This journal article presents new data on air change rates and real-time secondhand smoke particle levels for automobiles measured during over 100+ experiments. Air exchange values are given for a variety of car speeds, window positions, and air conditioner settings, which will be useful in exposure and risk assessments. The study confirmed that high concentrations of secondhand smoke can occur in automobiles during smoking, even if the windows are open and ventilation is used. If even two cigarettes are smoked with windows closed, the levels can exceed the 24-h USEPA standard for fine particles.