Rescue dogs searching for the 159 now missing in the wake of the Surfside apartment building collapse in Florida are looking specifically for two things: human breath or human odor, a local police and military K-9 instructor says. 

Sinead Imbaro said that once either of those things are detected, the dogs will begin to bark, which allows first responders to isolate locations where people may be trapped underneath the rubble. 

"It’s really that live breath that they’re looking for," Imbaro told Newsnation Now. "For confirmation that there’s somebody hidden in that pile, once they get, or once they locate that breath, or human odor, they will begin to bark for the alert." 

Search and rescue personnel with dogs search for survivors through the rubble at the Champlain Towers South Condo in Surfside, Fla., on Friday. (AP/Miami Herald)

RESCUERS PUT LIVES AT RISK IN SEARCH FOR SURFSIDE SURVIVORS 

Imbaro says it takes two and a half years for dogs to be trained for the challenging work. 

"It has to be a good relationship with the handler and dog in order to go in and seek out the victims," she told Newsnation Now. 

Surfside Mayor Charles Burkett told Fox News’ "America’s Newsroom" Friday morning that "we have one focus: and that focus is to bring people out of the rubble alive." 

Fire rescue personnel conduct a search and rescue with dogs at Champlain Towers South on Thursday. (AP/Miami Herald)

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"We don’t have a resource problem, we have a luck problem," he added. "We just need to stay focused and hope the weather cooperates with us – we have a small fire that keeps breaking out in the basement of the building that we are fighting, and we did have some debris that was loose up on top of the building, we needed to bring some heavy equipment to pick off." 

"But our first responders and our search and rescue teams are constantly eager to get back on the job, and they are on the job now," Burkett said.