Few studies have measured the effect of tobacco bans on secondhand smoke (SHS) exposure in prisons. From June 1, 2011, the sale of tobacco was prohibited in New Zealand prisons. One month later, the possession of tobacco was banned. We studied the indoor air quality before and after this policy was enforced.We measured indoor-fine-particulate (PM(2.5)) concentrations using a TSI SidePak photometer. The instrument was placed in a staff base of a New Zealand maximum-security prison, adjacent to four 12-cell wings. Measurements were made before the sales restriction, during this period, and after the ban. Data were summarized using daily geometric means and generalized least squares regression.A total of 7,107 observations were recorded at 5-min intervals, on 14 days before and 15 days after implementation, between 24 May and 5 August. Before the policy was implemented, the geometric mean was 6.58 μg/m(3) (95% CI = 6.29-6.58), which declined to 5.17 μg/m(3) (95% CI = 4.93-5.41) during the sales ban, and fell to 2.44 μg/m(3) (95% CI = 2.37-2.52) after the smoking ban. Regression analyses revealed an average 57% (95% CI = 42-68) decline in PM(2.5) concentrations, comparing the before and after periods.Our study showed a rapid and substantial improvement in indoor air quality after tobacco was banned at a prison. We conclude that prisoners have reduced their smoking in line with the ban, and that a significant health hazard has been reduced for staff and prisoners alike.