Rosa Vela is always on alert, watching for signs her five-year-old daughter, Lettie, is having trouble breathing. She knows the next severe asthma attack could send Lettie to the hospital for yet another round of treatment.

“I feel very sad,” Rosa Vela said. “Seeing her in the hospital, it makes me feel desperate.”

Lettie Vela’s family lives in the Duwamish River Valley that lies between Beacon Hill and West Seattle, where rates for hospitalization of children for asthma are the highest in King County.

Residents here face an onslaught of toxic airborne pollutants that, according to a recent study, exceed regulatory caution levels by up to 30 times.

According to a second recent study, the Puget Sound region is in the top 5 percent of communities nationally for air toxics. And the industrial neighborhoods of Georgetown and South Park have some of the dirtiest air in the Puget Sound region.

Where is this toxic air coming from? The answer may surprise you. The majority of the pollution, government regulators and scientists say, comes not from the large concentration of industrial facilities in South Park and Georgetown. Rather, it’s from the cars, trucks and buses whizzing by these neighborhoods — especially those with diesel engines. Fumes from ships in Elliott Bay and the Duwamish, as well as diesel-powered equipment at the Port of Seattle and elsewhere, add to the toxic mix. In the fall and winter, wood smoke from fireplaces becomes a significant contributor.

The problems here have implications in other neighborhoods, too: Anywhere people are living close to major roadways, they’re likely breathing unhealthy air, studies show. Anyone living within about 200 yards of a major roadway is thought to be at increased risk, with the first 100 yards being the hottest pollution zone.

That’s the situation the Vela family faces. Two major roadways choked with diesel traffic intersect within a block of the Velas’ home, which is also located near an entrance ramp to Interstate 5.

The white-picket fence in front of the Vela home cannot hold back the waves of soot from passing trucks and buses. The soot comes with toxic hitchhikers such as benzene and formaldehyde — toxics that the second recent study showed are present at levels that make the air here among the most deadly in the region. And state Ecology Department mapping shows air pollution levels along south Seattle highways are some of the highest in the state, said Matt Kadlec, senior toxicologist at the agency.

For people who live in Georgetown and South Park, one of the most visible sources of air pollution is the fleet of privately owned trucks that haul freight from the port to rail yards and other destinations in south Seattle. More than 8,000 port-related truck trips occur on an average weekday, the port estimates, based on Washington Department of Transportation traffic counts.

Residents have fixed much of their ire on these cargo-hauling semis.

“One of our neighbors counted 85 trucks within 30 feet of homes. Every morning. Every day,” Georgetown Neighborhood Council Chair Holly Krejci said at a meeting called by City Council members Nick Licata and Mike O’Brien.

Overall the Port of Seattle trucks contribute only a fraction of the air pollution generated by the port. An inventory of air pollution sources at the Puget Sound ports of Tacoma and Seattle found that Seattle trucks were responsible for only about 3 percent to 4 percent of the regional air pollution attributable to the two ports, said Stephanie Jones-Stebbins, manager of environmental affairs for the seaport. However, those trucks are heavily concentrated in the Duwamish Valley. Every time a truck makes a run, it pollutes — right there in someone’s neighborhood.

Port officials admit they don’t know what proportion of air pollution in the Duwamish Valley comes from the trucks.

“Obviously, what comes out of that tailpipe ends up in someone’s back yard pretty quickly,” said Michael Yost, the University of Washington researcher who was lead investigator on one of the two recent studies, measuring air toxics in south Seattle for the Puget Sound Clean Air Agency.

The two recent studies released in February come on top of a special federal study of the Duwamish Valley completed in 2008 that showed elevated cancer risks from airborne toxics occur across a wide swath of south Seattle from Seward Park to the western shores of West Seattle.

Liana Beal, who grew up in South Park and raised her asthma-plagued son there, still sees trucks frequently passing by her family home.

“Sometimes they have a lot of smoke coming out. They are in bad repair, bad shape,” she said. “There is always that diesel smell.”

Air pollution researchers all know that the dirtiest air is likely to be beside the biggest highways. So it’s a little surprising that Joel Kaufman of the University of Washington, one of the nation’s leading air-pollution researchers, has his lab located right underneath Interstate 5 on near the north shore of Lake Union.

However, there’s even dirtier air nearby: inside Kaufman’s lab, where scientists expose human test subjects to exhaust from a diesel engine to see what happens.

“All right — the exposure will be two hours, and I think we’re squared away here,” Kaufman said as he prepared to subject a volunteer to what he estimated would be about twice as bad as standing at a bus stop.

With a technician watching from the other side of a window, the human guinea pig was wired up to various special measuring instruments. The indoor diesel engine coughed to life and a maze of tubes guided the exhaust into a sealed glass room.

“Looks like he’s doing OK,” the technician reported. “His oxygen concentration’s fine.”

In the course of the test, the test subject breathed in tiny globs of diesel soot.

“They start a process of inflammation in the lungs, which trigger downstream effects on the respiratory system, cardiovascular, and maybe the neurological system,” Kaufman said. “It’s this inflammatory cascade (that) builds to become effects we can observe throughout the body.”

Diesel exhaust causes blood vessels to constrict and elevates blood pressure, an effect that lasts for hours, he said. “There’s something about traffic-related pollution that seems to have a way of accelerating heart disease and . . . asthma.”

“Most people are not able to perceive common urban air pollutants when they’re present in average amounts,” Ecology’s Kadlec said. “But sometimes the amounts of these pollutants get high enough for people with normal-sensitivity to notice. Cough, sore throat, eye irritation, and greater susceptibility to then longer-lasting sinus and common cold infections are typical responses to urban air pollution.

“People tend not associate these conditions with air pollution.”

Another condition people tend not to associate with air pollution: cancer.

Living with bad air significantly boosts the risk of contracting cancer, compared with living in an area with less air pollution, according to the 2008 federal study. Authorities have traditionally grown concerned about anything that caused a risk of more than one cancer in a million people exposed to a substance.

The 2008 study showed that air pollution raised cancer risks to as high as 400 extra cases per 100,000 people in Duwamish Valley hot spots. All along the highways, risks are elevated to between 40 and 80 extra cancers per 100,000 people exposed. And across the rest of south Seattle, from Seward Park to the western shores of West Seattle, the numbers were still elevated — from five to 40 extra cancer cases per 100,000 people exposed. (These numbers are based on 70 years of exposure, and are assumed to be a worst-case scenario.)