Bach et al. (1990) sought to determine whether humans reacted acutely to formaldehyde exposure, and if previous chronic-duration exposure to formaldehyde altered the responses noted in acute exposure. Thirty-two men with at least 5 years of occupational exposure to formaldehyde and 29 matched controls were exposed to formaldehyde at concentrations of 0, 0.12, 0.32, or 0.98 ppm for 5.5 hours. During the exposure period, subjects underwent a battery of performance tests designed to estimate the subject’s distractibility, short-term memory, and capability to understand and perform certain tasks. Controls tended to suffer from headaches, "heavy head", and physical tiredness more than the exposed workers. In both occupationally exposed subjects and nonexposed subjects, decreased performances in several tests were statistically significantly correlated with increasing acute concentration of formaldehyde. Occupationally exposed subjects showed significantly decreased performance, compared with nonexposed subjects, only in a digit span test, but not in variables for a graphic continuous line test, an addition test, or a digit symbol test. The authors noted that the typical dose-related symptoms of respiratory irritation were not seen in this study.