to prepare for the more stringent 2010 EPA regulations, which reduced levels even further. In 2008, the concerns about compliance shifted to the infrastructure for DEF distribution. The injection rate of DEF into the exhaust depends on the specific after-treatment system, but is typically 2–6% of diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimizes the tank's size and intrusion into vehicle packaging space. An electronic control unit adjusts the addition of fluid in accordance with parameters such as NOx level in exhaust gas (before catalytic converter, after catalytic converter, and possibly between catalytic converters if there is more than one), current ammonia filling level, engine operating temperature and speed. Chemistry DEF is a 32.5% solution of urea, . When it is injected into the hot exhaust gas stream, the water evaporates and the urea thermally decomposes to form ammonia () and isocyanic acid (HNCO): → + HNCO The isocyanic acid reacts with the water vapor and hydrolyses to carbon dioxide and ammonia: HNCO + → + Overall, thus far: + → 2 + Ammonia, in the presence of oxygen and a catalyst, reduces two different nitrogen oxides: 4 NO + 4 + → 4 + 6 ("standard SCR") and 6 + 8 → 7 + 12 ("NO2 SCR selective catalytic reduction") + + 2 → 2 + 3 ("fast SCR") The overall reduction of by urea is then: 2 + 4 + → 4 + 4 + 2 and 4 + 6 → 7 + 8 + 4 and + + → 2 + 2 + The ratio between and determines which reactions take place and how fast. The highest conversion rates are achieved if equal amounts of and are present, especially at temperatures between 200°C and 350°C. If there is more than , fast SCR and standard SCR take place sequentially. If there is more than , fast SCR and NO2 SCR take place sequentially, however, NO2 SCR is slower than standard SCR, and ammonia nitrate can form and temporarily deactivate the catalytic converter. Operation in winter time DEF freezes at . For the SCR exhaust cleaning system to function at low temperatures, a sufficient amount of the frozen DEF must be melted in as short time as possible, preferably on the order of minutes. For example, 2010 EPA emissions requirements require full DEF coolant flow within 70 minutes. In Europe, specified in