Updated: February 2010
"Do you have a woodstove," the doctor asks as I sit, barelegged in my too-small hospital gown, and give the respirometer a feeble puff. It’s my second try and I beg for one more, surely I can do better.
“Woodstove? Yes - but it’s one of those new ones,” I answer defensively, “you know, with a catalytic converter.”
Not one of those smoke belching dinosaurs, I’d like to add, the kind that blackens the cobwebs and sends clouds of smoke throughout the neighborhood as did the one in our old rental.
But I’m in denial. I ought to know better. Burning wood is dirty, pure and simple. No matter how hot the stove, no matter the catalytic converter devoted to reducing our share of wood smoke.
Chemically wood is about fifty percent carbon and forty-five percent oxygen, some hydrogen (around 6%) with a dash of nitrogen and assorted elements such as calcium, potassium and magnesium. That means that a cord of maple wood, roughly the amount we burn each winter, which weighs around 4,000 pounds, depending on how dry it is, contains roughly 4,000 pounds of carbon, oxygen and hydrogen. But, once we stuff the old pizza boxes, the Sunday Times, a little kindling from my husband’s workshop and add a few matches all that is neatly bound up in those logs up will be transformed into heat, light, gas and particles large and small. Some of those particles will end up in the ash pile at the bottom of our stove, and some, along with a mixture of hot gases will flow up the chimney and into the air. Technically, our little stove should release no more than 4 grams of particulates into the air per hour – a tenth of what stoves used to emit before the EPA stepped in. But is that good enough?
Even though we’re talking as little as four grams an hour (and upwards of 30 grams over a day), it is primarily those small particles which concern my doctor. As our wood burns, no matter how efficient or tight our stove, particulates and gases will leak out – if not into our home then up and out our chimney into the neighborhood, mingling with all of our neighbor’s wood gases and aromatic (in more ways than one) wood smoke.
The lovely smoky aroma that comes with wood burning not only indicates the return of crisp fall weather – but the slew of airborne chemicals from carcinogenic polyaromatics to volatile organic carbons (VOCs) to gases like carbon dioxide (the major gas), carbon monoxide and methane – and minerals like potassium, wafting around our "fresh country air." (I say this with some irony as our semi-rural valley sees its share of air pollutants hailing from NYC. And, depending on the weather, can have some of the worst air in the state, particularly in the summer.)
There are also very small bits of carbon in our wood smoke, known as particulate organic carbon, which make up in large part the particulate material or PM, released when wood is burned.
As with any science, the science of air pollutants like wood smoke evolves over time. What’s known to be released into the air when wood burns, and how much, is refined as technology allows scientists to measure increasingly smaller amounts and sizes of pollutants, as are the dangers of exposure to such pollutants.
The old adage you can’t condemn what you can’t measure (or something like that) often accounts for the all too common phenomenon of the dropping baseline in toxicology. The baseline being what was once considered “safe” or acceptable concentrations of exposure. Think lead, mercury, and radioactive chemicals like strontium and plutonium. All chemicals once treated more cavalierly, back in the day, than they are now. And all chemicals for which “allowable” concentrations have continued to decline over the decades.
When EPA first regulated particulates in 1987, they focused on PM10, or particulates 10 microns and smaller. Subsequent studies suggested that the much smaller particles were likely more dangerous, leading EPA to regulate PM2.5, (particulates that are 2.5 microns) nearly 10 years later in 1997. Flash forward nearly another ten years, and further concerns about these small particulates, caused EPA to reduce the acceptable amount of PM2.5 exposures in a 24 hour period by almost half. And, as technology provides scientists with the tools to study smaller and smaller particles, the studies that led to reductions in PM2.5 are being supplanted by studies revealing the toxicological importance of smaller and smaller particles. Some studies suggest that the majority or peak size of particulates released by wood smoke range from 0.15 to 0.4 microns – a few hundred nanometers in size.
Not only are researchers figuring out that bigger is sometimes better (much like FOX television which offered up a new Plus-sized reality show “More to Love,”) they’re also realizing that mass or weight isn’t everything.
The current U.S. EPA standard for PM2.5 considers only the combined mass, essentially the combined weight, of these little particles. Not the chemical composition nor the number of particles, nor the relative size of the particles. As scientists well know by now (or ought to) when it comes to very little things – like chemicals in the nanometer range (which include some of these particulates) – size does matter.
Typically the smaller things get the more surface area they have. Think about peeling a pound of granny smith apples, and a pound of crab apples. Which would you rather peel? More apple skin, more surface area on those little crab apples. Same with particulates. As these little particulates get smaller, they reveal more surface area. Same amount of mass but more area to react with a body’s cellular surfaces. Typically, the more reactive a particle, the more toxic it tends to be.
In fact, scientists are now linking the smallest of the small particulates, the ultrafine particles (particles smaller 100 nanometers in size) which comprise the smaller end of PM2.5 with a range of adverse health effects including asthma, chromosomal damage and cardiovascular effects linked to inhalation of particulate matter.
Using the woodstove is one of those lifestyle choices we make every day. As I swear up and down that my asthma tends to worsen with the leaf-mold season rather than wood smoke (although admittedly the two coincide - so who's to say) we rationalize that for each cord of the old maple that fell into our yard years ago, we avoid burning the imported fuel oil sitting below in our basement tank. Besides, we’re only burning a cord or two a year – and although could same can be said for our neighbors on either side, down the street and around the block, at least we’re not burning five hundred thousand tons of wood as proposed by Pioneer “Renewable” Energy....right? But that’s a story for another day.
For a good review of several recent studies on ultrafine particles check out Janet Raloff’s “Bad Breath.”
If you’d prefer primary literature, you can read all about it in Environmental Health Perspectives: