By 2020, the global shipping fleet will be required to slash the noxious emissions from thick, sulfur-laden “bunker” fuel, a move that is expected to sharply reduce air pollution and prevent millions of cases of childhood asthma and other respiratory ailments.

Along the Houston Ship Channel, a 52-mile waterway that spills into the Gulf of Mexico, giant vessels cruise beneath the blazing summer sun. Rusty tankers fill their holds with Texas specialties: refined oil products, petrochemicals, and plastic resins. Container ships arrive carrying corrugated boxes of imported T-shirts, electronics, and metals. Inside each freighter, however, one cargo is the same: the heavy fuel oil that drives their engines and winds up in the air as exhaust. Cargo ships are significant sources of air pollution globally, and their fuel oil is largely responsible. Pitch black and thick as molasses, “bunker” fuel is made from the dregs of the refining process. It’s also loaded with sulfur — the chemical that, when burned, produces noxious gases and fine particles that can harm human health and the environment, especially along highly trafficked areas. “There are lots of communities that live fenceline to the ship channel,” said Grace Tee Lewis, who is studying the public health effects of Houston’s ship pollution at the Environmental Defense Fund. Near Houston, boats are required to switch to low-sulfur fuels in an effort to limit air pollution, “but we also know emissions from ships can travel hundreds of miles inland.”

“There are very few examples of air quality regulations that have as broad a reach of benefits as this one,” says one scientist.

As more evidence points to the risks of burning bunker fuel, the global maritime industry is embarking on a major overhaul of its fuel supply. Starting January 1, 2020, the International Maritime Organization (IMO) will require that all fuels used in ships contain no more than 0.5 percent sulfur. The cap is a significant reduction from the existing sulfur limit of 3.5 percent and is well below the industry average of 2.7 percent sulfur content. Public health experts estimate that once the 2020 sulfur cap takes effect, it would prevent roughly 150,000 premature deaths and 7.6 million childhood asthma cases globally each year. “There are very few examples of air quality regulations that have as broad a reach of benefits as this one,” said James Corbett, a professor of marine science and policy at the University of Delaware, who has conducted seminal research on international shipping emissions. “This is going to benefit children and adults in coastal communities located along major shipping lanes — not only [near ports] where ships are delivering cargo.” The IMO is also taking early steps to reduce another form of harmful emissions: greenhouse gases. In April, the UN agency adopted a historic deal to curb carbon emissions from ships by at least 50 percent below 2008 levels by 2050. The non-binding agreement is expected to spur more investment in clean ship technologies, including fuel cells, biofuels, and advanced sail designs. For now, however, shipping companies are laser-focused on the impending sulfur cap. But switching fuels won’t be as simple as selecting a higher grade of gasoline at the pump.

Samples of heavy bunker fuel oil and distillate fuel at Viswa Labs in Houston. Maria Gallucci

Low-sulfur alternatives are generally more expensive and less widely available than bargain-rate bunker fuels. In some corners of the industry, this has led to much hand-wringing and anxious predictions of fuel shortages and spikes in cargo rates. But many companies are nevertheless working to comply with the cap, including by installing exhaust scrubber systems and switching to liquefied natural gas. “There’s strong public pressure forcing governments to impose these regulations,” Ram Vis, director of Viswa Group, said from his fuel quality testing laboratory in Houston. “You’re not going to escape it. Sulfur has to go. Will it go in a gradual way, or will it be abrupt? We think it will take some time.” The IMO doesn’t have authority to enforce the sulfur cap; that task falls to flag states, the countries to which vessels are registered. Uncertainties remain about how, and how well, authorities will inspect and monitor ships’ fuel usage. But in general, ships caught breaking the rules would risk steep fines, damage to their reputations, and the potential loss of insurance coverage. In Vis’s laboratory, acrid fumes wafted from beakers filled with bunker fuel. Technicians wearing blue rubber gloves hovered over their microscopes, looking for impurities that might damage ships’ diesel engines. Because bunker fuel is made from petroleum refining residues, waste byproducts sometimes wind up in the mix. Black and opaque, the fuel “looks like a nice receptacle for dumping whatever you want to get rid of,” Vis explained. Maritime regulators began discussing ships’ noxious exhaust gases in 1973, at the first MARPOL (short for “marine pollution”) convention. But it wasn’t until 1997 that the IMO adopted Annex VI, which established limits on sulfur oxide and nitrogen oxide emissions from ocean-going ships. Eight years later, those limits entered into force, with a 3.5 percent sulfur cap set for 2012 and the 0.5 percent cap for 2020.

Roughly 70 percent of shipping emissions occur within 250 miles of land, exposing hundreds of millions of people to harmful pollutants.

