The Dangers of Carbon Monoxide in Your Home

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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.

Recognizing and remedying a largely
unrecognized–and potentially deadly–form of
indoor air pollution, carbon monoxide.

The Dangers of Carbon Monoxide in Your Home

for a few minutes for a bedtime story?
After all, you deserve to put your bunions up after a full
day of insulating, weatherstripping, hauling firewood and
installing new storm windows. My, but isn’t it satisfying
to feel ready for winter? And the signs of approaching cold
are unmistakable. The sky was so blue and the air so crisp
today that you were tempted to look for stars at noon. Now,
as the sun nuzzles the ridge line, the inevitable chill is
settling in. It’ll probably drop into the teens tonight, so
the timing for the season’s first fire couldn’t be better.

Flames roll against the burned-clean back wall of
firebrick; shadows play from a light too subtle to be
electrical; the dog–settled at the foot of the
hearth–sighs. Today’s paper is on the end table, but
the firelight is a little too dim to read by, and a lamp
would spoil the mood. Besides, the fire is entertainment
enough–especially for one who wants so little to move
even a single muscle.

Eyelids grow heavy, and soon your chin is bouncing off’
your chest. On the edge of a snooze, you have a passing
thought about taking care of the fire and closing the
window you cracked when you lit it. But there’s still too
much flame to close the damper. And the window? No matter,
the new retention head-burner furnace will kick in and make
up for the heat lost.

Comforted, you fall asleep–for the last time.

Terminal Cabin Fever

A melodramatic story? Perhaps, but the scene it portrays
isn’t all that unusual. According to Jim White, an engineer
with the Canadian Mortgage and Housing Corporation Research
Division, around 220 North Americans die each year in
pretty much the way just described from carbon monoxide poisoning. Other scientists, such
as Joseph Lstiburek, of Building Energy Corporation,
Toronto, suggest that the toll may actually be much
higher–that hundreds more carbon monoxide-induced
deaths are being incorrectly attributed.

The cause? Backdrafting of combustion appliances located
inside houses. The details vary. It’s not always a conflict
between a fireplace and a furnace. A family of eight
expired near Memphis, Tennessee, in late summer last year
when their air conditioner and gas water heater got in a
tug of war.

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 CO Pollutants Coming From?

If the only indoor source of combustion generated 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:

1. Weatherization: Because of efforts to
reduce air leakage, it takes less to depressurize a modern
house than a turn-of-the-century residence. Homes built
between about 1940 and 1975 in northern climates are about
one-third as leaky as the previous housing stock. According
to Gory Nelson of the Minneapolis Blower Door Company
(which builds equipment for testing house leakage), the
average new Minneapolis home can reach a serious level of
depressurization under the influence of a fireplace or a
large exhaust fan. And a tight, energy-efficient home may
be depressurized by a single bathroom exhaust fan.

2. Fireplaces: When a
fireplace is burning actively, it draws as much as 600
cubic feet per minute (cfm) of air from a home–easily
enough to backdraft other combustion appliances. Even more
serious, the fireplace’s natural draft drops as the fire
burns down. It may win the battle of depressurization early
in the burn, backdrafting other appliances, and then begin
backdrafting itself later, during the charcoal phase, when
the carbon monoxide concentrations are highest.

Fireplaces are one thing that all the experts dealing with
backdrafting agree on: If a modern house is to have one
(and if it is to be used), a fireplace should have an ample
outside air source and should be equipped with
tight-fitting doors that are closed as soon as the flames
begin to die.

3. Indoor barbecues and
kitchen-island exhaust fans:
Modern houses have
many exhaust fans, some of which are very powerful. Jim
White says that the two most potent inhalers of house air
are indoor barbecues and kitchen-island exhaust fans.
Typically, neither type of device is fitted with an outside
air source, so they suck 400 to 600 cfm of air out of the
house. No natural-draft burner can overcome this level of
depressurization in even an average home. When the exhaust
fan is operating, the furnace, gas water heater, woodstove,
and the fireplace, too, will work backwards. White cites
examples where the suction has bowed picture windows in to
the extent that images distort.

