The report includes emissions data from residential natural-gas furnaces and compares selected data to emissions data from residential oil furnaces and wood-stoves. Natural-gas furnace emissions data are given for carbon monoxide (CO), unburned hydrocarbons, aldehydes, volatile and semivolatile organic compounds (including mono- and polyaromatic hydrocarbons), total particulate, and total extractable organics. Natural-gas furnace combustion emissions were collected in a laboratory setting under simulated residential conditions and usage. The study was part of the Integrated Air Cancer Project, a multidisciplinary, cross-EPA laboratory effort to apportion sources of mutagenicity in ambient emissions. Both high and low efficiency gas furnaces were tested. The results indicate that significant levels of CO, unburned hydrocarbons, aromatic volatile organic compounds, and aldehydes can be emitted. Based on emission factors relative to thermal input, residential natural-gas emissions are at least a factor of 10 to 100 less than comparable emissions from residential oil furnaces and woodstoves.