Air carrier flight attendants reported a persistent 'strong odor' in the aft galley during boarding. Passengers were deplaned and flight attendant's physiological symptoms were evaluated by medical personnel.

Narrative

When I went into the AFT galley I immediately smelled a strong odor. The Flight Attendant (FA) B also could smell it as well as the cleaners who were onboard. They tried to clean the area and it didn't go away. We thought it was something in the trash so we waited for the caterers to take the trash and then we were going to re-evaluate. Once they took it away; the smell was still there and it began to make the FA B and me feel sick. Our heads started to hurt; our mouths were becoming dry and our eyes were becoming red and itchy. The FA D came to the back and confirmed the odor and she said it was making her nauseous. The back row of the airplane was also feeling itchy and nauseous. We told the pilots and the FO came to the back and confirmed the odor. That's when we made the call to deplane and seek medical attention. We talked to supervisors and had medics check our vitals. The odor had made its way to the jet-bridge by the time we exited the boarding area. We should have notified the Captain right away but we thought the odor would go away on its own it did not.