California Air Resources Board

Burning Wood Produces Wood Smoke and Air Pollution

CalEPA and your local air district are asking you to help clear the air of wood smoke. In this handbook you will find information about the air pollutants in wood smoke, health effects of smoke, how wood burns, why it smokes and how you can reduce wood smoke pollution.

Smoke from neighborhood stoves and fireplaces, a common source of both odor and reduced visibility, greatly contributes to the air pollution problems people complain about most. When you include the health-related problems caused by inhaling smoke pollutants, health costs for individuals and the community can be significant. To be a good neighbor, eliminate wood burning. If you do burn, learn to limit the amount of wood smoke produced.

Sources of Wood Burning and Air Pollution

Air pollution affects millions of Californians every day. It damages our health, our crops, our property and our environment. In neighborhoods everywhere across California, residential wood burning is a growing source of air pollution. Most wood heaters, such as woodstoves and fireplaces, release far more air pollution, indoors and out, than heaters using other fuels. In winter, when we heat our homes the most, cold nights with little wind cause smoke and air pollutants to remain stagnate at ground level for long periods.

Burning Wood Causes Indoor Air Pollution

High levels of smoke pollutants leaking from stoves and fireplaces have been measured in some wood burning homes. If you or family members suffer from chronic or repeated respiratory problems like asthma or emphysema, or have heart disease, you should not burn wood at all. If you must burn wood, make sure your stove or fireplace doesn't leak and that you operate it correctly.

Remember - If you can smell smoke, you are breathing smoke.

What Happens When Wood Burns?

Complete combustion gives off light, heat, and the gases carbon dioxide and water vapor. Because when wood burns complete combustion does not occur, it also produces wood smoke, which contains the following major air pollutions, regulated by State and federal rules because of their known health effects:

• Carbon monoxide (CO): An odorless, colorless gas, produced in large amounts by burning wood with insufficient air. CO reduces the blood’s ability to supply oxygen to body tissues, and can cause stress on your heart and reduce your ability to exercise. Exposure to CO can cause long-term health problems, dizziness, confusion, severe headache, unconsciousness and even death. Those most at risk from CO poisoning are the unborn child, and people with anemia, heart, circulatory or lung disease.
• Oxides of nitrogen (NO_x): NO_x impairs the respiratory system and its ability to fight infection. NO_x also combines with VOCs to make ozone and with water vapor to form acid rain or acid fog.
• Volatile organic compounds (VOCs): Evaporated carbon compounds which react with NO_x in sunlight to form ozone (photochemical smog). Ozone injures the lungs and makes breathing difficult, especially in children and exercising adults. NO_x and VOCs also form particulate matter through reactions in the atmosphere.
• Toxic pollutants: Wood smoke also contains VOCs which include toxic and/or cancer-causing substances, such as benzene, formaldehyde and benzo-a-pyrene, a polycyclic aromatic hydrocarbon (PAH). Manufactured fireplace logs, for instance, are not recommended for burning because they produce toxic fumes, including PCBs (polychlorinated biphenyls). Researchers are now studying these and other smoke products to learn more about their effects on human health.
• Particulate matter less than 10 microns in diameter (PM10) are very small droplets of condensed organic vapors of wood tar and gases. These particles are a result of unburned fuel and have a diameter of 10 microns or smaller (the diameter of a human hair is about 50 to 100 microns), which allows them to be inhaled into the lungs. Exposure to PM10 aggravates a number of respiratory illnesses. PM10 includes a smaller group of particles called PM2.5, particles with diameters of 2.5 microns and less. These finer particles pose an increased health risk because they can lodge deep in the lungs and contain substances that are particularly harmful to human health, contributing to lung diseases and cancer. Exposure to PM2.5 may even cause early death in people with existing heart and lung disease.

Fireplaces and Old Woodstoves are Inefficient, Expensive Heaters

Why? Because of the way wood burns - As the fire temperature rises, different stages occur:

• Stage 1 - Water Boils Off: As the log heats, moisture contained in the log vaporizes, and escapes through the log's surface as water vapor. More energy is used up vaporizing the moisture than is used to burn the log. That heat energy could be warming your house instead of drying your wood before it burns.
• Stage 2 - Vaporizes Wood Gases: Before burning, firewood "cooking" creates and releases hundreds of new volatile organic gases, which contain VOCs, tars and charcoal or carbon. Because the log temperature at this stage is too low to burn gases and tars, they escape up the flue. As they cool, some of the gases will combine with water vapor to form highly flammable creosote that sticks to the flue walls; other gases condense into smoke particles.
• Stage 3 - Log Charcoal Burns: At temperatures above 600 degrees Fahrenheit the escaping gases start burning, ignited by nearby flames. As the temperature reaches 1000 degrees, the log charcoal burns and emits heat. Burning the charcoal produces most of the fire's usable heat.

