Commercial ships emit almost half as much particulate pollution as do cars and trucks around the world, according to a federal study announced Thursday.
Researchers for the National Oceanic and Atmospheric Administration and the University of Colorado concluded that ship pollutants affect both the global climate and the health of people living along coastlines.
"Since more than 70 percent of shipping traffic takes place within 250 miles of the coastline, this is a significant health concern for coastal communities," said the study's lead author Daniel Lack from Boulder, Colo., where he's a researcher with NOAA's Earth System Research Laboratory.
Particulate is tiny solid or liquid particles. Particulate from vessels can stay in the air seven to 10 days. Particulate suspended can enter the lungs and bloodstream, causing respiratory and cardiovascular problems including asthma and heart attacks.
"It really doesn't matter that the ship is not visible," Lack said. "The pollution is always there."
The study authors estimate that commercial vessels emit about 2.2 million pounds of particle pollution each year.
"It's a pretty big number when you think about how many ships there are out there compared to cars," Lack said of the world shipping fleet, which consists of more than 100,000 vessels.
Previous studies have estimated ship pollution based on fewer vessels surveyed. The results give researchers more confidence in those studies, Lack said, and should be useful to policy makers considering regulations.
Lack and colleagues in summer 2006 sailed on a research cruise from Charleston, S.C., to Houston on the NOAA ship Ronald H. Brown. They analyzed exhaust from more that 200 commercial vessels in the Gulf of Mexico, Galveston Bay and the Houston Ship Channel.
The research pulled in pollutants through a pipe that directed samples to 15 different instruments. Lack described the setup as a snorkel that fed samples to a shipping container retrofitted into a laboratory.
Researchers linked samples to the particular kind of vessel they were passing: cargo ships, tankers, ferries, cruise ships or other vessels.
With characteristics broken down by vessel type, researchers applied the numbers to the world fleet, making the assumption that ships operating in Houston or Galveston would have the same characteristics as vessels operating in Los Angeles, Europe or other world ports.
Ships put out three types of particulate: sulfates, tied to the amount of sulfur in their fuel; organic material, such as oil or tar; and soot or black carbon. Sulfates tend to absorb water, form clouds and fall out of the air faster.
Earlier research by one of the study's authors, James Corbett of the University of Delaware, linked particle pollution to premature deaths among coastal populations.
Ships also emit carbon dioxide, which has an opposite effect on the climate than particulate, according to the study. While carbon dioxide warms, the particles have a cooling effect that is at least five times greater than the global warming effect from the ships' carbon dioxide emissions.
The primary cooling effect is from the formation of clouds, which have a shading effect.
The findings appear online this week in the Journal of Geophysical Research.