This is sounding less and less 'natural' to me. I can't envision anything suddenly overtaking people to that extent. CO and CO2 are not likely to overtake conscious adults. Most cases are when gasses build up in areas where people are asleep.
Some excerpts from:
https://www.seattletimes.com/nation...nia-hiking-trail-and-officials-dont-know-why/
Gerrish was in a seated position with the baby and dog beside him, according to the Chronicle. Chung was a little farther up the hill.
Authorities still don’t know how it happened. An autopsy recently completed on the bodies yielded no conclusive results about the cause of the deaths, CNN reported. A toxicology report, which could take several weeks, is pending.
A necropsy is being performed on Oski, the dog, but the Mariposa County Sheriff’s Office said no other animals were found dead in the immediate area.
..........................
There was no suicide note or indication that the deaths were intentional, according to CNN. Although temperatures reached as high as 109 degrees the day the family hiked, dehydration was deemed unlikely because there was still water in the family’s hydration pack, the Chronicle reported.
.............................
But reported human deaths from freshwater algal blooms are rare, the Chronicle reported. David Caron, a University of Southern California biological sciences professor, told the newspaper that while freshwater bacteria are a threat to people and animals, it would take high concentrations to kill humans rapidly.
“It’s conceivable that it is the cause,” Caron said. “But a lot needs to be done forensically to tie it to toxins.”
.....................................
“We have not found any old mine shafts near the area,” he added. “There are some mine shafts, but we can’t confirm if that’s the cause yet.”