A colorless, odorless, flavorless, nonirritating gas, carbon monoxide (CO) causes more poisoning deaths today than any other substance. In my work as a professional engineer with Iowa State University Extension, I've investigated more than 65 indoor air quality cases in the last three years, many of them related to CO production and venting problems. Inadequate installation and maintenance of heating appliances and improper venting can cause serious health problems, mental deterioration, and death to residents exposed to the gas. Preventing indoor CO problems isn't impossible. Proper installation and regular maintenance of equipment by trained and qualified heating contractors reduces the probability of CO emissions and venting failures. Preventive Measures Yearly service by a qualified heating contractor is vital to reducing the risks posed by