I’ve been hearing about fume events where engine oil containing tricresyl phosphate as an additive can leak onto the HVAC packs and enter cabin air. These organophosphate are incredibly toxic and are used in nerve gases. There are even cases where pilots have died such as here (AVHerald) and here (BBC).
The AVHerald article you linked to pretty much as the answer to your question:
- How common are they?
Nearly 2,000 events a year in the US alone.
[According] to the Kansas State University Research there have been 5.3 fume events per 24,000 daily flights in the USA or 1955 fume events a year. However, only 6 fume events per year get reported to the FAA.
- How dangerous are they?
In extreme cases, they can lead to death, but even mild cases diminish flight crew capabilities.