I had a toxic air incident on an a 320 a month ago. Luckily it happened at the gate prior to departure. My flight attendants had had it happen before and were familiar with the smell. We ask the passengers to deplane, called maintenance and the aircraft was taken out of service.
Maintenance found the APU oil level greatly over-serviced and oil lying in the duct-work leading from the APU air plenum. There is a duct "dry out" procedure in the maintenance manual. It's a lengthy procedure. Run the APU and air-conditioning at high temp. The aircraft was out of service overnight. There was no push-back from the company.