Since that time, global shipping activity has continued to soar as populations rise and economies expand. Cargo ships unloaded about 10.3 billion metric tons of goods worldwide in 2016 — a 300 percent jump from 1970 volumes, according to UN trade statistics. Today, the shipping industry is one of the world’s largest emitters of sulfur oxides, behind the energy industry. Roughly 70 percent of the maritime industry’s emissions occur within 250 miles of land, exposing hundreds of millions of people to harmful pollutants. Yet cargo ships, tucked within industrial ports or cruising far offshore, are often out of sight, leaving the public oblivious to the health hazards posed by shipping emissions. Sulfur oxides (SOx) can harm people’s respiratory systems and cause breathing difficulties, particularly for children, older adults, and asthma sufferers. Sulfur pollution also contributes to airborne particulate matter (PM), the tiny particles that enter the bloodstream and damage the lungs and heart. That can lead to heart attacks, aggravated asthma, increased hospital admissions, and premature deaths. Chronic exposure to shipping-related fine particulate emissions results in about 400,000 premature deaths each year from lung cancer and cardiovascular disease, according to a 2018 study. Among children, acute exposure to ship emissions contributes to roughly 14 million asthma cases annually, researchers from the United States and Finland said. The study found that switching to lower-sulfur marine fuels would dramatically improve those health outcomes — particularly in parts of Asia and Africa with heavy shipping activity. Sulfur caps have already proven effective in certain parts of Europe and North America, where governments have established Emissions Control Areas. In these zones, cargo ships are required to use low, or ultra-low, sulfur fuels as they approach the coastline. But because these cleaner fuels are more expensive, ships often switch back to burning cheap bunker fuel once they leave designated waters.

Concentrations of air pollution along global shipping routes. Until recently, most of the world's 10 busiest ports — nine of which are in Asia — had no limits on emissions. Nature

China, home to seven of the world’s 10 busiest container ports, started rolling out its own low-emissions zones in 2017. Ships entering about a dozen major ports must now switch to 0.5-percent sulfur fuel while at the dock. A government report found the rule has already reduced sulfur emissions in certain regions. When the global sulfur cap takes effect, it will improve air quality for all of China’s coastal residents, said Freda Fung, a Hong Kong-based consultant for the Natural Resources Defense Council. “It’s very encouraging to see what’s happening,” she said. In 2014, Fung co-authored a report that found one container ship off China’s coast emits as much diesel fuel pollution as 500,000 new trucks in a single day.

As of now, global shipping companies have no silver bullet for complying with the stricter sulfur limits. Any alternative will require large investments and alter ships’ operations. And none of today’s leading solutions makes a clean break from fossil fuels. While “zero-emissions” technologies, such as hydrogen fuel cells and battery-electric systems, are now powering ferries and research vessels, they are not yet ready for deployment at the massive scale of ocean-going freighters. Initially, the majority of ship owners are expected to replace heavy bunker fuel with lower sulfur fuels such as marine gas oil, a petroleum product that’s already used in Emission Control Areas. This fuel is light and nearly transparent, dyed yellow or red like sugary sports drinks to distinguish it from other types of oils. With the switch, global shipping fuel costs may rise by at least $24 billion in 2020, in part because the cleaner fuels cost more to produce, Wood Mackenzie, a consulting firm, said in April. That’s up nearly 25 percent from today’s shipping fuel bill of roughly $100 billion a year.

A small percentage of ships are expected to run on liquefied natural gas, which has only recently made inroads in the marine market.

Around 10 to 15 percent of ships are projected to keep burning high-sulfur fuels and install scrubber systems, which capture SOx and fine particulate emissions before they escape exhaust funnels. Scrubbers allow companies to keep using cheap bunker fuel, but the technology can have drawbacks, beyond the high upfront cost. Some models require significant power and freshwater supplies to operate. The collected pollution must be neutralized and dispersed into the ocean or deposited on land. In Houston, the Viswa Group has created the first-ever horizontal scrubber that it says weighs less, and consumes less energy, than conventional vertical scrubbers. Its laboratory is also in the early stages of developing a “desulfurization” process to clean up heavy bunker fuel still sold on the market. A small percentage of cargo ships are expected to run on liquefied natural gas (LNG), a fuel that has only recently made inroads in the marine market. Replacing heavy bunker fuel with LNG would reduce SOx and nitrogen oxide emissions by 90 to 95 percent and curb carbon dioxide emissions by 20 to 25 percent, according to industry estimates. “Compared to other solutions to meet the sulfur cap, LNG is sort of a one-stop shop when it comes to emissions [reductions],” said John Adamo, who studies the sustainability of LNG marine fuels at the University of Texas at Austin’s Energy Institute. Yet LNG faces a chicken-and-egg dilemma, he said. Facilities to supply LNG are few and far between. That may change as more companies purchase ships that can run on LNG to comply with the 2020 sulfur cap. About 120 LNG-capable ships are already on the water, and shipping giants such as CMA CGM have ordered more to join their fleets.