For each study site, personnel records were obtained including a work history for each exposed worker. For current workers, smoking history was obtained by questionnaire and carboxyhemoglobin measured both pre-shift and post-shift. The feasibility of obtaining smoking history on previously employed workers was explored. Vital status ascertainment included the use of records from the Social Security Administration, drivers license bureaus, the Internal Revenue Service, and the National Death Index. Death certificates were obtained from appropriate state vital statistics offices. Statistical analyses were performed using a modified life table analysis system frequently used by National Institute of Occupational Safety and Health (NIOSH) in similar studies. In addition, industrial hygienists from NIOSH visited each study site to measure current occupational exposure levels and to gather information which allowed estimation of past exposure levels. Previous carbon monoxide monitoring data was available for some tunnels.