The hygienists' group is expected to use 13,608 hotel room nights during its 2018 convention here.
The group last visited in 2007. "We had a great meeting," O'Neil said.
Like other Convention Center customers, O'Neil praised Philadelphia and its convention center for easy transportation access and an ample supply of hotels and restaurants within walking distance.
But, O'Neil said, the exhibitors were unhappy.
"You couldn't unplug a computer," O'Neil said. "You couldn't carry in your own box - not a dolly of cartons, but just a box of brochures from your hotel room. You couldn't plug in your Ethernet cable. Silly stuff. Union rules."
Many associations rely on booth fees from exhibitors to raise money, either for the organization's general coffers or to defray the convention costs.
O'Neil said each convention center has its foibles. The work rule situation at Philadelphia's center was the talk of the industry.
"Everyone knew the labor situation had to change," he said. " Philadelphia and Chicago were words you spit out of your mouth."
Chicago's situation, which had been similar to Philadelphia's, was the first to change, he said.
O'Neil credits SMG, the West Conshohocken management firm that now runs Chicago's McCormick Place, with shepherding the change there.
In December, SMG took over management of the Convention Center and quickly began negotiations with the center's six unions over a new customer satisfaction agreement.
The agreement gave exhibitors more leeway in setting up their booths.
The agreement was signed by four of the six unions before a May 5 deadline imposed by SMG.
Two other unions, which signed May 9, have filed complaints with the National Labor Relations Board saying they thought May 10 was the deadline.
O'Neil said that once SMG took over the center and negotiations were underway, his group tentatively decided to return.
Two weeks after the work rules were changed, the group finalized the deal for 2018.
Since 2007, O'Neil's group has been to Minneapolis, Montreal, and Denver among other cities. The group met last week in San Antonio and will be in Salt Lake City next year.
Even though it appears the American Industrial Hygiene Association rarely repeats a convention city more than once a decade, O'Neil said the group would have returned sooner were it not for the work rule issues.
With the changes, he said, "Philadelphia is definitely back on the map."