In 2013, transportation (road, rail, air and marine) contributed the most to Canada's NOX emissions, representing 43% (889 kt). The oil and gas industry emitted the next largest proportions of national NOX emissions, representing 24% (499 kt), followed by off-road vehicles with 12% (254 kt). Between 1990 and 2013, the source that had the largest reduction (531 kt) in NOX emissions was transportation (road, rail, air and marine) while the oil and gas industry experienced the largest increase (145 kt).