depending on how the fire is burning. Combustion appliances in use today create highly variable conditions, which means the particles coming out of one fireplace can be chemically quite different from those produced by another burning the same fuel.6PubMed Central. Health effects of residential wood smoke particles: the importance of combustion conditions and physicochemical particle properties
At a structural level, flaming fires produce soot clumps built from roughly a thousand very small primary particles, each around 0.025 to 0.035 micrometers across. Smoldering fires work differently: their clumps contain far fewer primary particles, around a hundred, but each individual particle is larger, about 0.07 micrometers.7Heat Transfer: Volume 2. Fractal Properties of Smoke Produced From Smoldering and Flaming Fires Particle size determines how deep into your lungs smoke can travel, and the optical properties of the two types also differ, changing how they absorb and scatter sunlight once they enter the atmosphere.
How Flaming and Smoldering Change the Output
A single wood fire typically passes through both flaming and smoldering phases, and the chemistry of the output shifts with each transition. During flaming combustion, temperatures are high and volatile gases ignite as they leave the wood surface. This burns off a large portion of the organic compounds before they can escape, so the smoke carries less total mass per kilogram of fuel consumed. But what does escape tends to be more mutagenic and more toxic to lung tissue on a per-mass basis.8PubMed Central. Mutagenicity and Lung Toxicity of Smoldering vs. Flaming Emissions from Various Biomass Fuels: Implications for Health Effects from Wildland Fires
Smoldering is the opposite trade-off. The fire is cooler, oxygen supply is limited, and combustion is far less complete. Per kilogram of wood consumed, smoldering produces much more total smoke, more carbon monoxide, and more partially burned organic compounds. On a per-mass-of-particles basis the toxicity may be lower, but the sheer volume of emissions can push the overall health burden higher.
Moisture is one of the biggest levers on this balance. Wet wood forces the fire to spend energy evaporating water before the wood can reach combustion temperatures, pushing the burn toward smoldering conditions. Burning wet fuel results in lower combustion efficiency and higher emissions overall.9PubMed. Effect of low-density polyethylene on smoke emissions from burning of simulated debris piles This is why seasoned (dry) firewood produces noticeably less smoke and creosote than freshly cut logs: more of the fuel’s energy actually goes into heat rather than into half-burned byproducts.
What Ash Is Made Of
Everything that does not vaporize stays behind as ash, the mineral skeleton of the original tree. Ash is predominantly inorganic: calcium, potassium, magnesium, and phosphorus compounds dominate. Mineral analysis of wood ash from domestic furnaces shows high proportions of calcite (calcium carbonate), along with monetite, fairchildite, and quartz.10International Journal of Environmental Science and Technology. Chemical and mineral composition of ashes from wood biomass combustion in domestic wood-fired furnaces
The chemistry of ash shifts with burning temperature. Potassium, sodium, and zinc content decrease at higher temperatures because those elements are volatile enough to escape with the smoke. Other metals tend to stay put or concentrate as surrounding material burns away. When ash meets water, the resulting solution is strongly alkaline, about 92% hydroxide, which is why wood ash has been used for centuries to make lye for soap and to condition acidic garden soil.11Bioresource Technology. Physical and chemical characteristics of wood ash
Hardwood Versus Softwood
Not all wood burns the same way, and the differences show up in the smoke. Hardwoods like oak, maple, and birch and softwoods like pine, spruce, and fir have different proportions of cellulose, hemicellulose, and lignin, and their lignin types differ structurally. When lignin breaks down during burning, phenolic compounds are the most abundant chemical group in the resulting smoke. Both hardwood and softwood lignin produce phenols, but the specific phenol mix differs: hardwood lignin contains a higher proportion of units with two methoxy groups on the aromatic ring, while softwood lignin leans toward one.12Energies. Characterization of Fast Pyrolysis Bio-Oil from Hardwood and Softwood Lignin
In practical terms, this is partly why the aroma of burning oak smells different from burning pine, and it is one reason barbecue pitmasters are picky about their wood species. Softwoods also tend to be richer in resin, which burns with more visible soot and deposits more creosote in chimney flues. Hardwoods are generally denser, meaning they burn hotter and longer per log, delivering somewhat less smoke per unit of useful heat.
How Stove Design Changes What You Breathe
The technology you burn wood in can change the emissions picture by an order of magnitude. A study testing different residential stove types found that an older uncertified cordwood stove emitted about 14.6 grams of particulate matter per kilogram of wood burned. A modern non-catalytic stove cut that roughly in half. A catalytic or hybrid stove dropped it to about 3.4 grams per kilogram, and a pellet stove was the cleanest at under 1 gram per kilogram, roughly 17 times less particulate than the uncertified unit.13PubMed Central. Criteria, Greenhouse Gas, and Hazardous Air Pollutant Emissions Factors from Residential Cordwood and Pellet Stoves Using an Integrated Duty Cycle Test Protocol
Catalysts make a particularly dramatic difference for specific pollutants. A residential stove catalyst was shown to remove about 87% of carbon monoxide and 91% of polycyclic aromatic hydrocarbons from the exhaust stream.14PubMed Central. Abatement of Emissions of a Residential Wood Stove: Effect of the Catalyst on Gaseous and Condensable Pollutants Concentration and Their Toxicity The catalyst provides a surface where incompletely burned gases can finish their reactions at lower temperatures than an open flame would require. If you heat your home with wood, upgrading to a certified catalytic or pellet stove is one of the most effective single changes you can make for both indoor and outdoor air quality.
