Short-term rental marketplace Airbnb has an incentive to break the rules set by Portland—and cities around the country—a new analysis finds.

Portland as well as San Francisco require that the people live in any Airbnb property at least nine months out of the year, in order to prevent tourists from taking over homes that could otherwise house local residents.

But according the the data website FiveThirtyEight, 40 percent of the revenue Airbnb hosts earn in Portland are from entire apartments or homes rented for more than 180 days a year, such as a Boise townhouse and a San Francisco condo listed by an Airbnb manager.

And Portland is not alone. These commercial properties account for 32.7 percent of the revenue from Airbnb's top-25 markets.

They represent 15.6 percent of the listings in Portland, which ranks second, after Honolulu among the 25 cities, for having the highest percentage of properties that are commercial operations.

Airbnb spokeswoman Alison Schumer told WW that the tech company, which has a call center in downtown Portland, is trying to get its clients to comply with city rules.

"As our host community has grown over the past two years, we have taken multiple steps to encourage hosts to get city-required permits," Schumer says. "We have hired additional staff and held 29 registration events, including five Q&A sessions with city staff."

Still, these listings, at least to date, represent a small if growing portion of the city's overall housing stock.

Airbnb took issue with the analysis. Here's how FiveThirtyEight summarized the company's objections:

Company spokesman Christopher Nulty said some of the units FiveThirtyEight identified as “commercial” might instead be boutique hotels or guest houses. He also called the 180-days-booked standard “arbitrary,” and said an approach based on days-booked could never accurately identify units that would otherwise be long-term rentals.