Ammonia slip is the portion of ammonia (NH3) that leaves an SCR system unreacted. Ammonia forms in the exhaust from the injected urea solution and is meant to react completely with the nitrogen oxides on the SCR catalyst. If more reductant reaches the catalyst than it can convert, the excess passes through with the clean gas. Common causes are dosing that is too high or responds too slowly to load changes, uneven distribution of reductant across the duct cross-section, exhaust temperatures that are too low, or an aged catalyst with declining activity. Ammonia slip causes odour, can combine with sulphur oxides to form ammonium sulphate deposits and is itself a relevant emission. Remedies include a properly designed mixing section, dosing control matched to load and temperature and, where needed, a downstream ammonia slip catalyst. Rising urea consumption without a corresponding improvement in NOx reduction is often an early sign that the SCR stage needs checking.