Burning wood with lichen on it is generally safe for occasional use in a fireplace, wood stove, or campfire. The small amounts of lichen found on typical firewood don’t produce meaningfully different smoke than the wood itself. That said, lichen does carry some characteristics worth understanding, especially if you burn wood regularly or in enclosed spaces.
Why Lichen on Firewood Is Usually Fine
Lichen is a thin, slow-growing organism that coats bark surfaces without penetrating deeply into the wood. On a typical piece of firewood, you’re looking at a few grams of dried lichen mixed in with several pounds of wood. At that ratio, the lichen contributes almost nothing to the overall combustion. Most experienced wood burners treat it the same way they treat bark: it’s part of the package and not worth worrying about.
If you properly season your firewood (split, stacked off the ground, and covered on top for at least a year), the lichen dries out along with the wood. After two or three years of seasoning, much of it crumbles away or falls off on its own, particularly on split pieces. Unsplit rounds tend to hold onto lichen longer, but the drying process still kills the living organism and reduces it to a thin, brittle layer.
The Heavy Metal Factor
Lichen is one of nature’s most effective air pollution sponges. Because it absorbs nutrients and moisture directly from the atmosphere (it has no roots), it also absorbs whatever contaminants are floating in the air. Research on lichen used as pollution monitors shows it accumulates aluminum, chromium, copper, iron, nickel, lead, zinc, and mercury from its surroundings.
This matters because burning releases those accumulated metals into the air as part of the smoke. If your firewood comes from an area near highways, industrial sites, or heavily trafficked roads, the lichen on it may carry higher concentrations of these elements. For a backyard bonfire or occasional fireplace use, the quantities involved are tiny. But if you’re burning lichen-covered wood daily in an indoor stove with poor ventilation, the cumulative exposure adds a small, avoidable risk on top of normal wood smoke exposure. Ensuring good airflow and a properly functioning chimney or flue handles this effectively.
A Few Lichen Species Are Genuinely Toxic
Most of the lichen you’ll encounter on firewood is harmless. However, a handful of species produce compounds that are toxic even before burning. The most notable is wolf lichen, a bright chartreuse-yellow species common on conifer bark in western North America and Europe. Wolf lichen was historically used to poison wolves and was applied to arrowheads by some Native American tribes. Another species, ground lichen, was identified as the cause of death for 300 elk in Wyoming in 2004 and has also killed sheep and cattle.
These species are uncommon on typical firewood, but if you notice unusually bright yellow-green lichen on conifer wood, it’s worth brushing or peeling it off before burning. You can do this with a stiff brush or gloved hand once the wood is dry.
Cyanobacteria in Some Lichens
Lichen is actually two organisms living together: a fungus and either an alga or a cyanobacterium (sometimes both). The varieties that partner with cyanobacteria are sometimes called cyanolichens, and they raise a more specific concern. Some cyanobacteria produce a neurotoxin called BMAA that has been linked to neurodegenerative diseases. Research has confirmed that cyanotoxins can become airborne and that inhaling them is a plausible route of human exposure. One study found that BMAA travels along olfactory pathways to the brain after nasal exposure in mice, suggesting the nose and brain may be particularly vulnerable.
To be clear, this research focused on direct cyanobacterial exposure, not specifically on burning lichen. Whether combustion destroys BMAA or releases it intact in smoke hasn’t been conclusively established. But cyanolichens are a reasonable thing to minimize in your smoke if you can. Cyanolichens tend to be darker, often appearing in shades of dark brown, black, or dark gray, and are more common in wet, old-growth forest environments. The pale gray-green crusty lichens typical on stacked firewood are usually algal lichens and don’t contain cyanobacteria.
Practical Steps for Burning Lichen-Covered Wood
- Season your wood properly. Split it, stack it off the ground, and cover the top. After one to two years, most lichen dries out and falls away naturally.
- Scrape off heavy growth. If a piece of wood is thickly coated, knock the bulk of it off with a brush or the back of a hatchet before tossing it in. This is most relevant for brightly colored or unusually thick lichen.
- Ensure good ventilation. This applies to all wood burning, but especially when your fuel has surface organisms. A well-drawing chimney or an outdoor fire disperses smoke effectively.
- Consider the source. Firewood gathered near roads or industrial areas carries lichen with higher pollutant loads. Wood from rural forests is a cleaner bet.
- Don’t burn piles of pure lichen. Using lichen-covered wood is different from scooping lichen off rocks and throwing it on a fire. The dose makes the difference.
For the vast majority of people burning seasoned firewood with some lichen on the bark, there’s no meaningful added risk beyond what wood smoke already presents. The precautions above are simple, and most seasoned wood burners follow them without thinking about it.