SYNOPSIS-ABSTRACT A national survey was conducted in 1969-1972 for the purpose of determining the range of carboxyhemoglobin (COHb) in various segments of the American population. Venous blood samples were obtained from 29@000 adults at arbitrarily chosen blood bank collection sites in the following geographical areas: Anchorage@ Chicago@ Denver@ Detroit@ Honolulu@ Houston@ Los Angeles@ Miami@ Milwaukee@ New Orleans@ New York City@ Phoenix@ St.Louis@ Salt Lake City@ San Francisco@ Seattle@ Washington@ D. C and small cities in Vermont and New Hampshire. These blood samples were then analyzed for COHb content. From every tenth participant@ an alveolar breath sample was obtained for CO analysis and correlation with the COHb. At the time of blood collection@ detailed questionnaires were completed so that the influence of the following variables could be assessed: age@ weight@ height@ sex@ race@ health status@ smoking habits@ place of residence@ occupation@ place of work@ meteorological conditions@ background CO concentration at sampling site@ and location and time of sampling. For comparative purposes@ COHb measurements were also made on samples from 1@525 blood donors in 13 smaller communities in Vermont and New Hampshire@ and on samples obtained from volunteers breathing CO-free air and air with known concentrations of CO. There was excellent agreement between the two independent analytical methods used to analyze COHb. The mean COHb saturation of 0.45% of the adults breathing CO-free air demonstrated the sensitivity of the analytical method in detecting low concentrations of COHb. The experimental exposures to fluctuating and discontinuous concentrations of CO demonstrated the accuracy of the analytical methods to measure COHb over the range of saturations most frequently encountered in the study. Forty-five percent of all the non-smoking blook donors tested had COHb saturations gt; 1.5% and indicated that exposure to CO in excess of that permitted by the Air Quality Standards is widespread and occurring regularly. Tobacco smoking was the single most important factor responsible for the highest COHb saturations observed. The other chief factors influencing the COHb saturation were the geographical location of the individual@ his occupation@ and the existing meteorological conditions. None of the large urban communities had CO concentrations low enough to comply with the EPA ambient Air Quality Standards for CO.