100 drivers aged 23-65, 53 male and 47 female, were recruited to drive an apparently unmodified passenger car for one hour in city traffic. They were randomly assigned either to a control condition, or to ionised air containing 20 000-30 000 negative ions/ml, but were unaware of the presence of an ioniser in the vehicle. A computer initiated unprepared signals to which drivers should normally be alert. Drivers responded by pressing a foot-switch and reporting verbally. Signals were selected at random from 21 possible signals, and were presented for up to 3 minutes, with a random delay of 30-180 seconds after each response or failure to respond. Subjects reported subjective symptom intensity by marking a set of 11 visual-analogue scales. Eye symptoms were alleviated by ionisation for subjects aged < 40 years (P<0.05), but older female subjects experienced more eye discomfort, headache and fatigue in the ionisation condition (P < 0.05). Detection of 10 of the 21 signals was better (P < 0.05, 2-tail) in the ionisation condition. The effect was more often significant in the second half-hour, for subjects < 40 years, and for women. A significant negative effect (P < 0.05) was observed for the signal indicating speed below true speed.