As you can see, most of your investment in wood goes up in smoke. This is an expensive way to produce a little heat.

Most Fireplaces are Not Good Heaters

Most fireplaces rob your house of heat because they draw air from the room and send it up the chimney! Yes, you'll be warmed if you sit within six feet of the fire, but the rest of
your house is getting colder as outdoor air leaks in to replace the hot air going up the chimney.

The key to burning clean and hot is to control the airflow. Most fireplaces waste wood because of unrestricted airflow. A lot of air helps the fire burn fast, but a load of wood will last only one or two hours.

Some older fireplaces actually pollute more if you install glass doors on an old fireplace insert that is not a certified clean-burning model. Restricting the air supply causes the fire to smolder and smoke. Make sure you install a new, certified clean-burning fireplace insert.

Where Does Your Heat Go? Check your Insulation and Weather-Stripping

Warm air is always escaping from your house, and is replaced by unheated outdoor air. The typical house has one-half to two air exchanges per hour, and more on windy and/or very cold days. If your house has little insulation and many air leaks, you are paying to heat the outdoors. And if the outside air is smoky, soon your air inside will be too.

Some air exchange is necessary because of the many sources of air pollution in the home (wood heater, gas stove, consumer products, cigarettes, etc.) Sufficient fresh air inlets are needed to replace air forced out of the house by exhaust fans, dryers, furnaces, water heaters, or wood fires.

Here are some suggestions to minimize excess air exchange:

• Install ceiling insulation. When hot air rises, much of the heat is lost through the ceiling and roof. Wall and floor insulation also reduce heat loss. Recommended amounts of insulation have increased in recent years, so be sure your house has all it needs.
• Caulk around all windows, doors, pipes, and any opening into the house.
• Weather-strip all door and window openings. Consider installing double-paned glass, outdoor or indoor storm windows, and/or insulated curtains.
• Close the damper tightly when the heater is not in use. Stoves and fireplaces allow air to leak out of the house even when they are not operating, unless they are literally airtight.
• Close off unused rooms if you do not use central heating – Don’t waste the heat!

Clean up your Air Guzzling Fireplace by Trying Alternate Heating Methods

Check for the latest from the CARB Residential Woodsmoke Reduction Program.

Use an electric fireplace: Electric fireplaces can be installed anywhere, and no vent is required. They can be plugged into any standard household electrical (120V) outlet and can operate with or without heat. Most fireplaces are made with an adjustable thermostat that maintains room temperatures. The fireplace glass does not absorb heat, so is safe to touch whether or not the heater is operating.

If you Still Must Burn Wood, Follow These Tips on Clean Burning – To Heat More Efficiently and Reduce Air Pollution

