Carbon Monoxide CO Hazards

Carbon monoxide (CO) is released by combustion appliances, automobiles, and cigarettes as a product of incomplete combustion. CO is the largest cause of injury and death from gas poisoning, resulting in more than 500 deaths per year. Many more people are injured by high concentrations of the gas, or sickened by lower concentrations of 5-to-50 parts per million (ppm). The symptoms of low-level CO exposure are similar to the flu, and may go undiagnosed.

CO blocks the oxygen-carrying capacity of the blood’s hemoglobin, which carries vital oxygen to the tissues. At low concentrations (5-to-50 ppm), CO reduces nerve reaction time and causes mild drowsiness, nausea, and headaches. Higher concentrations (50-to-3000 ppm), lead to severe headaches, vomiting, and even death, if the high concentration persists. The effects of CO poisoning seem to be largely reversible, except for exposure to very high levels, which can cause brain damage.

The EPA’s suggested maximum 8-hour exposure is 9 ppm in room air. Room levels of CO at or above 9 ppm are usually associated with the use of malfunctioning combustion appliances within the living space. These include: unvented combustion space heaters, gas ranges, leaky wood stoves, and backdrafting vented space heaters. Backdrafting furnaces and boilers may also lead to high levels of CO. CO is a common problem in low-income housing, affecting 20% or more of residential buildings in some regions.

The most common CO-testing instruments are electronic sensors with a digital readouts in parts per million (ppm). Unvented combustion appliances should operate with virtually no CO production and vented appliances should produce no more than 100 ppm of CO in the flue gas, measured before the dilution device. CO is normally tested near the flame or at the exhaust port of the heat exchanger. CO is usually caused by one of the following:

Flame interference from a part of the heating device (a pan over a gas burner on a range top, for example)

  • Flame interference from dirt and debris
  • Misalignment of the burner
  • Inadequate combustion air
  • Backdrafting of combustion by-products onto the flame