Air carrier Flight Attendant reported a chemical odor in the cabin during preflight which resulted in passenger deplaning and all crew members being assessed for medical treatment. The Captain informed the cabin crew that this aircraft had multiple fume events previously and that the APU was replaced after this event.

Narrative

At the end of boarding; the #1 Flight Attendant requested water pressure to use the coffee pot in his galley. The Captain turned on the APU and immediately chemical odors permeated the aircraft. I immediately walked to the back of the aircraft to assess the situation. Passengers seated in XX row noticed the odor and stopped me to inquire about it. The fumes were worse in the aft galley of the aircraft and the #2 Flight Attendant was coughing profusely. I instructed her to go to the front door to get fresh air. I called the Captain and informed him of the odor; he said he noticed it too; and was in the process of starting an evacuation of passengers.I made sure the galley was secured and moved as far away from the aft galley as I could without leaving the doors unattended. The #2 and I took turns trading places on the aircraft so we could each get fresh air until the aircraft was free of all passengers; approximately XX minutes; and the required checks were finished. All 5 crew members went to the ER for treatment; this is my second major cabin fumes event in four months.The Captain informed us this particular aircraft was written up for similar fumes events on DATE; DATE1; and DATE2 in all phases of flights and marked that maintenance could not find the source so they marked it as a NFF (No Fault Found) and put it back in service. Our event was treated differently as they changed the filters; which are not designed to remove chemical toxins from the air. Seven days later on DATE3; after many concerned emails; regarding; the aircraft had a full APU change and was returned to service on DATE4.