• Start your fire with softwood kindling. Softwoods (pine, fir) are generally low in density, ignite easily, burn fast and hot and will heat the firebox and flue quickly. They are ideal for kindling and starting your fires, but form creosote easily due to the high resin (sap) content.
• Burn longer and cleaner with hardwood. Hardwoods (oak, cherry) are denser and take longer to ignite, but burn slower and more evenly, producing less smoke. They also provide more heat energy than softwood logs of the same size.
• Burn only "seasoned" firewood. Firewood should dry, or "season" a minimum of 6 to 12 months after splitting. Hardwoods dry more slowly than softwoods and may take over a year to dry. Seasoned firewood by definition contains 20 percent moisture or less by weight. Wood dries faster in a warmer storage area with more air circulation.
• Speed drying with split and stack. Logs dry from the outside in, so split big logs right away for faster drying. Stack loosely in a crosswise fashion to get good air circulation. Store high & dry. Stack a foot or more above the ground and away from buildings in a sunny, well-ventilated area. Cover the top to keep dew and rain off the wood, but leave the sides open to breezes.
• Be careful when buying wood advertised as "seasoned". Look for:
  • Dark colored, cracked ends, with cracks radiating from the center like bicycle spokes.
  • Light in weight, meaning there is little moisture left; hardwood logs will weigh more than softwood.
  • Sound - Hit two pieces together. Wet wood makes a dull "thud" sound. Dry wood rings with a resonant "crack," like a bat hitting a baseball.
  • Easily peeled or broken bark. No green should show under the bark.
• Build a small, hot fire first:
  • Open damper wide - allow in maximum air to fuel the fire. And leave it and other air inlets open for 30 minutes.
  • Start small and hot - leave a thin layer of ash for insulation. Crumple a few sheets of newspaper and add some small pieces of kindling, then light. Add bigger kindling a few at a time as the fire grows. Get it burning briskly to form a bed of hot coals. Now add 2 or 3 logs.
  • Position the next logs carefully - place logs close enough together to keep each other hot, but far apart enough to let sufficient air (oxygen) move between them.
• Refuel while the coals are still hot: If a fireplace insert or glass door is present, open it slightly for a minute to prevent back puffing of smoke into the room. When smoke subsides, then open the door fully. Preheat again by placing a few pieces of kindling onto the red-hot coals. Add more as they catch fire, then add a few larger pieces. Small, frequent loading causes less smoke than a big load in most older stoves. After refueling, leave the dampers and inlets open for about 30 minutes. The fire will get plenty of air and burn hot, retarding creosote formation (which forms early in a burn).
• Light and refuel your fire quickly and carefully. These are the times it will smoke the most.
• Don’t burn anything but clean, seasoned wood, fireplace logs, and non-glossy white paper. Burning these materials can produce noxious, corrosive smoke and fumes that may be toxic. They can foul your catalytic combustor, your flue, and the lungs of your family and neighbors.
  • No garbage - No plastics
  • No rubber - No waste
  • No particleboard - No plywood
  • No glossy paper - No colored paper
  • No solvent or paint - No oil
  • No coal or charcoal - No painted/treated wood
  • Warning: Kiln-dried lumber vaporizes too rapidly, causing creosote buildup.
• Overnight heating: When using an open fireplace, Do not burn overnight unattended - it's a major fire hazard. This can also lead to a back draft of the smoke into your own home, causing very hazardous indoor air pollution. Build a small, hot fire and let it burn out completely. Rely on your home's insulation to hold in enough heat for the night. When the fire is out, close the damper tightly.
• Heating in warmer weather: If you do need extra heat in warmer weather, and a small space heater will not suffice, open the air controls wide, build a small, hot fire, using more finely split wood, and let it burn out. Do not try to reduce the heat from a big fire by reducing its air supply because this leads to smoldering, creosote buildup and air pollution.
• Maintain your fire properly and watch the temperature
  • Do not close the damper or air inlets too tightly - The fire will smoke from lack of air.
  • Follow the Wood Stove or Fireplace Manufacturer's Instructions Carefully - Be sure that anyone who operates it is also familiar with these instructions.
  • Your Actions Determine How Efficiently Your Fireplace or Wood Stove Will Operate - A good wood stove/fireplace is designed to burn cleanly and efficiently, but it can not do its job right if you do not cooperate.
• Watch for smoke signals: Get into the habit of glancing out at your chimney top every so often. Apart from the half hour after lighting and refueling, a properly burning fire should give off only a thin wisp of white steam. If you see smoke, adjust your dampers or air inlets to let in more air. The darker the smoke, the more pollutants it contains and the more fuel is being wasted.
• Inspection and upkeep - For safety’s sake: Periodic inspection of your wood stove or fireplace is essential to ensuring its continued safe and clean-burning operation. Keep in mind the following points when performing your fireplace inspection:
  • Chimney caps can be plugged by debris, which will reduce draft.
  • Chimneys should be cleaned professionally at least once a year to remove creosote buildup. Remember – creosote can fuel a chimney fire that can burn down your house!
  • Catalytic combustor holes can plug up; follow instructions to clean.
  • Stovepipe angles and bolts are particularly subject to corrosion.
  • Gaskets on airtight stove doors need replacement every few years.
  • Seams on stoves sealed with furnace cement may leak. Eventually the cement dries out, becomes brittle, and may fall out.
  • Firebricks may be broken or missing.
  • Grates or stove bottoms can crack or break.

Resources

• What are combustion pollutants that can impact indoor air quality and how do they affect health?
• What you can do to reduce combustion pollutants in your home?