BACKDRAFTING YOUR LAST GASP

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To be sure, even the higher estimates of fatalities are not decimating the population as effectively as, say, the automobile or religious wars. At least not yet. What does cause alarm is that the trend is upward—steeply.

Where Are Pollutants Coming From?

If the only indoor source of combustiongenerated pollutants was the outdoors, you wouldn't be reading this article. Unfortunately, most households don't have to go shopping for combustion products, since they are built around their own pollution sources. Carbon-based fuels such as natural gas, fuel oil, coal, wood, kerosene, alcohol, tobacco—practically every substance that burns other than hydrogen—leave behind a mixture of carbon dioxide, carbon monoxide, nitrogen oxides, particles, volatile organic compounds and water when they burn. A short list of common residential refineries includes furnaces; gas water heaters; gas, oil and kerosene space heaters; gas ranges and ovens; woodstoves; and fireplaces.

Among the many noxious products these appliances produce, carbon monoxide (CO) is of the most immediate concern. This deadly gas (see the sidebar for more about its workings) is the main reason that most combustion devices have chimneys. (Unvented gas appliances and kerosene space heaters produce very little CO when they're working right and are operated correctly, although the long-term health effects of even small amounts of the pollutant are uncertain.) Indeed, as long as chimneys and burners work as designed, people 'don't die of carbon monoxide poisoning in their own homes. But chimneys and appliances don't always work right, and people are dying. Many more are sick.

Why Is CO Staying in Houses?

One of the first questions that comes to mind when one learns of the increase in the incidence of carbon monoxide poisoning is: Why now? Houses have had furnaces for more than a century and fireplaces for most of a millennium. What has changed?

As you'll soon learn, there are many factors that can conspire to foul indoor air, but the root of the problem—the basic design rule that has changed—is that modern houses are prone to operate at lower air pressure than older ones. Often, the air pressure indoors is significantly lower than that outdoors, and if the level of depressurization exceeds the strength of chimney draft—at best, a modest force in a natural-draft chimney—the flue will work backwards. The chimney will serve as the air inlet, and the deadly gas will be exhausted into the house—a phenomenon called backdrafting. What sorts of influences can bring depressurization to a crucial point? Here's a partial list:

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