Nitrogen dioxide is a ubiquitous air pollutant resulting from the combustion of fossil fuels. Indoor levels of nitrogen dioxide are often higher than outdoor concentrations, especially in homes where there are unvented heating and cooking appliances that utilize natural gas, kerosene, coal, or wood. Drs. John Spengler, Jonathan Samet, and their colleagues determined the impact of housing characteristics and the type and use of cooking ranges on nitrogen dioxide levels in infants' bedrooms in Albuquerque. Housing characteristics included the age, size, and type of construction of the house, the type of furnace, and the presence of a gas or kerosene space heater, fireplace, evaporative cooler, or attached garage. They also determined whether the cooking appliance was a gas range with a continuously burning pilot light, electronic ignition, or manually lit pilot light, or an electric range; whether the cooking range was used for space heating; and the minutes of stove use per season or year.