Cabin Pressure Problem or Medical ?
 

A United Airlines flight en route to Los Angeles from Denver was forced to make an emergency landing in Grand Junction, Colo., when several passengers complained about low cabin pressure shortly after takeoff, causing the pilot to deploy oxygen masks from the plane's ceiling.
"After receiving reports of multiple ill customers, our crew deployed the oxygen masks and elected to divert to Grand Junction," Jennifer Dohm, a spokeswoman for United Airlines, told Yahoo News. "Medical personnel met the flight upon arrival, and we'll fly our passengers on a new aircraft to Los Angeles."
The Airbus A320 carrying 150 passengers and six crew members took off at approximately 8:25 a.m. from Denver International Airport, according to FlightAware.com, landing in Grand Junction just over an hour later.
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Both.

Pressure problems causing passengers to be/feel ill.

Reading between the lines...."chain reaction....nausea....nearby passengers...no 'abnormal' O2/CO2 levels found (by emergency responders) after landing...zero air quality deficiencies" - it sounds to me like an "uncontained stomach failure" filled part of the cabin with vomit fumes.

United Airlines flight bound for LAX diverted in Colorado after passengers become ill - LA Times

It is, of course, possible that some other fumes triggered the initial event.

Perhaps a smuggled...
 

https://en.wikipedia.org/wiki/Durian#Flavour_and_odour

SOMEBODY owes me a new keyboard!

Airtoxic ?