THE REXALL PHARMACY building at the corner of NE 24th and Alberta was home for Megan Holmes and Ana Briseño for almost five years. In July, the pair received notice they'd have to move out by September 30.

There are eight long-term apartments above the former pharmacy. Each apartment has two rooms—a living room and full kitchen with a bedroom attached. The tenants share two bathrooms.

Then there's the ninth unit, a short-term rental available for $125 a night on Airbnb. Holmes says the former tenants—a mother and her young daughter—were served with a no-cause eviction notice last year. When the apartment was empty, the building owners remodeled it, added a bathroom, and put it up for rent on the popular rental site.

Holmes and Briseño think that might be the future of their own beloved apartment. At a time when Portland's housing market has reached a point of crisis, they worry lucrative services like Airbnb are poised to snatch units off the residential rental market.

"It's super competitive right now, finding a place to live," Holmes says. "Other tenants in the building are upset, and just waiting to get evicted."

Airbnb got started in 2007 when a couple of San Francisco roommates decided to raise some money by putting three air mattresses in their apartment and charging people to stay there. Today, the rental site is worth an estimated $20 billion and has more than 1.2 million rentals available worldwide.

In Portland, where vacancy rates sit at record lows and rents are at all-time highs, many people worry that Airbnb is exacerbating the problem.

"Renters face substantial rent increases throughout Portland, especially in the amenity-rich inner neighborhoods where short-term rentals are more popular," says Justin Buri, executive director of the Community Alliance of Tenants. "There has been much discussion, but very little empirical research or data, on how short-term rentals impact vacancy rates and affordability."

In fact, there is some data on Airbnb's foothold here. New York City resident Murray Cox started a website called Inside Airbnb that pulls data from Airbnb to glean specifics on things like how many nights a home is available per year, whether a host appears to live onsite, and which hosts have multiple listings. Cox says he started the site out of curiosity. He knew of people in his Brooklyn neighborhood who rented entire apartments to short-term tenants and suspected the problem might be widespread.

"Airbnb isn't the root cause of inequality and gentrification, but they're enabling property owners to rent to tourists and that has a direct impact on the housing supply," Cox says. "This includes not only renting apartments and rooms on a short-term basis, but also buying and renovating real estate for the purpose of renting as Airbnb units."

In July 2014, after the service had already emerged here, Portland City Council gave Airbnb the legal go-ahead to operate within the city. For $180, Portlanders can buy a permit from the Portland Bureau of Development Services and rent out one or two bedrooms in their home.

Cox found 2,461 available Airbnb units for rent in Portland. Of those, 1,445 are entire homes or apartments. And about 84 percent of those homes or apartments are available for more than 95 days per year, according to Cox, a violation of city policy.

"Places that are highly available year-round for tourists probably don't have the owner present and could be illegal," he says. "But more importantly, they're displacing potential long-term tenants."

Holmes and Briseño paid $400 a month for their apartment. A $125 per night charge, the fee for the Airbnb unit down the hall, would make more than that in four days.

Cox and Buri have a point—at a time of widespread displacement and rent increases, every apartment counts in Portland. But while it's certainly true that a unit that's being rented as an Airbnb is removed from the long-term rental market, numbers-wise the service's activity is a drop in the bucket.

Patrick Barry of Barry & Associates, a Portland commercial real estate appraisal firm, estimates there are about 120,400 multifamily units in Multnomah County. (Portland-specific numbers are hard to come by.)

While 1,445 sounds like a lot of potential units being taken off the market, it's not even two percent of what's available. So is Airbnb getting a bad rap?

Ed Valik, a Portland-based general contractor who's also an Airbnb host, thinks so.

Valik's got a detached studio apartment, also known as an accessory dwelling unit (ADU), on his property. He built it in 2012 and rented it to long-term tenants for the first three years. Then in 2015, Valik decided to put it up for rent on Airbnb.

He went that route, he says, because it gives him flexibility. He can make extra money from having short-term guests stay for a few nights a month and still keep the unit available for friends and family when they visit.

"ADUs aren't the answer to Portland's lack of rentals," Valik says. "In 2008, with the recession, all construction just shut down and didn't get going again until about 2013. But people kept moving here."

According to data from Metro, Portland is the 15th fastest-growing metro area among the country's 50 largest. Between 2013 and 2014, 33,500 new residents arrived, while just 6,800 new units were added to the rental market in 2014, according to data compiled by Jonathan Clay of Multifamily NW.

"What the city needs is to catch up on the fact that there was nothing built for five years," Valik says.

Still, adding another 1,445 units to the mix could help a lot of potential tenants find homes.

"Clearly Airbnb has had a negative impact on the housing supply," says Commissioner Nick Fish. "That's part of the reason why I voted against allowing short-term rentals in multifamily buildings. I was concerned about the impact it could have on the market."

Last year, Fish proposed taking some of the money the city receives from Airbnb permits and placing it in a housing investment fund to create more affordable housing. Fish had Housing Commissioner Dan Saltzman's support, but wasn't able to get the third vote needed for passage. The two are planning on reintroducing the idea to city council, and Fish thinks they'll be successful this time around.

Of course, it won't do much good to pass legislation if hosts aren't complying with Portland's rules. Cox's data suggests that only 11 percent of Airbnb hosts in Portland have the required permit. He says that's up from five percent, which is what he found in May when he first ran Portland's numbers.

"It's still a pretty dismal amount," Cox says.

Fish agrees, and says it's up to Airbnb to require its hosts to follow local laws, including passing a safety inspection.

"I don't understand why the industry has been so reluctant to voluntarily comply," Fish says. "The fact is that the company is making money by advertising rentals that are illegal. It's a safety issue and I hope it doesn't take a tragedy to get them to step up."