I thought the general understanding of inert gas asphyxia was that it was painless as gasses like nitrogen don't trigger the hypercapnic alarm response, which i

I thought the general understanding of inert gas asphyxia was that it was painless as gasses like nitrogen don't trigger the hypercapnic alarm response, which in humans arises mostly from blood carbon dioxide levels rising. This report makes it sound like that isn't the case. Does anyone know why?