There are three basic forms of "pure Alcohol"; Ethanol, Methanol and Isopropanol. All of them are various combinations and quantities of Hydrogen, Oxygen and Carbon in different molecular chains, (as is common table sugar and corn syrup!). And all of the "alcohol" forms of these molecules are "poison" at some level of ingestion over time; some more so than others.
Ethanol is also known as "Grain Alcohol" and is derived from the fermentation (ethanol is a byproduct of bacterial growth!) of just about any fruit or grain. It is a very mild poison that the body can convert to fat or excreted rather quickly if ingested in small amounts and causes little or no damage. Larger amounts affect the nervous system, mostly temporarily and produces a mild euphoria that many people quite enjoy, but does cause some damage to the nervous system and the liver. Larger quantities produce loss of judgment which often leads to death because the intoxicated (note the route word is "Toxic") individual is at that point lacking in the intelligence to not attempt to drive a car while under the influence of the alcohol. Ingesting extremely large quantities leads to nervous system shut down and death. It is not easily absorbed into the body through the skin.
Methanol is similar to Ethanol, but is much more poisonous in even small quantities. It can be derived in several ways, including the fermentation of wood (hence its common name of "Wood Alcohol") or from the distillation of crude oil. When ingested, the body converts it to formaldehyde which destroys synapses, which, depending on the amount ingested, usually leads to blindness prior to the general shut down of the nervous system and death. It should be noted that it can be absorbed through the skin, thus skin contact IS dangerous.
Isopropanol is also derived from the distillation of crude oil (and some other sources). It does not get absorbed through the skin the way Methanol is, but it is still poisonous if taken internally.
Ethanol is mostly used as an ingestible drink as noted above and because of its intoxicant nature tends to make the person ingesting it to believe they are not being harmed by it... and that feeling of euphoria leads the user to desire more (that old "if it feels good, do it" thought). Due to its addictive nature, the government controls the production and sale of the intoxicant by taxing it at a very high rate (easy money!) and imposing severe penalties for attempting to avoid the tax.
Ethanol is also an excellent solvent, and in that role is often used to dissolve flakes of "Shellac" to produce a durable surface finish for wood furniture. But pure Ethanol is too expensive (due to the tax) to use for this purpose.
Methanol, being a poison is not taxed as heavily. It also can be used as a solvent, but it is not as good as Ethanol. Because it is poisonous and blends neatly with Ethanol (and it is hard to separate the two), most alcohol solvents are Ethanol "denatured" (made poisonous) with Methanol (and often other "solvents" that the manufacturer thinks improves the solvent properties) so it can be sold without the "drinking tax" applied and is thus cheaper. The exact proportions of the various components of any brand of "denatured alcohol" is treated as a Trade Secret, but the government has set a minimum percentage of Methanol in the mix to make the concoction non-drinkable/poisonous and thus avoid the tax.
Isopropanol is often mixed with glycerin as a skin emollient and other oils and sold as "rubbing alcohol". It is a solvent to cleanse the skin of oil, and when it evaporates it leaves the glycerin and other "cleaner" oils.
Complete combustion of Ethanol or Methanol produces only Carbon Dioxide and Water (CO2 and H2O). BUT, incomplete combustion can produce some level of Carbon Monoxide (CO) as well as leave some of the chains of the alcohol in the form of vapor in the exhaust. AND, depending on what else is in the alcohol, can produce some other vapors that might be very dangerous.
For me, I stay away from Methanol...
I noted that doctors swab your skin with "alcohol" to kill germs before giving you a shot. And you can buy "waterless" hand cleaners that are gelatin'd alcohol to clean and sanitize. I did not understand the differences or that Methanol can be absorbed through the skin and I made the mistake washing my hands too many times with "Denatured Alcohol" and as a result damaged my liver. I did not relate the use of "alcohol" to wash my hands with the pain in my gut. The Dorktor kept sending me to various labs for tests trying to prove I had hepatitis, but the results kept coming back negative (he made the mistake of making the diagnosis based on my generation and symptoms before testing was complete and then kept trying to prove his diagnosis... I got better in spite of his ego!). It took a while to achieve the correct diagnosis, but by then I was better and apparently have not suffered great damage and over time the liver might repair itself (if I don't die of old age first).
Relevant to the question posed by Thumper in this thread... I also had an "event" when firing my Mike and using "Sunnyside" alcohol from True Value Hardware (also sold at other places). It has more adulterants than other brands (and supposedly is a better solvent). I was leaning over my loco doing the traditional "oiling around" while steam was building and got a heavy whiff of the exhaust of the engine. If it weren't for a lawn chair that I had taken out there with me I doubt if I would have made it to the house before being on the ground. I sat in that chair for a long time before I could get up to shut the fire off and put everything away. I didn't get to run that day.
I now use S-L-X brand of alcohol from Home Depot (and other places) because it has less of the additional solvents than the other brands (at least it used to). The odor of it is not nearly as bad as the Sunnyside brand, but I am not about to go take a good deep breath of it to find out if it will knock me down. I have read that the governmental regulations are slowly forcing the various brands of "Denatured Alcohol" to use similar proportions and I have not bought S-L-X recently to know if it now stinks as bad as the Sunnyside brand.
Regardless, I am very careful about breathing the vapors from my engine...
But it should be noted that many people burn "Sterno" (which is basically unflavored gelatin reconstituted with Denatured Alcohol) under a Chaffing dish when serving elegant dinners or making Fondue and nobody seems to be the worse for wear. Granted those burners are pretty much open for complete combustion and are not "forced" via a draft so there SHOULD BE fewer unburnt or incomplete burnt byproducts in the exhaust. Note also that people burn butane in "candles" on the dinner table and plumbers often use a propane or butane torch in confined spaces for short periods of time with no (presently known) untoward effects.
I think there is a higher probability that you will burn your house down when running a live steamer indoors than being harmed from the fumes, but that uncommon form of intelligence known a "common sense" should prevail when you are playing with trains, whether indoors or out.
