Over half a million cars pass through Hong Kong’s tunnels every day. Commuters, taxi drivers, and other drivers who pass through these tunnels may be exposed to high levels of particulate matter and toxic gases present in the air. There is limited data on in-cabin exposure to pollutants, especially while the vehicle is driving through a tunnel under different cabin ventilation conditions. This study reports in-cabin measurements of fine particles (FP, dp < 2.5 μm) -- which includes nanoparticles and ultrafine particles (UFP, dp < 100 nm) -- in a 1998 Nissan Sunny EX passenger car while driving through Hong Kong’s Tseung Kwan O Tunnel. The vehicle tested did not contain a particle filtration system or an activated carbon filter, and was fueled with unleaded gasoline. The measurements were taken using a water-based condensation particle counter (WCPC) under different conditions consistent with driver behavior. The particle count readings were generally highest with the windows closed and air conditioning on. On average, these readings were more than three times higher than readings with the windows closed and the air conditioning off, and 68% higher than readings with the windows open and the air conditioning off. In-cabin particle concentrations inside the tunnel were up to twenty-one times higher than in-cabin particle concentrations outside the tunnel under comparable traffic conditions. The highest in-cabin particle count concentration reading, 1.94 million particles/cm3, was taken with the windows closed and air conditioning on while the test car was passing a double-decker diesel bus.