One in four Minnesota middle school students report that they have ridden in a car with someone who was smoking cigarettes in the preceding week (Minnesota Youth Tobacco and Asthma Survey, 2011), yet only seven U.S. states have policies prohibiting smoking with youth in vehicles.This study expands on previous research by measuring secondhand smoke particulate concentrations under a comprehensive set of conditions that affect passenger exposure. A total of 171 trials were conducted, including duplicate trials to determine reliability. The monitoring included continuous photometer measurements of fine particles (PM2.5) before, during, and after a participant drove and smoked a cigarette. The instruments were installed in 3 to 5 locations inside the vehicle and 1 location outside to measure and compensate for ambient air particulates. The monitoring was conducted for three vehicle types (sedan, minivan, SUV), two driving speeds, four window positions, and multiple ventilation operating conditions over both summer and winter conditions.When the windows were closed the peak 30 second average PM2.5 concentrations ranged from 359 to 5612 with an average of 2013 μg/m3. Aftersmoking stopped, it took from 4 minutes to over 25 minutes for the particulate level to decrease to the background level. When the active smoking and post-smoking periods were combined, the passenger’s total PM2.5 exposure averaged 12,250 min*ug/m3, which is about equal to sitting in a typicalsmoky bar for three hours (Bohac et al. 2010). The most significant factors on SHS exposure were window position, driving speed, ventilation operation, and smoking behavior. Opening the driver’s window 2” reduced exposure by 93% and fully opening the window reduced exposure by another 62%. The average exposure was 63% lower for highway driving (60 mph) than city driving (30 mph). Operating the ventilation in recirulating mode did not have a significant effect, but operating in fresh air mode reduced exposure by 42% when driving 30 mph. Holding the cigarette and blowing smoke towards the window reduced exposure by 34% to 75% compared to holding the cigarette and blowing smoke toward the inside of the vehicle. There was little difference in exposure between interior locations except one of the far back seats of the minivan had significantly lower exposure.