These products are generally more harmful than CO or CO2, but are present at much lower levels. Propane, like methane, is fairly clean-burning. Carbon dioxide is only slightly poisonous. It is the waste product of animal metabolism, so animals have a pretty high tolerance for it. While it is an odorless, tasteless gas, it does combine with water to form carbonic acid which has an odor and taste.
Carbon monoxide is another animal.
It is a potent poison, with an action rather like cyanide. Apparently its action is somewhat less severe than cyanide, but since you are more likely to be exposed to CO than to CN, that won't comfort your next of kin much. Symptoms of mild CO poisoning include headache. While both CO and CO2 are environmental pollutants, the quantities that a forge will produce is of no particular concern to anyone but you. In General 1.