Low sulfur fuel is any petroleum-based fuel that has been refined to contain significantly less sulfur than conventional fuel. For highway diesel in the United States, that means a maximum of 15 parts per million (ppm) of sulfur, down from the 5,000 ppm that diesel contained before federal regulation. The goal is straightforward: sulfur in fuel becomes sulfur dioxide when burned, which triggers acid rain, creates smog, and causes serious respiratory harm. Removing it at the refinery prevents those problems at the tailpipe, smokestack, or ship exhaust.
Sulfur Limits by Fuel Type
The term “low sulfur” covers several tiers depending on the fuel’s intended use. For on-road vehicles in the U.S., the EPA phased in ultra-low sulfur diesel (ULSD) starting in 2006, capping sulfur at 15 ppm. By 2014, that same 15 ppm cap applied to all nonroad, locomotive, and marine diesel sold domestically. Before the ULSD standard, the intermediate step was 500 ppm diesel, which was formally labeled “low sulfur diesel” at the time.
Gasoline follows a parallel track. The EPA’s Tier 3 standards lowered gasoline sulfur to an annual average of 10 ppm, making modern gasoline essentially ultra-low sulfur as well.
International shipping operates on a different scale entirely. The International Maritime Organization’s IMO 2020 rule cut the maximum sulfur content in marine fuel from 3.5% to 0.50% (5,000 ppm) for ships operating in open waters. Inside designated emission control areas, including the Baltic Sea, North Sea, and coastal waters off the U.S. and Canada, the limit drops further to 0.10% (1,000 ppm). The Mediterranean Sea is being added as an emission control area, with the stricter limit taking effect in May 2025. The Canadian Arctic and Norwegian Sea will follow in 2027.
How Sulfur Gets Removed
Refineries strip sulfur from fuel through a process called hydrodesulfurization. The fuel passes through a reactor packed with a metal catalyst while hydrogen gas is pumped in at high temperature and pressure. Sulfur compounds in the fuel react with the hydrogen, converting into hydrogen sulfide gas, which is then captured and removed. The result is fuel with sulfur content low enough to meet regulatory limits.
This process is effective but not without trade-offs. The same chemical treatment that removes sulfur also strips out trace compounds, particularly heavy aromatics and molecules containing oxygen or nitrogen, that naturally lubricate fuel system components. Without those compounds, fuel injectors and pumps can wear out faster. Common rail injection systems, which operate under extremely tight tolerances, are especially sensitive to this loss of lubricity. Refiners and fuel blenders compensate by adding lubricity-improving additives, typically fatty acids, synthetic amides, or small amounts of biodiesel. Concentrations as low as 10 to 100 ppm of these additives are enough to restore adequate protection.
Why Sulfur in Fuel Is a Health Problem
When fuel containing sulfur burns, it produces sulfur dioxide. In the atmosphere, sulfur dioxide undergoes chemical reactions, combining with oxygen and water to form sulfate particles. These particles are a major component of PM2.5, the fine particulate matter small enough to penetrate deep into lung tissue and enter the bloodstream. PM2.5 exposure is linked to heart attacks, strokes, lung cancer, and worsening of asthma and chronic lung disease.
Sulfur in fuel also poisons the catalytic converters and particulate filters that vehicles use to control other pollutants. High-sulfur fuel coats these devices with sulfur compounds, reducing their effectiveness. The push to 15 ppm diesel in the U.S. was driven partly by the need to enable advanced emissions control technology, including catalyzed particulate filters and systems that reduce nitrogen oxide emissions. Those devices simply cannot function properly with high-sulfur fuel running through them.
Measurable Health Benefits
The shift to cleaner fuels has produced quantifiable results, particularly in shipping. A study published in Nature Communications estimated that the IMO 2020 marine fuel standard would prevent roughly 266,300 premature deaths per year globally, a 34% reduction in ship-related mortality from cardiovascular disease and lung cancer. Childhood asthma cases linked to shipping emissions were projected to drop by 54%, from 14 million affected children to 6.4 million. These numbers reflect how large the maritime contribution to air pollution was before the rule took effect, especially in coastal cities and port communities.
On-road standards have delivered similar benefits at a local level. Cities near major highways saw measurable drops in fine particulate pollution after ULSD replaced conventional diesel, though the effect is harder to isolate because vehicle emissions standards tightened simultaneously.
What Ships Actually Burn Now
The maritime industry adapted to IMO 2020 primarily through a fuel called Very Low Sulfur Fuel Oil (VLSFO), which meets the 0.50% sulfur cap. VLSFO accounted for about 56.8% of the global marine fuel market in 2024 and continues to grow. Some operators chose alternative compliance routes: installing exhaust gas cleaning systems (scrubbers) that let them keep burning cheaper high-sulfur fuel while capturing sulfur from the exhaust, or switching to liquefied natural gas, which contains virtually no sulfur. A smaller share of the market uses Ultra Low Sulfur Fuel Oil (ULSFO), which meets the stricter 0.10% emission control area limits.
For everyday drivers, compliance is invisible. Every gallon of diesel and gasoline sold at U.S. pumps already meets ultra-low sulfur standards. The transition happened nearly two decades ago, and modern engines are designed around it. The only practical consideration is for operators of older diesel equipment or boats, who should verify they’re using ULSD to protect both their emissions systems and their engine components.
California’s Early Lead
California adopted diesel sulfur limits in 1988, years before federal rules caught up. The state’s original standard capped sulfur at 500 ppm and restricted aromatic hydrocarbon content to 10% by volume. California then moved to 15 ppm in 2006, applying the limit to both on-road and off-road diesel simultaneously rather than phasing in the nonroad sector later as the federal program did. This pattern of California setting stricter standards first and the EPA following has repeated across several fuel and emissions regulations.