How common and dangerous are fume events?
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.