An 18% reduction in the carboxyhemoglobin (HbCO) saturation in nonsmoking Chicago blood donors occurred between 1970 and 1974, indicating that current donors are being exposed to a lower average carbon monoxide (CO) concentration than had been experienced by 1970 donors. In contrast to the situation in 1970, when it was discovered that 74% of the nonsmokers in Chicago were being exposed to CO in excess of the amount permitted by the federal air quality standards, in 1974 only 41% of the nonsmokers were being overexposed. The observed reduction in HbCO correlates well with both the ambient CO levels recorded at the air monitoring stations and the reduction in CO emission from automobiles. If the current trend continues, Chicago should reach compliance with air quality standards for CO by 1985. The measurement of HbCO in a representative urban population is an accurate index of actual CO exposure and supplements the air pollution data provided by air monitoring stations.