John D. Spengler’s research has led to smoking bans on airplanes and heightened awareness of childhood asthma in public housing. Yet his pioneering focus on indoor air quality got its start while he worked on a landmark study exposing the health risks of outdoor pollution.

Spengler and other researchers working on Harvard’s “Six Cities” study of 8,000 Americans in six cities — launched in the 1970s — considered subjects’ smoking history and made a surprising discovery.

“Seventy-five percent of the kids lived with smoking parents or they were cooking with gas for nitrogen dioxide particles,” Spengler said during a recent Harvard talk reflecting on his 50-year career. “So Topeka, Kansas, had as much air pollution that the kids and adults were breathing as a dirty city, because of indoor sources. That made it very complex. So that got us really curious about the indoor environments.”

Spengler, the former Akira Yamaguchi Professor of Environmental Health and Human Habitation, retired on Jan. 2 to take up a research professor role. Last week he sat down with longtime collaborator Linda Powers Tomasso, a research associate in environmental health in the Harvard T.H. Chan School of Public Health’s Leadership Studio, to discuss five decades of environmental health progress.

The Six Cities fieldwork began during the oil embargoes of the 1970s, which prompted moves toward energy efficiency. But sealing up cracks and better insulating homes just made indoor air worse, Spengler said.

“All of a sudden, people were tightening up homes, they were shutting off ventilators for schools to save money,” Spengler said. “Air pollution indoors got worse, so that those things sort of converge to say this is an important area and many doctoral students’ dissertations later, it’s still important.”

Six Cities is credited with prompting Congress to adopt the Clean Air Act Amendments of 1990. Those amendments tightened restrictions on particulate pollution, sulfur dioxide, and nitrogen oxides, which cause acid rain, and set tougher pollution controls on vehicles. In the years since, the study has come under assault by industry groups and some politicians looking to roll back the Clean Air Act restrictions.

Spengler’s work on indoor air pollution continued through the 1980s and the decades since. In 1983, he co-authored an influential report that investigated, among other things, air pollution on airplanes. After bringing air quality monitoring equipment on flights, they found pollution levels could top 1,000 micrograms per cubic meter of air — smokier, Spengler said, than “the smokiest bar you ever went to.” Of the 10 recommendations in the report, the FAA took only one and banned smoking from airplanes.

“The one that the FAA took — and the airlines were happy — was to get rid of smoking,” Spengler said. “The flight attendants gave me a bottle of wine.”

Also in the 1980s, Spengler, who played recreational hockey, found himself wondering why ice rinks smelled like garages, and brought his air pollution instruments to investigate. He found high levels of carbon monoxide, carbon dioxide, and nitrogen oxides due to the internal combustion engines on the ice-smoothing Zamboni machines. That coincided, he said, with cases of hockey players passing out during tournaments due to carbon monoxide. He studied the problem regionally and internationally and reached out to the machines’ manufacturers, who began to explain to rink owners safe use of the machines.

In recent decades, as his work on childhood asthma in Boston’s public housing progressed, Spengler worked with the Boston Housing Authority to reduce what were found to be triggers: cigarette smoke, dust mites, pets, and cockroaches.

Spengler was also instrumental in making changes on Harvard’s campuses. He created the master’s program in environmental management and sustainability at the Harvard Extension School. And, with former Vice President for Administration Thomas Vautin, he founded the Harvard Green Campus Initiative, which was a hub for sustainable operations on campus. With the support of several Harvard presidents, the Green Campus Initiative leveraged current research to make Harvard’s operations more sustainable and evolved into today’s Office for Sustainability.

“It is embedded in everything the University does and we should all be proud that this University has more green buildings certified than any campus in the world,” Spengler said.

If there’s a unifying theme, Spengler said, it’s that these indoor air quality issues affect virtually everyone and, though materials may change, they have similar causes and require a systemic solution, taking into account ventilation, filtration, and sustainability. For example, in his studies of public housing, many business managers who found pests on their property would spray pesticides without considering how that would affect the home and its occupants.

“The common denominator is that everyone lives somewhere. We all have residences and, as our time activity studies say, we spend a lot of time indoors and a lot of time in our houses,” Spengler said. “The issues there might change with modernity of products and outgassing, but the issues are pretty much the same. How do you treat water, dampness, mold, infestation, insects? These are everywhere but it’s never thought of as a whole system. How does the house handle this?”

In explaining his success, Spengler praised the strong teams he was part of at the Harvard Chan School; his family, who supported his work; and his students, many of whom he’s kept in touch with.

“They have so much to teach me, to watch their careers change, watch how they’ve raised their families, where they have impacts in their communities, their colleges, and on the global stage,” Spengler said, adding that some former students lead schools of public health, and one is the first woman president of a major university in Taiwan. “Who wouldn’t want to see this unfold in front of your eyes?”