You can cause them to give a yellow flame at any time by blowing air against them, and that yellow flame means imperfect combustion, and a probability of the deadly carbon monoxide.
"The Book of Life: Vol. I Mind and Body; Vol. II Love and Society"
Upton Sinclair
If your fire does not "draw," you will have smoke or coal-gas in the house, and this is bad for the lungs; but worse for the lungs is a gas that you can neither see nor smell nor taste, the deadly carbon monoxide.
"The Book of Life: Vol. I Mind and Body; Vol. II Love and Society"
Upton Sinclair
118. Carbon Protoxide, called also carbon monoxide, carbonic oxide, etc.
"An Introduction to Chemical Science"
R.P. Williams