Brain displaying necrosis due to carbon monoxide poisoning
In the past, carbon monoxide toxicity was not unheard of, especially during the winter. Cold nights and CO-producing fires lead to a silent death while asleep.
Though people were aware that they needed ventilation when fires were burning, frigid cold outside leading to people keeping too much air out, and having a stove burning through the night (so as not to freeze to death…), led to at least several dozen documented deaths throughout the Midwestern United States in the 1890s. 
A Text-Book of Pathology. W. G. MacCallum, 1916.

Brain displaying necrosis due to carbon monoxide poisoning

In the past, carbon monoxide toxicity was not unheard of, especially during the winter. Cold nights and CO-producing fires lead to a silent death while asleep.

Though people were aware that they needed ventilation when fires were burning, frigid cold outside leading to people keeping too much air out, and having a stove burning through the night (so as not to freeze to death…), led to at least several dozen documented deaths throughout the Midwestern United States in the 1890s. 

A Text-Book of Pathology. W. G. MacCallum, 1916.