This question is constantly asked and in my honest opinion, just read the MSDS of Butane put out by actual trained scientists. Butane is always listed in the MSDS as being Non-toxic in inhalation tests, and the only harm comes from the short term impact of asphyxiation and oxygen depravation (as well as a cold liquid turning into a gas freezing your lungs) if you're inhaling literal concentrated butane from the can.
http://www.gasinnovations.com/literature/MSDS-Sheets/MSDS-BUTANE.pdf
Even if you have butane soup, butane by itself is the least of your worries arm unless you're inhaling unrealistic amounts of it (even the most butane filled BHO will not have enough butane to cause the short term harmful effects of butane inhalation, the only thing (IMO) to worry about is the shortness of breath and harshness from badly purged tane .
Still, one should still be concerned about the source of butane and other potential contaminates that can be found in poorly made BHO - Butane leaches chemicals so using improper containers for the oil can lead to contamination, and then there is the whole issue of lubes from canned butane producing a mystery oil of death after a ton of empty runs onto a pyrex pan (which is mitigated by ensuring your BHO is created with proper tane from a professional source).
Oh how I love not having to deal with any of this thanks to the Rosin tek coming out big in public knowledge within the past year...