 Modern diesel engines (Euro 6 & US EPA standards) use exhaust gas recirculation into the intakes to reduce excess oxygen present in combustion, and urea injection to turn NOx into N2 and water.Small particles (PM 2.5) can penetrate deeply into lung tissue and damage it, causing premature death in extreme cases.Inhalation of such particles may cause or worsen respiratory diseases, such as emphysema or bronchitis, or may also aggravate existing heart disease.