What Wood Smoke Does to Your Cardiovascular System
Wood smoke is not just an irritant. It carries measurable cardiovascular effects even at relatively brief exposures. In a controlled experiment, healthy volunteers who breathed wood smoke for a few hours showed increased arterial stiffness, reduced heart rate variability, and an elevated heart rate compared to sessions when they breathed filtered air.15PubMed Central. Exposure to wood smoke increases arterial stiffness and decreases heart rate variability in humans These are the same kinds of changes that predict higher risk of heart attack and stroke over time.
In communities where wood is the primary cooking fuel, the effects are chronic and can partly reverse with intervention. A study among women in Guatemala found that switching from open wood fires to improved chimney stoves reduced signs of cardiac electrical stress on electrocardiograms.16PubMed Central. Intervention to lower household wood smoke exposure in Guatemala reduces ST-segment depression on electrocardiograms The takeaway is straightforward: the combustion products from wood are not benign just because the fuel is natural. Reducing exposure matters for heart health, and the stove or fire design is one of the main tools for achieving that.
Brown Carbon and the Atmosphere
Once wood smoke enters the atmosphere, it does more than degrade local air quality. The organic carbon in smoke absorbs sunlight, especially in shorter wavelengths near the blue and ultraviolet end of the spectrum, which is partly why thick biomass smoke can look brownish. Atmospheric scientists call this light-absorbing organic fraction “brown carbon” to distinguish it from black carbon (pure soot), which absorbs across all wavelengths.
How much brown carbon a fire produces depends on both the fuel type and the burn conditions. Research on prescribed burns and laboratory fires found that the light-absorbing properties of the organic carbon were strongly influenced by fuel species, and the relationship between brown carbon and the ratio of soot to organic carbon held within individual fuel types but broke down when different fuels were lumped together.17PubMed Central. Light-absorbing organic carbon from prescribed and laboratory biomass burning and gasoline vehicle emissions Brown carbon complicates climate models because it warms the atmosphere by absorbing sunlight but can also change how reflective clouds and other aerosols are. The net climate impact of wood smoke is still being worked out; it is not simply a warming or cooling agent but both, in proportions that shift with the fire and the sky.
Why You Should Never Burn Treated Wood
Everything discussed so far assumes you are burning natural, untreated wood. Treated lumber is a different beast entirely. Wood preserved with chromated copper arsenate, widely used in decking and outdoor structures before regulatory restrictions tightened, releases arsenic when burned. Between 11 and 14% of the arsenic present in CCA-treated wood escapes into the air as fine, respirable particles, while less than 1% of the chromium and copper does.18PubMed. Emissions of chromium, copper, arsenic, and PCDDs/Fs from open burning of CCA-treated wood People who burn construction timber in their home wood burners show higher arsenic levels in their hair than those who burn only logs or coal.19PubMed Central. Arsenic in Hair as a Marker of Exposure to Smoke from the Burning of Treated Wood in Domestic Wood Burners
The problem goes beyond older CCA-treated lumber. Even newer “arsenic-free” wood preservatives that use organic biocides like tebuconazole or permethrin can produce extreme levels of dioxins and furans when burned. In laboratory tests, wood treated with combinations of organic and copper-based preservatives generated dioxin yields exceeding those from untreated wood by roughly a factor of a thousand.20PubMed. Formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) in fires of arsenic-free treated wood: role of organic preservatives
Dioxins also form, in far smaller quantities, when completely untreated wood burns. Wood naturally contains trace chlorine, and wherever carbon, chlorine, and heat come together, dioxin formation is essentially unavoidable. The amounts from clean firewood are low enough to be a minor concern in a well-maintained stove, but the point stands: dioxin emissions from wood combustion can be minimized but never entirely eliminated.21Biomass and Bioenergy. Dioxin levels in wood combustion—a review This is why burning scrap lumber, painted wood, or demolition timber in a backyard fire or home stove is a genuinely bad idea — you have no way of knowing what preservatives or coatings were applied, and the combustion products can be orders of magnitude more hazardous than anything clean firewood would produce.
Biochar and Incomplete Burning on Purpose
When wood is heated in conditions with little or no oxygen, it does not fully combust. Instead of converting all the carbon into CO2, this process (pyrolysis) leaves behind a carbon-rich solid called biochar. It is essentially charcoal made with modern precision: controlled temperatures, controlled atmosphere, and a consistent product.22Journal of Agriculture and Food Research. Biochar application: A sustainable approach to improve soil health
Biochar is interesting because it flips the carbon equation. Normal combustion sends wood’s carbon into the atmosphere as CO2, where it contributes to warming. Pyrolysis locks a large fraction of that carbon into a solid form that resists decomposition for centuries. When mixed into soil, biochar can improve water retention, nutrient availability, and microbial activity. The same chemistry that produces dangerous smoke when burning goes wrong can, under controlled conditions, produce a stable soil amendment and a form of long-term carbon storage. Several startup companies and agricultural programs are now scaling biochar production as a way to pull carbon out of the cycle altogether, turning a waste product into a climate tool.