Walk Softly – Why are forest fires so stinky?

by Geoffrey Carpentier

It’s not hard to imagine why we might be speaking about forest fires right now. Our skies are clouded with smoke which has travelled thousands of kilometres, homesteads in the north are gone or threatened and people displaced to cities and towns far from home. I think we understand why our skies are smoke filled, as it’s relatively simple – lots of fires, lots of smoke and winds which have driven them both towards us.

The other day, as my wife and I stood on our deck, looking at the eerie gray-orange sunrise, we could smell the strong burnt wood odour, but it seemed ‘off’. Why didn’t it just smell like a campfire? It made me think, why do forest fires smell the way they do?

Let’s start at the beginning – a traditional campfire relies on the natural breakdown of the components of the wood and logs we burn for heat or cooking. When wood, which is composed of cellulose and other complex organic molecules, is heated, its inherent moisture vaporizes first. As the fire burns, combustible gases (Volatile Organic Compounds (VOCs)) are released into the atmosphere through the process of pyrolysis. The smoke we see, coming from our fire, is really the result of those volatile gases cooling off and condensing before they can fully combust. In essence smoke is simply condensed, unburned gases and particulates. As the fire continues to burn and heat up, embers become apparent at its base. These are the result of the bonding of carbon and oxygen into carbon monoxide (if oxygen is lacking) or carbon dioxide (if oxygen is available). Finally, the ash, evident in the fire pit once the fire is done, is really a mixture of non-combustible inorganic minerals (e.g. potassium, calcium and magnesium). Any unburned carbon is left behind as soot or charcoal. So, there you have it – a fire is a truly complex process.

Smoke from wildfires, however, is not a single, uniform substance. Its makeup depends on what is burning, how hot the fire is, whether it is flaming or smoldering, the humidity and how far the smoke has traveled. As the smoke moves, sunlight and atmospheric oxidants can change the nature of the smoke, by breaking down some compounds while creating other compounds.

Okay, so now let’s add the complexity of forest fires near urban settlements into the mix. In natural uncontaminated systems, it’s just a matter of scale – it’s no longer a campfire but now a massive fire covering vast areas. But really, they’re not that different. Where the difference comes in is, when houses, cars, boats, industrial buildings and other man-made products burn. The synthetic materials (e.g. vinyl siding, polyurethane foam, ABS plastics, electronics, rubber tires, treated lumber, insulation, carpeting, etc.) in these items release various contaminants and therefore smell distinctly different from wood smoke. Depending on the heat generated, many nasty compounds can be released into the environment. These can include VOCs (e.g. benzene and formaldehyde), aldehydes, nitrates (NO₃), heavy metals, Polycyclic Aromatic Hydrocarbons (PACs), and dioxins & furans. Many of these are not only odorous but dangerous, from an environmental and health perspective. To complicate this a bit more, sunlight reacting with smoke causes secondary organic aerosols and ozone to be released into the atmosphere. This means smoke which has traveled hundreds of kilometers, can contain higher levels of certain irritating chemicals than it did at the source. This is certainly a toxic soup!

Several of these compounds, when heated, have sharp, unmistakable odours. Formaldehyde smells pungent; acrolein is intensely irritating, even at low concentration; and styrene, which gives off gasses from burning polystyrene and other plastics, has a sweet but synthetic smell. These combustion products have no natural equivalent, so the resulting smell is immediately recognizable as artificial.

According to AccuWeather, on smoke-impacted days, air monitoring stations consistently detect spikes in hazardous compounds. Formaldehyde concentrations jump by 46%, compared to clean-air days, acetaldehyde rises about 36%, and acrolein (a potent irritant) increases around 34%.

What we experience when we smell the smoke from wildfires generally reminds us of burnt plastics or chemicals, with a smoky base. Okay, so there you have it – it’s not your imagination – forest fires really can be stinky and nasty!

Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line at www.avocetnatureservices.com and on LinkedIn and Facebook.