Whats the issue with the Toxic Twins

Carbon Monoxide attacks the bloods ability to carry oxygen to the organs, Hydrogen Cyanide impacts on the organs and cells abilities to use oxygen. This is why they are considered toxic twins, because together they are widely regarded as acutely toxic. Levels of 50ppm are general regarded as immediately dangerous to life and health. Workplace exposure levels are set as low 0.5ppm for HCN. Gas Monitoring is the only way to detect these.

Both exist at ALL fires, but the concentrations are worse during the initial development stage and the decay stage of a fire.