Carbon Monoxide Poisoning: Ventilation (AEN-209)

People may think that carbon monoxide is no problem because they live in a “loose” old house. Or that carbon monoxide is not a problem because they “opened a door” in the garage. Or that carbon monoxide is not a problem because they are in a huge building with lots of open space, like in an ice skating arena. Unfortunately, a “loose” building, or an open door, or a large space does not guarantee that a person will not become ill from carbon monoxide or even die. CO is a highly toxic gas, that can kill at small concentrations of 0.03 percent (300 parts per million) or less. High toxicity at low concentrations means that even in relatively loose spaces, or large buildings, concentrations can quickly build to dangerous concentrations when large sources of CO, such as a malfunctioning furnace or a gasoline engine, are operated indoors. The fact that CO is a colorless, odorless, tasteless, and non-irritating poison compounds the problem. When exposed to high concentrations persons become dizzy, are unable to stand or move out of the space, and often collapse. The brain does not received sufficient oxygen during a CO exposure resulting in confusion. Because it takes several hours for the body to remove carbon monoxide, CO is a cumulative poison. The amount of CO in the body continues to increase while breathing CO in the air.

How can carbon monoxide poisoning be prevented? There are three primary means to prevent CO poisoning.

  1. Prevent carbon monoxide from being produced by proper design, installation, and maintenance of gas appliances.
  2. Vent combustion products outdoors.
  3. Install warning devices (carbon monoxide detectors).

What about dilution (for example, having a “loose” house)? Dilution with outside air is an inefficient method of removing pollutants. For toxins such as CO providing more outside air while CO is still being produced does not solve the problem and is dangerous. Even with high volumes of dilution air the air quality is degraded when pollutants are dumped into a space. Providing high volumes of dilution air greatly increases heating and cooling costs.

Removing toxic materials at the source is safer and more effective. A properly burning gas flame does not produce excessive amounts of carbon monoxide, so it is critical that the burner be properly adjusted. Electronic carbon monoxide instruments available to heating contractors make it easy to properly adjust gas flames for minimum CO. To remove any small amount of CO that might still be produced, and all other products of combustion, the heating appliance should be vented to the outdoors. The new, direct-vent sealed combustion heating appliances give positive venting of all combustion products to the outdoors, while increasing efficiency and lowering heating costs.

What is the advantage of a “loose” house or ventilation? Many pollutants, such as those generated from normal body processes including breathing, can not be reasonably removed at the source. Dilution by ventilation air is then the correct and proper means for removal. Certainly, ventilation will reduce the concentrations of even toxic material. For example, an improperly operating kitchen range operated without a range hood might produce a CO concentration of 60 parts per million (ppm) in a “tight” (0.2 air change per hour) house. The same range in a “loose” (1.0 air change per hour) might only raise CO concentrations to 12 ppm, a more tolerable concentration.

Are there other advantages of a “loose” house or ventilation? In addition to the dilution effects, adequate ventilation provides the “make-up” air needed when operating exhaust fans and clothes dryers, air needed for proper combustion of gas appliances, dilution air needed at the draft hoods of natural draft appliances, and air needed for operation of open fireplaces. Without adequate air from the outdoors, chimneys and vents do not operate properly – wood burning fireplaces smoke into the room, and natural draft furnaces and water heaters downdraft.

Will ventilation reduce the concentrations of CO to safe levels? Carbon monoxide is a toxic material, and there is no “safe” concentration, only a “tolerable” concentration, which depends on the health and age of the victim. Certainly ventilation can reduce the health risk, but at a high energy cost. In one case, a faulty water heater raised the concentrations of CO in a mobile home above 660 ppm, which led to the death of one man and poisoning of six others. Doubling the ventilation rate in the home would have reduced the CO concentration to 330 ppm, which is still a lethal concentration. Only a few grams of carbon monoxide are needed to raise the entire concentration in a home to dangerous levels, and faulty appliances can produce hundreds of grams of CO. Every home, even a “loose” one, needs a carbon monoxide detector.

Can “loose” houses produce CO problems? Yes. Besides the obvious problems with “loose” houses (they are hard to heat and cool, drafty, and uncomfortable) a “loose” house can lead to venting problems. When the holes in a “loose” house are located high on the house, for instance in the ceiling or on the second story, the air leaking out of the house must be replaced by air sucked into the house through the basement and lower levels. If enough air leaks out at the top of the house, the suction can pull air down the chimneys and vents, causing downdrafting and combustion gas spillage into the home.

When I work in the garage, I keep all the doors open. Is that okay? No. The National Institute for Occupational Safety and Health (NIOSH) ran a 5.5 horsepower gasoline-powered pressure washer in a double garage with both doors open, the window open, and a vent open. In only 12 minutes CO concentrations in the garage rose to 658 parts per million (ppm). The rate of emission from a typical gasoline engine is so large (30,000 to 100,000 ppm) that it is very difficult to provide sufficient ventilation. NIOSH warns, “Do not use equipment and tools powered by gasoline engines inside buildings…”

Do large buildings dilute carbon monoxide enough to eliminate the risk of CO poisoning? No. NIOSH investigated a case where a worker in a 48 x 88 x 14 foot room was poisoned by carbon monoxide. He was using an 8-horsepower pump and had fresh air entering the room through the forced-air heating system. Ten minutes after the pump engine was started. CO concentrations as high as 395 ppm were measured. In an Iowa case, an entire six- story hotel was filled with carbon monoxide from a single malfunctioning water heater located in the basement. Concentrations were as high as 600 ppm in a sixth-story room, a potentially lethal level.

Larger spaces decrease, and smaller spaces increase, the concentration of carbon monoxide. For instance, operation of a defective unvented gas heater in a 1680 square foot home that might raise CO levels to 30 ppm would raise CO concentrations in a mobile home half the size (840 square feet) to 60 ppm. If the mobile home was also tighter, CO concentrations would rise even higher. For those reasons, mobile homes are required to have only vented space heating appliances.

Even large auditoriums, with ventilation, can quickly fill with CO from malfunctioning equipment. Use of LP-powered ice polishing machines in skating rinks have led to several cases of carbon monoxide poisoning in the U.S. and Canada, with concentrations above 100 ppm. A Canadian city prohibited indoor tractor pulls after they found that even with all outside doors open and ventilation fans operating they could not prevent carbon monoxide concentrations from rising above 400 ppm.


Prepared by
T.H. Greiner, Ph.D., P.E.
Extension Agricultural Engineer

The Iowa Cooperative Extension Service’s programs and policies are consistent with pertinent federal and state laws and regulations on nondiscrimination regarding race, color, national origin, religion, sex, age and disability.

File: sep98\AEN-209