People who live in Philadelphia Housing Authority apartments — many of them children and the elderly with asthma and other lung conditions — are taking in a lot less secondhand smoke these days, researchers are reporting.

Exposure to others' cigarette smoke, connected with a long list of health woes, is about half what it was before the Housing Authority went smoke-free 18 months ago, according to a study that comes as smaller housing agencies prepare their own tobacco bans.

No-smoking rules have expanded rapidly in recent decades, in public places including airplanes and airports, schools, government buildings, and restaurants. As public distaste grew along with evidence of harm from secondhand smoke, the indoor bans morphed into smoke-free policies that included outdoor campuses and parks.

Yet places where people are exposed the most — and could benefit most from smoking bans — often changed last. 

"Public housing is a very, very important place to change," said Ann Klassen, a professor at Drexel University Dornsife School of Public Health who led the new study. Compared with the rest of the city, a larger share of subsidized housing residents are very young or very old.  And low-income Philadelphians smoke at far higher rates than the general population.

"Even if you keep a smoke-free home you can't protect yourself from other smokers in the building," said Klassen,  "because about 65 percent of the air in apartments in multiunit buildings comes from outside the apartment."

The Philadelphia Housing Authority, with 80,000 residents, is the largest public housing agency in the country to implement a comprehensive ban. It covers apartments, hallways, parking lots, and every other part of the properties, inside and out.

For the study, to be published in the spring issue of Tobacco Regulatory Science, Klassen's team interviewed residents in 172 households and hung nicotine monitors in dozens of locations at four PHA sites, twice before the ban took effect in August 2015, and once after. A survey at the beginning of the study found that 56 percent of residents smoked, more than double the city average and triple the national rate. A follow-up survey to see how many are now smoking has not been done.

Nicotine did not disappear from any of the sites. But average levels in common areas dropped by half from 2013 to 2016, with smaller decreases inside the units.

A previous study after the Boston Housing Authority went smoke-free found an even greater decline in airborne nicotine there but an increase in cotinine, a tobacco metabolite found in saliva. The authors speculated that smokers (accompanied by their nonsmoking neighbors) might have continued their habit away from the property, although other evidence did not support that notion.

The Philadelphia study did not collect saliva for analysis. But even if it turns out that smokers took their cigarette breaks elsewhere, Klassen said, changes in the environment like PHA's ban create "a teachable moment" that can have a broader impact on habits.

That's what happened after the Housing Authority of Chester County became the first public housing agency in the Philadelphia region to go smoke-free, in 2013,  said director Dale Gravett. "Over time it has become accepted by the residents in the way that [similar policies] have become accepted when going to a restaurant," he said.

Two residents were evicted for violating the ban, Gravett said, and some others have decided not to move in. But overall the policy has been "a great marketing tool" with senior citizens, who make up a majority of the authority's 330 units, "happy to see that it is a health-conscious site."

It also is cheaper. Units whose residents smoked take "an inordinate amount of money, triple or quadruple the cost to rehab," said Karen Blumenfeld, executive director of the New Jersey-based Global Advisers on Smokefree Policy (GASP). And "unfortunately there are many people who have health issues that are in public housing," she said, noting the dangers of smoking near portable oxygen.

The Montgomery County Housing Authority went smoke-free on July 1.

Housing authorities in Bucks, Delaware, and Gloucester Counties are in the early planning stages of smoke-free policies (other counties in the region don't have public housing agencies); a spokesman for the City of Chester Housing Authority said it hopes to go smoke-free soon.

All were being pushed to act by a new rule from the U.S. Department of Housing and Urban Development expected to require smoke-free public housing within the next two years.

Like multiple other rules finalized in the waning hours of the Obama administration, however, this one appears to be on hold, at least temporarily, while Republicans conduct a review in an effort to reduce bureaucracy.

Thurman Brendlinger, tobacco program director at the Clean Air Council in Philadelphia, said he is optimistic that it will be approved because President Trump's nominee for HUD secretary, Ben Carson, is a physician who has seen firsthand "the impact of smoking."