One possible theory ruled out...
>>"Authorities haven't determined the cause of the deaths and illnesses, but ruled out carbon monoxide poisoning. The incident happened at the Angel Valley Retreat Center just outside scenic Sedona."<<
http://www.foxnews.com/story/0,2933,563541,00.html
From one source I had read that they may have been using ritualistic herbs as well... guess we wait and see.
One other forum questioned whether the lodge coverings were cotton/natural material blankets (they would offer some ventilation), or plastic (which would not offer ventilation)... which would still give Zoebess's guess value as a possible even with the rule out of carbon monoxide poisoning.
Has anyone read what they used to cover the structure? I would guess some synthetic materials, if used (or if the coverings were washed and dried with having chemical soap/drying sheet/dry cleaning residues) would also be in question.
I suspect that this could have been a combination of things as well now that more information seems to be slowly emerging - and I would bet it was...
At minimum, individual health and electrolyte imbalances/deficiencies.
grz-