To estimate the impact of a high concentration of environmental tobacco smoke (ETS) during a typical workday, ten female (age 28-46) and ten male (age 20-44) non-smokers were exposed, in same-sex groups of 5, to fresh diluted sidestream smoke (FDSS) at a mean respirable suspended particle (RSP) concentration of 200 µg/m3. During the period from 8 am to 4:30 pm, subjects were exposed to FDSS for 7.33 hours in a controlled environment room (CER). Throughout exposure, subjects provided sensory ratings of the room air, took a timed numeral-scanning test (measure of information processing speed) and completed the 30-item state portion of the State-Trait Personality Inventory. Results were compared to those recorded during control exposures (clean air only) conducted on the day before, and on the two days after, FDSS exposure. Ratings of eye irritation rose over the first hour of FDSS, then remained stable for the remainder of the day. Odor and nasal irritation ratings were greatest at the beginning of exposure and immediately after the 30-minute lunch break, and declined with prolonged exposure. Sensations of odor, nasal irritation and eye irritation averaged approximately 18, 15 and 13%, respectively, of the strongest that the subjects experienced prior to participating in this study. Numeral-scanning performance improved with successive days in the CER but was unaffected by FDSS exposure. Similarly, no effect on State Anger, State Anxiety or State Curiosity was seen. Although the non-smokers in this study were well aware of the presence of second-hand smoke, it did not alter either their psychological state or their ability to perform mental work. The smoke concentration employed is at least 1400-fold higher than those typical of current workplace environments in the US. where smoking is permitted.