Dead monkeys
Monkeys and dogs don't have too much brain to damage, so their status isn't of much interest here.
I believe there are two key steps in altitude - one at about 25,000 feet above which oxygen saturation cannot be maintained high enough to ensure long-term survival - the mountaineer's death zone - and about 60,000 feet, when the ambient pressure reduces beneath the vapor pressure of water in the lungs, "ebullition" as the macabre link suggests.
Above 50,000 feet, a pressure suit is required. Below that, oxygen allows hours of survival… but what about the sickest oldest passengers out of the 500? What about the few percent of passengers who can't work their masks, due either to a lack of natural talent, or to low-oxygen-induced senselessness?