The Toxic Air enters the cabin, through the bleed air system. Bleed air system supplies hot air for the A/C system.
As engined get older, seals do not seal as good as they do when new. It is a well known fact for the seal manufacturers, however Airlines and Aircraft manufacturers, Airbus in this case, tend to kick the bucket down the road.
Airlines will generally bully and intimidate crews that take action due to Toxic Cabin Air Contamination, if this fails, go a long and costly legal process that renders crews' bankrupt unable to pursue workplace compensation for illnesses.
From what I understand, Qantas do take this seriously and pull aircraft off line to further investigate.
https://www.aerotoxic.org
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