Attempts to fit indoor barbecues and island exhaust fans
with their own air inlets are impractical unless the
retrofit incorporates an intake fan comparable to the
exhaust fan. To provide natural make-up air to the average
indoor barbecue fan, you’d need a hole about 32 inches in
diameter. When 23 Canadian building code and standard
committees convened in Ottawa in October to consider
regulations to control backdrafting and spillage, indoor
barbecues and other large exhaust fans were high up on the
list for discussion.

4. Smaller exhaust fans acting in concert:
In order of the volume of air they expel from a building
(after indoor barbecues and island fans), clothes dryers
(about 200 cfm), furnaces (about 120 cfm), standard range
hoods (about 100 cfm), bathroom ex haust fans (50 to 80
cfm), woodstoves (less than 50 cfm) and gas water heaters
(about 40 cfm) all can contribute to depressurization.

5. More-efficient and smaller furnaces:
Most of the improvements in furnace efficiencies achieved
in the last decade have come from more-effective heat
extraction. As a result, flue gas temperatures are lower
and draft is reduced proportionately. In fact, draft is so
threatened by low exhaust temperature that the
most-efficient furnaces now have fans to remove the waste
products. It’s the middle ground where problems can arise.
A chimney that was marginal with a 65%-efficient
furnace–because it was too large or was leaky or was
on a cold outside wall–may not work at all at 80%

The problem can be magnified when a house is extensively
weatherized, reducing the need for heat, and a smaller and
more-efficient furnace is installed. The flue gases will be
cooler and their volume will be too little for the
chimney’s capacity. To cope with lower flue temperatures
and smaller furnaces, natural-draft chimneys should be
built from insulated stainless steel or, even better,
insulating refractory in a size appropriate to the furnace

In the U.S., assuming that federally mandated
minimum efficiency levels are instituted on schedule in
1992, the majority of new furnaces will have induced-draft
fans, effectively eliminating the possibility of

6. Leaky forced-air furnace ductwork:
Furnace installers and homeowners usually devote a lot of
attention to sealing up furnace supply ducts, the tubes
through which hot air is pushed by the fan. Typically,
though, the return lines, through which cool air is pulled
back to the furnace, get much less attention. Consequently,
if the return ducts happen to be leaky in the furnace room,
which isn’t unusual, the furnace’s own fan can depressurize
the area near the furnace. In this case, you get a double
whammy: The chimney backdrafts because of the fan, and then
the fan distributes the poisons to the living area.

Well-sealed ductwork is particularly important on gas
air-conditioning systems. While in the heating mode, the
supply fan won’t come on until the plenum has heated to a
set level, allowing the chimney time to establish good
flow. In air-conditioning mode, however, the distribution
fan already may be on when the burner cycles. Bear in mind,
too, that natural chimney draft is low in summer because of
warmer outside temperatures.

In general, it’s unwise to run a furnace blower constantly
for any reason, though it won’t be a problem as long as
neither the furnace burner nor any other combustion
appliance operates while the fan is on.

7. Leaks in the wrong parts of a house: 
Wind blowing against a house creates high pressure on the
windward side and low pressure on the lee. So, even leaky
houses can become seriously depressurized if the
predominant leaks are on the downwind side. These leaks
could be intentional. Fresh-air inlets for a fireplace or
furnace or even a dryer exhaust can allow wind to pull air
from a house. And any house can become seriously
depressurized if there’s a window open on the downwind

Leaks at the wall-ceiling junction are also likely to
encourage depressurization, since they help the house to
act like a good chimney. This effect is more pronounced in
a multistory house, simply because it’s taller. Likewise,
open windows on the second story (and particularly
downwind) may further increase depressurization.

Gary Nelson has also noticed an interesting phenomenon in
three Minnesota houses he’s investigated. All three were
fitted with power attic ventilators in attempts to solve
moisture problems in insulation. According to Nelson, the
moisture was coming from the house through holes in the
ceiling, so exhausting air from the attic only increased
the leakage rate. Not surprisingly, increasing the leakage
rate increased depressurization, and all three houses had
backdrafting problems.

