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Highlights • Spatiotemporal distribution of in-tunnel toxic gases, visibility, and air velocity was analyzed. • Correlations between traffic volume, toxic gas, and visibility were quantified. • CO and visibility are unsuitable as the primary control indexes for mountain tunnel ventilation systems. • Stricter emission standards and more intelligent ventilation-control technologies should be adopted in future. Abstract Owing to their ability to dilute pollutant concentrations and enhance visibility, which significantly improves driving safety, ventilation systems have become one of the critical subsidiary facilities in extra-long highway tunnels. The spatiotemporal distribution characteristics of toxic gases, visibility, and air velocity in the Qin Mountains No. 1 tunnel (uphill bore, 6.02 km) were analyzed by combining a traffic survey with an analysis of environmental monitoring data. The analysis indicated that CO and visibility are unsuitable as primary control indexes for mountain tunnel ventilation when heavy goods vehicles (HGVs) represent a fairly large proportion of the traffic composition. The maximum in-tunnel NO 2 concentration was 2.3 ppm, which is 2.3 times the design standard. Differences in the NO 2 /NO x ratio proved that gaps still exist between the diesel vehicle exhaust gas after-treatment technologies used in China and Europe. Consequently, stricter diesel exhaust emission standards and more intelligent ventilation-control technologies should be adopted in future tunnel designs. The results not only offer data that support the establishment of in-tunnel air quality control standards but also provide a feasible method for predicting the levels of various pollutants and developing ventilation-control strategies. [ABSTRACT FROM AUTHOR]