Carbon monoxide (CO) is a colorless, odorless, and tasteless gas, making it impossible to detect without specialized equipment. This deceptive nature means carbon monoxide can accumulate to dangerous levels without warning, making an understanding of its presence and effects important for safety.
Understanding Carbon Monoxide Measurement
Carbon monoxide levels are typically measured in “parts per million” (PPM). This unit indicates the volume of carbon monoxide present within a million parts of air. For example, 1 PPM means one part of carbon monoxide for every million parts of air. This measurement system provides a standardized way to quantify the amount of gas, allowing for consistent evaluation of exposure risks.
Defining Acceptable and Hazardous Levels
In outdoor ambient air, regulatory bodies like the U.S. Environmental Protection Agency (EPA) set standards, with a federal standard of 9 PPM averaged over an 8-hour period and 35 PPM over a 1-hour period, not to be exceeded more than once a year. For indoor residential spaces, a maximum indoor safe carbon monoxide level is generally considered to be 9 PPM over an 8-hour period.
The World Health Organization (WHO) suggests indoor limits of 9-10 PPM for no more than 8 hours, 25-35 PPM for no more than 1 hour, and 90-100 PPM for no more than 15 minutes. Levels near properly adjusted gas stoves might range from 5 to 15 PPM, while poorly adjusted stoves could produce 30 PPM or higher. Carbon monoxide alarms, certified by organizations like Underwriters Laboratories (UL), are designed to activate at specific thresholds, such as 70 PPM for one to four hours, 150 PPM for 10 to 50 minutes, or 400 PPM for four to 15 minutes.
The Occupational Safety and Health Administration (OSHA) sets a permissible exposure limit (PEL) for carbon monoxide at 50 PPM averaged over an 8-hour time-weighted average. The National Institute for Occupational Safety and Health (NIOSH) recommends an exposure limit of 35 PPM as an 8-hour time-weighted average and a ceiling of 200 PPM. The American Conference of Governmental Industrial Hygienists (ACGIH) assigns a threshold limit value (TLV) of 25 PPM for an 8-hour workday. Levels exceeding 100 PPM typically prompt recommendations for evacuation in workplace environments.
Health Effects of Exposure
Carbon monoxide exerts its harmful effects by interfering with the body’s ability to transport and utilize oxygen. When inhaled, CO binds to hemoglobin in red blood cells, forming carboxyhemoglobin (COHb). This binding is 200 to 250 times stronger than oxygen’s, reducing the blood’s oxygen-carrying capacity and making oxygen difficult to release to tissues, leading to cellular hypoxia.
At lower concentrations, such as prolonged exposure to 9-50 PPM, individuals might experience fatigue, headaches, nausea, or chest pain, particularly those with pre-existing heart conditions. Chronic exposure during pregnancy, even at low levels like 25 PPM, has been associated with potential damage to fetal brain cells.
As concentrations increase to moderate levels, such as 100-200 PPM, symptoms can progress to impaired vision, coordination difficulties, more severe headaches, dizziness, confusion, and vomiting. At very high concentrations, such as 400 PPM or more, carbon monoxide exposure can lead to severe headaches, convulsions, unconsciousness, and permanent damage to the heart and brain. Exposure to 800 PPM or greater can be fatal within minutes. Young children, the elderly, and individuals with existing heart or lung conditions are particularly susceptible to the effects of carbon monoxide poisoning.
Common Sources and Prevention Strategies
Carbon monoxide is produced whenever fuels containing carbon, such as natural gas, gasoline, kerosene, oil, propane, coal, or wood, undergo incomplete combustion. Common sources in and around homes include faulty furnaces, gas stoves, ovens, water heaters, and clothes dryers. Leaking chimneys, fireplaces, and wood-burning stoves can also release carbon monoxide into indoor spaces. Other significant sources are portable generators, car exhaust from attached garages, and the indoor use of charcoal grills or portable camping stoves. Tobacco smoke also contributes to indoor carbon monoxide levels.
Installing carbon monoxide alarms is a primary safety measure, with recommendations to place them outside each separate sleeping area, on every level of the home, and near attached garages. Regularly check the alarm batteries.
Regular professional inspection and maintenance of all fuel-burning appliances, including furnaces, water heaters, stoves, and fireplaces, are necessary to prevent carbon monoxide buildup. Ensuring proper ventilation and keeping vents and chimneys clear of obstructions is equally important. Portable generators, charcoal grills, and any gas-powered equipment should never be used indoors or in attached garages; they must be operated outdoors and away from windows and doors. Additionally, gas ovens or ranges should not be used for heating purposes, and cars should not be idled in attached garages.