8. Unsafe practices: 
There aren’t, and aren’t likely to be, regulations
prohibiting a homeowner from a foolhardy practice such as
heating the kitchen with a gas range turned on high.
Likewise, heating systems about which many scientists are
skeptical, such as unvented space heaters, are quite

(Bruce Eugene Davis, Housing Director of the Economic
Opportunity Agency of Washington County, Arkansas, reports
that 12% of the houses in his state have unvented heaters,
and he has found CO in the exhaust streams of 19% of the
units he’s tested.)

As for misusing an appliance such as a gas range to heat a
space, we can adamantly say don’t! But whether you want to
have an unvented combustion appliance in your house at all
(be it a range, an oven or a gas, oil or kerosene heater)
is another matter entirely. You should be aware, however,
that even in the absence of carbon monoxide, such an
appliance could be contributing to chronic health problems
your family members may have. (Canadian researchers are
finding out alarming things about certain fungi that
flourish in the damp environment often produced by unvented
space heaters.) And should an unvented heater malfunction,
it could be life-threatening.

What to Do

If you have a woodstove or an oil furnace that backdrafts,
you’re probably aware of it. Wood smoke is visible and
fuel-oil furnaces provide a telltale whiff of sulfur to the
nose. Unfortunately, gas appliances don’t offer such ready
hints. In any event, if backdrafting has been occurring
often in a fossil-fuel burner, there should be some signs
on the appliance itself. On an oil-burner, look for soot
smudges around the draft control (a T in the exhaust from
the furnace that has a freeswinging flap).

If they’re extensive, you’ve been having at least some
spillage. On a gas furnace or water heater, look at the
outlet from the unit. There will be a cone-shaped hood
there that admits extra air to the chimney to stabilize
draft, and soot will show up around it if the appliance has
been backdrafting significantly. Even if you find no signs
that your furnace or water heater has been backdrafting,
it’s a good idea to perform the simple tests described at the end of the article.

If one or more of your combustion appliances
is backdrafting for longer than about 30 seconds at
start-up, you should fix the problem. As long as the heater is correctly tuned, the situation
isn’t life-threatening. But poor air quality may already be
causing health problems–sore throats, irritation to
nostrils and eyes and asthma-like symptoms–and may be
making the indoor environment overly humid. If you ignore
the problem, the performance of the burner will
deteriorate, and the situation could well become deadly.

Experts take two approaches to solving backdrafting
problems: One method goes after the mechanism of
backdrafting; the other eliminates sources. The potential
for backdrafting can be controlled by balancing indoor and
outdoor air pressure with a fan that pushes air into the
house. Jim White points out that air will get into the
building anyway, so it may be best to heat it and push it
in intentionally.

Source control, as advocated by Lstiburek
and Nelson, is done by a procedure called aerodynamic
uncoupling. Fortunately, this isn’t as complex as it first
sounds. It simply means that anything that burns fuel gets its
supply air from the outdoors and dumps its waste back out
there. For the purposes of combustion, the furnace, water
heater, gas dryer and woodstove or fireplace all operate a
“they were outdoors. The first three can be exhausted
with a power venter, which can be plumbed to handle all the
burners at once. (Tjernlund Products offers such equipment,
as well as inexpensive sensors that will shut off a
backdrafting appliance.) Fireplaces, if they’re used, should have outside air inlets
inside the fire chamber and well-sealed glass doors.
Woodstoves are difficult to retrofit with outside air
supplies, but models built for mobile homes come so
equipped. One advantage of utilizing aerodynamic uncoupling
over pressure neutralization is that major exhaust devices
(like indoor barbecues) are no longer a problem, since
there are no pollutants to be drawn in. In addition, in
colder climates, infiltration (air moving into the house
through the walls) is preferred to exfiltration (air moving
out through the walls) because water vapor in the indoor
air can cause problems if it condenses inside a wall.


By now you should have a pretty good idea what causes
backdrafting and how to stop it. But, as a final exam, see
if you can figure out what happened to you back in our
bedtime story. Don’t peek below until you’ve figured out
your explanation.

