Air Carrier Flight Attendant reported a fume event during boarding. The rest of the flight crew also noticed the odor and fumes. After maintenance arrived; the passengers and flight crew deplaned and the aircraft was taken out of service.

Narrative

I was FA (Flight Attendant) #4. Flight and cabin crew boarded the aircraft at approximately XA:45. The #2 and I went to the rear of the aircraft. Just prior to boarding the #2 commented that the cabin looked hazy and I concurred. It was the entire cabin but appeared more profound mid cabin. A slight smell of burning plastic and a musty odor combination followed. At that point; boarding had begun and the smell worsened but the haze seemed to clear. After boarding was in progress for about 10 minutes; the smell began to give me a headache as well as the #2. At that point we notified the #1 who then notified the cockpit. From what I was told; the FO (First Officer) also began to notice the smell. The Captain then walked through the cabin and noticed the smell as well. At that point the Captain stopped the boarding. Mechanics were called and showed in about 5 - 10 minutes. After the mechanics walked through the cabin the decision was made to have the boarded passengers and crew exit the aircraft. That was at approximately XB:45 - XC:00; from my recollection. At that time I was experiencing a full blown headache. The aircraft was taken out of service and the crew waited for another replacement aircraft. My headache lasted a good part of the day until early evening.[I suggest to] determine and address the cause and not the symptom. After years of reporting on symptoms; i.e. what it smells like; what it looks like; and when it happens; a cause needs to be determined. And whatever the cause is; what is contained in these fumes needs to be determined. Short term effects are headache etc.; but what are the long term effects. A carbon dioxide test is useless if the culprit is not carbon dioxide.