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While diesel vehicles have many benefits, continued growth in the diesel population raises concerns regarding the health and environmental effects of the high NOx and particulate emissions which could result. The concern over diesel particulate has increased in recent years. Firstly, the evidence indicating adverse effects from all particulate in urban air has grown stronger. Secondly, regarding diesel's cancer risk, evidence has grown that there is no threshold, i.e., there is no level below which one could consider diesel particulate to be safe. As a result, countries around the world are increasingly tightening diesel regulations with the result that technology for reducing emissions continues to advance. Engine and combustion improvements have substantially reduced NOx and particulate from modern engines. Oxidation catalysts have also become widespread especially on passenger cars in Europe and trucks in the US.