Here’s how we stacked the deck against
your surviving the first four paragraphs. First, you spent
the day messing with the air leakage characteristics of the
building. That new furnace might have worked acceptably in
the old leaky building, even though it’s not cold enough to
produce really strong draft, but the addition of caulk and
storm windows has thrown it a curve. There’s a hearty fire
in the fireplace and that chimney is drawing correctly as
you go to sleep. The cracked window on the downwind side is
fighting against it–but it isn’t winning yet.

in the evening, the balance in the tug of war changes. The fire burns down some, enough to let the thermostat call
for heat from the furnace. Sure enough, it backdrafts, but
it’s in good tune and there’s not much CO being produced.
(Later in the heating season, should someone else take up
residence, the performance will deteriorate under the
adverse conditions.) You’re asleep, so you don’t notice the

Later still, the fire dies to its charcoal phase,
producing lots of carbon monoxide and very little draft.
It’s chilly with that open window encouraging infiltration,
though. Soon the flow reverses in the fireplace chimney,
spilling CO-laden air into the room. Less than an hour
later it’s all over.

C0: The Invisible Killer

WHEN CARBON-CONTAINING FUELS oxidize (burn) with air, they
create hundreds of new compounds. Many of these waste
products are dangerous, but only the simplest of
them–carbon monoxide (CO)–is likely to occur in
concentrations that immediately jeopardize health. Because
of the deadly gas’s tremendous affinity for blood in the
lungs (an affinity more than 200 times that of oxygen), it
readily steps in the way of the life-giving gas and denies
cells their breath. People who die from exhaust fumes in
cars or from smoke inhalation in fires suffocate from
carbon monoxide poisoning. Luckily for us, carbon monoxide
normally makes up only a very small part of Earth’s
atmosphere–about 1/10 of a part per million (ppm).

Because there are so many devices in urban areas that burn
carbon fuels, though, CO usually lingers there in the 5- to
10-ppm range and may exceed 10 times that level near busy
highways. CO begins to affect hu mans after we’ve been
exposed to as little as 15 ppm over eight
hours–causing minor confusion and loss of sense of
time–and at 1070 (that’s 10,000 ppm) it kills in 10
to 20 minutes. Combine carbon monoxide’s extreme toxicity
with the difficulty of detecting it (the gas is colorless,
odorless and tasteless at all but the highest
concentrations) and the subtlety of its symptoms (victims
of acute CO poisoning typically lose consciousness without
recognizing the seriousness of their distress), and you
have a prescription for accidental death. But even
comparatively small concentrations of carbon monoxide
present serious long-term health hazards.

Testing for Backdrafting

Though sophisticated instruments are required to accurately
measure the potential for backdrafting in a house, you can
perform a “worst-case” test of your own with nothing more
than a finger for a test instrument. There are two stages
to the test, the procedures for which we’ve adapted from
recommendations by CMHC, Gary Nelson and Joseph Lstiburek.

Furnace Return Test

Close all outside doors and windows. Shut the door between
the furnace room and the rest of the building, and turn on
the furnace fan (no need for the burner). Using a portable
smoke source (an incense stick or cigarette) inside the
furnace room, look for air movement under or around doors.
If the smoke trails toward the furnace, you’ve got return
ductwork leaks that need repair.

Basic Depressurization Test

On a mild, still day, close all exterior doors and windows.
At a time when the furnace hasn’t run for several hours,
turn on every exhaust device in the house (kitchen and bath
fans, dryer, water heater, attic and wholehouse fans) and open all possible doors between
the furnace and the rooms where the exhausts are located.
Keep all other interior doors closed. Go to the furnace
room and have someone else turn on the furnace at the
thermostat. Feel at the draft control on an oil furnace or
the draft hood on a gas furnace for the heat of a backward
flow in the chimney. Redo the test on gas water heaters and
gas dryers after they’ve had a chance to cool thoroughly.
If spillage continues for longer than about 30 seconds on
any combustion appliance, you could have a problem.

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