FORT RUCKER, Ala. (July 9, 2019) - Our mission was to play "elevator" for pararescue personnel training and qualifying with the square parachute. Our task was to pick up as many personnel as we could, take them up to 13,000 feet and have them jump out. It was a cool day in the California high desert, and we weren't worried about power limits due to the temperature. The mission was pretty simple to execute -- that is until it took a bad turn.We started the day with a pre-mission brief that included all the jumpers and my aircrew. The jumpers then rode a bus to the jump site while we prepared the aircraft for the mission. The preflight, start up and flight to the practice area all went without incident, and we arrived for our first "stick" of jumpers. It took us some time to climb to altitude, as we were loaded with gas and had a full cabin. Each follow-on jump went just as the one before and we were soon low on fuel and had to return to base. Once we'd refueled the aircraft and gotten a meal, we returned to the jump site for another set of lifts.As we started our second hour of lifts, boredom set in. While we were leveling off for the jump, my co-pilot rogered my call of 13,000 feet and then I heard a very weird laugh. That caught my attention, and I looked over at him to see what was so funny. What I saw wasn't funny in the least. My co-pilot looked back at me with bluish lips and a big grin that made me wonder if he was suffering from hypoxia. I then pulled off my glove to look at my nail beds, which were also a blue shade. I said we were finished once the jumpers were away and my co-pilot looked at me, laughed and asked, "Why?"As the jumpers departed, I took control of the aircraft, started a descent and called the ground party to notify them we were returning to the airfield for fuel and a crew change. I then contacted the operations center and arranged for another crew to go out and finish the final two hours of lifts. Once we were on the ground, I pulled my crew together for a debrief and told them that once we were finished, we were all going to the flight surgeon's office to get examined.I explained to the flight surgeon what had transpired and he examined us. Once we received a clean bill of health, we returned to the unit, where I talked with the commander and safety officer about crew limits on high-altitude flying. I recommended each crew be limited to only two lifts of 2 1/2 hours per day and that those flights be separated by another crew flying a 2 1/2-hour flight.What had seemed like a simple flight requiring little skill on the aircrew's part took an interesting, potentially deadly turn. As in this case, the effects of hypoxia can be cumulative. Because of that, keep an eye on your crew and use situational awareness to watch for hidden hazards in each mission and flight. The risk management worksheet showed an overall low risk for this mission -- until we found good reason to elevate the risk. Initially, we believed the risk of hypoxia was removed as long as we limited ourselves to a maximum of 30 minutes above 10,000 feet. However, the cumulative effects of repeatedly operating above that altitude on a cold day unexpectedly increased our risk. Fortunately, we learned that lesson without having to sacrifice an aircraft and crew to pay for it.Do you have a story to share? Risk Management is always looking for contributors to provide ground, aviation, driving (both private motor vehicle and motorcycle) and off-duty safety articles. Don't worry if you've never written an article for publication. Just write about what you know and our editorial staff will take care of the rest. Your story might just save another Soldier's life. To learn more, visit https://safety.army.mil/MEDIA/Risk-Management-Magazine.