Frank Eliason didn't know any better.
Lunch in hand, he pushed open the wrong door of the new Comcast tower. He used the swing door, not the revolving door - not the right choice for someone aiming to be environmentally correct.
Which he is.
When I pointed this out to him, he was aghast.
"I care!" he said. "I drive a Prius!"
More than 100 years after Philadelphian Theophilus Van Kannel invented the revolving door, many pedestrians still don't know that it's better to spin than swing.
Van Kannel, in prescient anticipation of future climate concerns, had conservation in mind when he invented his door.
" ... as the door fits snugly in the casing," his 1888 patent states, "it is perfectly noiseless in its operation and effectually prevents the entrance of wind, snow, rain or dust either when it is closed or when persons are passing through it.''
The beauty of the revolving doors is etched in Van Kannel's company motto: "Always open, always closed."
Of course the amount of air, hot or cold, that's exchanged through a swing door varies from building to building, season to season.
The calculation factors complexities like gravitational acceleration and pressure coefficients, but also simple things, like whether it's windy outside.
Generally, however, eight times more air is exchanged when someone uses a swing door instead of a revolving.
This information comes to us thanks to four students at M.I.T. who studied door use for a class project. They found that a swing door wastes the equivalent of 4.3 hours of light from a compact fluorescent bulb, or driving the average car 306 feet.
At one oft-used M.I.T. building, the researchers calculated that if everyone - instead of the 23 percent they observed - used the revolving doors, it could save $7,500 in natural gas payments and spare the planet 15 tons of CO2 emissions a year.
So how come more don't?
Dan Wesolowski, 28, who has just about finished a doctorate in materials science and engineering, and his cohorts spent nearly 50 hours watching the comings and goings of students.
They noted that people tended to follow the leader or take the shortest route. (One set of swing doors was two steps closer than the revolving doors.)
They surveyed students. (Nonusers felt the revolving doors were stressful and took more effort. Users knew about the energy savings. Or simply thought the doors were more fun.)
They tested signage. (Explaining the eco-benefits didn't help much. It made door selection seem burdensome and, Wesolowski said, some people "don't like environmentalists pushing their will." But putting up a large, simply worded sign - "please use the revolving door" - roughly doubled the usage.)
In the end Wesolowski could only shrug: "I'm not a psychiatrist."
Even in Philadelphia's greenest office building, One Crescent Drive, getting everyone to use the revolving doors has been tough.
Brian Cohen, vice president of Liberty Property Trust, which manages the building, put up signs. They didn't work.
About a year ago, he installed a lock that required people to swipe their IDs to get through the swing door - and sent an e-mail telling everyone why.
That helped. But some people still use the swing doors.
At a time when many of us are trying to reduce our carbon footprints, every little step counts.
"By just walking through that door, you've blown your whole green savings," says John Kelly, director of property management at Brandywine Realty Trust, which manages One and Two Logan Square.
"Whatever you're doing in your office trying to be green for us, you've just basically wasted."