I knew that the best way to time the capture of the image would be to make it automated. My human reflexes would not have been reliable enough, and my 60D's shutter isn't fast enough to just continuously capture images and hope that one of them is the golden shot.
To automate the shot, I initially tried to detect the gust of air coming from the subject's mouth. I hung a tissue gently in front of them from a stand and aimed a laser at the bottom of it. When the tissue is sneezed on and it deflects even just a little bit, the laser beam would no longer be interrupted, shining through at a photoresistor that fire the shutter. This kind of worked, but turned out to be not nearly fast enough.
I dug around my bin of things looking for another solution. I found an accelerometer that my brother had given me for Christmas years ago that I had never had a use for. I wondered if I could attach this to the subject's head and detect the sudden acceleration of the head caused by a sneeze. It kind of seemed like a wild idea, but I gave it ago. We found a a bandanna that we could use to strap it to a head. Remarkably, once I figured out the thresholds and appropriate delay, it turned out to work pretty well.