Sierra Oliver, age 2, fell asleep after having lunch with her dad, Andre Foster, in the lobby at the Days Inn motel. The motel is being used as temporary housing for the family which is homeless. (Michael S. Williamson/The Washington Post)

Washington is a city that wheezes. It has one of the highest rates of asthma in the country. Those who suffer from the disease cluster in the city’s poorest communities, where asthmatic children are 10 times more likely to visit the emergency room than in wealthy neighborhoods. Some of the most dangerous air for these children to breathe, studies say, is laced with exhaust.

So advocates were alarmed when officials announced this year that they’re planning to open a shelter for homeless families within a football field’s length of the District’s largest bus garage — one of the city’s biggest producers of exhaust, a known asthma trigger.

The proposal, announced in February, is part of a controversial plan to close D.C. General, a former hospital that serves as the city’s shelter for 247 homeless families, and disperse its residents into seven new facilities spread evenly across the city.

Some of those facilities would materialize in affluent, tree-lined neighborhoods anchored by Whole Foods markets and Metro stops. The Ward 5 site, however, sits behind barbed wire off Bladensburg Road, in the shadows of a concrete facility and a gargantuan warehouse where Metro houses 260 buses, boilers, large generators and a hazardous waste shed. In 2014, the Environmental Protection Agency cited the facility twice for improper handling of hazardous waste, including failure to properly mark two waste containers.

But it’s the quality of the air near the garage that is of particular concern to environmental health advocates, attorneys for the homeless and pediatricians.

Elizabeth Gates, a Unity Health Care pediatrician who has treated dozens of children staying at D.C. General, estimates that one-third suffered from asthma, a condition she said is often more prevalent among homeless children.

“Before you put people there, and families there, you really need to check” the air quality, said Janet Phoenix, an assistant professor at George Washington University and an official with Breathe DC, which tracks asthma in the city.

The question of how polluted the air is at the Ward 5 site has been frustratingly difficult to answer. The city hasn’t specifically tested the air quality at the location, on which the shelter is slated to open in January 2018 and house up to 50 families. Yet officials say there’s no evidence suggesting it’s unsafe.

Mayor Muriel E. Bowser’s administration considered two sites for the shelter, one of which was excluded because it was too small. Officials have acknowledged that they stopped looking for an alternative site early last year once they considered the bus depot site to be of an appropriate size and the proximity to a bus stop on Bladensburg Road to provide adequate access to public transit.

Community groups have fiercely opposed the site for environmental and other concerns. They have since proposed a dozen alternative sites that the administration has quickly dismissed for various reasons. But Bowser’s team has yet to respond to a proposal by another property owner to lease a parcel for less money a mile away, near Langdon Park.

The site near the bus depot “is an appropriate site,” spokesman Michael Czin said. “It has the square footage we need and indoor and outdoor play areas, and it’s available. . . . There’s no evidence that there would be environmental issues there that would impact health.”

Seven air quality stations spread throughout the District have not detected increased pollution as a result of the garage, said Julia Christian, public information officer for the District’s Department of Energy and Environment.

But critics say the air quality stations, which produce an average air pollution rating for the entire city, are too far away to accurately detect airborne pollutants near the garage. The closest one is miles away.

The stations “are not intended to assess exposure for something very specific in a specific section of the city,” Phoenix said.

And the data that is available, advocates say, isn’t encouraging. Jerry Paulson, a professor of environmental health at George Washington University, pointed to the federal government’s Air Quality Index, which designates a score from 0 to 500 for degree of pollution. The District’s citywide Air Quality Index is sometimes in the 80s. Anything between 100 and 150, according to the index, is “unhealthy for sensitive groups” — like children with asthma.

“We know that kids who have asthma are likely to have exacerbations of asthma if they are in areas with air pollution,” Paulson said. “. . . Any high volume area with a mix of traffic will have high air pollution, but now you’re talking about an area with a lot of diesel buses and a concrete facility.” It’s likely, he said, that the Air Quality Index in the area around the bus garage is above 100.

The concern over the air quality at the site highlights yet another division between rich and poor in the city. Those with asthma in the District are overwhelmingly African American and live south and east of the Anacostia River — the same poor communities that produce the bulk of the city’s homeless families. According to the Urban Institute, African American children in the District are three times more likely to have asthma and nearly seven times more likely to die because of it.

“It’s a perfect storm,” in the District, said Stephen Teach, chairman of the Department of Pediatrics at Children’s National Hospital, where he supervises an asthma program servicing Washington’s poor. Not only is there a high concentration of minority residents in the District, he said, but there’s also “adverse housing conditions, low access to primary services, high rates of smoking and poor indoor air quality” — all of which exacerbates asthma.

“We try to minimize exposure to triggers,” he said. “And particulate matter” — such as that found in exhaust spewed by buses at a depot — “is a well-identified asthma trigger.”

One study conducted in Los Angeles and published in the Journal of Allergy and Clinical Immunology in 2014 found that air pollution near roadways resulted in childhood asthma. Another, published in Environmental Health Prospectives in 2012, discussed an “emerging consensus” that exposure to roadway traffic leads to asthma. A 2007 Lancet study tracked nearly 3,700 children over eight years, finding that living within 500 meters of a freeway had “detrimental, and independent, effects on lung function.”

Some parents at D.C. General with asthmatic children are already fretting.

April Williams, 25, who has lived at D.C. General for almost a year, said that her 2-year-old son, Antwain, has a bad case of asthma. Twice a day, Williams fits a nebulizer mask around his tiny features to help him breathe. She remembers the first night she knew something was wrong.

“He was wheezing, throwing up, coughing,” she said of that evening last autumn. “He couldn’t really breathe.” She ultimately had to call an ambulance. He’s been taken to the hospital for asthma 10 times since then, she said. “I don’t want to keep going back to the hospital,” she said. “When you keep going back to the hospital they think you doing something, and it’s not that. It's the environment you’re in.”

She’s worried about continuing to raise him at D.C. General, which she thinks has exacerbated his asthma. But she also fears living somewhere that could make his condition worse.

“I would be scared for my son,” she said.

This story has been updated.

Aaron C. Davis and Luz Lazo contributed to this report.