In rare instances, carbon monoxide (CO) levels in houses can reach dangerously high concentrations, causing adverse health effects ranging from mild headaches to, under extreme conditions, death. Hundreds of fatal accidental carbon monoxide poisonings occur each year primarily due to the indoor operation of motor vehicles, the indoor use of charcoal for cooking, the operation of malfunctioning vented and unvented combustion appliances, and the misuse of combustion appliances. Because there is a lack of simple, inexpensive, and accurate field , sampling instrumentation, it is difficult for gas utilities and researchers to co_nduct field r~search studies designed . to quantify the concentrations of CO in residences.Determining the concentration of CO in residences is the first step towards identifying the high risk appliances and high-CO environments which pose health risks.