Date: Aug 6, 2026 PDF: 553 KB
Selective Catalytic Reduction (SCR) systems are installed in diesel-powered vehicles to reduce emissions of nitrogen oxides (NOx). Leveraging Diesel Exhaust Fluid (DEF) to regenerate the catalyst, these systems have helped reduce NOx emissions in the United States by 79% since 2010. Operator concerns regarding requirements that engines be “derated” in the event vehicles run out of DEF or the available DEF does not meet ISO quality standards, have led the U.S. Environmental Protection Agency to issue guidance and proposed regulations to change the requirements regarding DEF.
The Transportation Energy Institute convened a diverse task group of interested stakeholders, including those representing vehicle operators, vehicle and engine manufacturers, diesel fuel and diesel exhaust fluid producers and distributors, public health and environment advocates, to develop a resource that presents the current state of the market objectively. This new TEI Market Report provides facts about SCR systems and the DEF market to help guide discussions about the proposed regulations. It does not advocate for an outcome but presents facts to help interested stakeholders better understand the state of the market and explore opportunities to preserve emissions reductions while ensuring vehicle operators are not subject to unreasonable conditions.
Executive Summary:
- Nitrogen oxides (NOx) can contribute to the formation of ozone, which inflicts billions of dollars of damage to American industries and agriculture and can cause significant respiratory illnesses.
- In 2010, diesel engines in on and off highway vehicles accounted for 65% of U.S. NOx emissions.
- Installation of Selective Catalytic Reduction (SCR) systems and use of Diesel Exhaust Fluid (DEF) on nearly all heavy-duty vehicles and nonroad equipment has contributed to a 76% and 42% reduction in NOx emissions, respectively, accounting for 79% of total NOx reductions in the U.S. since 2010.
- More than two-thirds of commercial diesel trucks in operation are equipped with SCR systems, and DEF is available at more than 100,000 retail locations in North America.
- By shifting the burden of NOx emissions control from the engine to the aftertreatment system, engines have been optimized for maximum fuel efficiency.
- Guidance issued by the U.S. Environmental Protection Agency (EPA) in 2025 and 2026 provides more time for operators to make repairs to the SCR/DEF systems before derating, addressing a primary user concern about these systems.
- SCR systems can experience significant and costly damage if operated without DEF.
- The continued supply and use of DEF are critical to ensure lasting air quality improvements, and the durability, performance, and efficiency of diesel engines and equipment.
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