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The time was when garages, mimicking the stables of earlier years, were built separate from the house. That changed after World War II, when architects began including garages in the house structure. Attached garages make life a little more convenient for homeowners, but they also have an unwelcome effect: They can, and frequently do, pose a health hazard for the home's occupants. A recent study in Anchorage, Alaska, found that attached garages readily leak or funnel airborne pollutants, most notably carbon monoxide (CO) from idling cars, into adjacent living quarters.
The study, conducted by John Freeman of Sunrise Energy Works for the Alaska Housing Finance Corp., confirms similar findings in a Minnesota study a few years earlier. While showing that there is cause for concern, particularly during winters in the colder regions of the country, the findings also indicate that homeowners can take relatively simple steps to eliminate the danger. CO is a colorless, odorless gas that binds with hemoglobin and prevents oxygen from being carried by red blood cells. It is, in fact, the most commonly encountered and pervasive poison in our environment, responsible for more deaths than all other poisons combined and considerable suffering in those who survive high-level exposures. Lower concentrations produce symptoms that are often confused with flu or food poisoning. Over four consecutive days during the winter of 2000, Freeman studied 12 houses selected to represent current building practices and common house-to-garage attachments. He traced CO movement using data loggers, one in the garage and two to three others in the homes, depending on their size. The results showed that CO moved readily into all but one of the homes.
Most surprising - almost startling, in Freeman's view - was "how quickly and widely the gas dispersed into the homes. In one instance, the farthest sensor would read the same as in the garage within four minutesOthers took longer but, with only one exception, "the house levels consistently tracked the garage release Freeman said, though at reduced levels of concentration. Exposure levels in the houses ranged from zero to 115 percent of the EPA's exposure limit for CO, which is nine parts per million. In four of the houses, the level exceeded 60 percent of the EPA standard. While such levels may have little or no effect on healthy occupants, exposure at the EPA limit can produce ill effects in at-risk residents, such as those with angina, Freeman said. The culprit is the so-called stack effect, in which the warmer air in the home - leaking to the outdoors through a multitude of cracks, windows and door frames - works like a vacuum cleaner, sucking the CO and any other garage-based air pollutants into the house. The situation is aggravated when exhaust fans - in the bathroom, over the kitchen range or elsewhere in the house - are operating and exhausting air directly outdoors. The most rapid CO transfer rates occurred in homes with forced-air heating ducts located in the garage. "Leaks in the ducts provide a ready path for the CO to enter the home" Freeman said. The stack effect is evident even in tightly constructed, energy efficient homes because of the considerable heat differential between the home and garage. Even if garage doors are opened before starting the car, Freeman said, "the pull from the house is enough to draw the pollutants inside.
He also notes that homes with a tuck-in design - in which garages are sited beneath living quarters - are the most rapidly affected of all house styles because of the much greater shared surface area through which the CO can be drawn. In the only home showing no CO incursion into the living quarters, the garage was under negative pressure relative to the house - no stack effect, in other words. Freeman found that this was due to the operation of a water heater and unit heater in the garage that vented outdoors. The combined effect of these two units, drawing air needed for combustion from the garage and venting it safely out the chimney, overcame the stack effect in the house. This problem of CO transfer is most noticeable in winter when cold exterior temperatures maximize the stack effect in warm houses. Also, cars are left idling much longer in winter to allow the engines to warm up before driving off. After the car drives away and the garage doors are shut, the garage remains as a high-CO reservoir for the house for some time. Even as CO begins to decline in the garage, levels continue to rise in the house, sometimes peaking as much as three hours after the car has gone. Acute CO poisoning from attached garages is unlikely.
However, the findings are worrisome, Freeman said, because other garage-based contaminants - benzene, a constituent of gasoline, along with fumes from paints and other stored chemicals - often are drawn into the homes along with the CO. Benzene is a particularly hazardous chemical that causes drowsiness, headaches, unconsciousness and - after long exposure - a variety of blood and immune disorders. Garages are more than just homes for the car. "Snow blowers, snowmobiles and lawn mowers also sit there out-gassing into the garage. So do stored paint cans and other household chemicals, Freeman said. In fact, it was the Alaska Housing Finance Corp.'s concern about the high levels of benzene in most Alaskan garages that prompted Freeman's study in the first place. His "considerable concern for the long-term health impact of garage-based pollution is tempered by the fact that tackling the problem is relatively straightforward - the first and obvious step being the elimination of the source. His advice: If the stack effect in the house can be negated by a fan, then garage-based pollutants will not be drawn in the living quarters and the problem will be solved unless a power failure or some mechanical breakdown occurred. Removing the source of the contamination from the garage remains the first and wisest move.