Endogenous ethanol levels in the blood from fermentation in the gut do not normally exceed 0.04 mg/dl in healthy individuals. The FAA limit for ethanol in the blood is 40.00 mg/dl, which is 1000 times above the usual level of endogenous ethanol. Thus, the claim that this limit is necessary to account for endogenous ethanol, or that endogenous ethanol makes any significant contribution to intoxication in comparison to exogenous ethanol, is baseless.
The aviation limits for ethanol intoxication are extremely generous in this context. They are that way only because ethanol is a legal drug, and because it is so deeply ingrained in socially acceptable practices of Western society that hardly anyone seems able to do without it. This high limit, however, does not mean that the impairment caused by alcohol is any less severe quantitatively than that produced by many other illegal or less socially acceptable drugs, such as cocaine or nicotine. Additionally, the impairment due to alcohol is qualitatively different from that produced by many other substances. Caffeine, for example, is primarily a stimulant that enhances alertness, and does not necessarily impair a pilot in a way that makes him less safe (although this should not be construed to mean that taking caffeine is okay or a good idea); but alcohol impairs pilots in a number of ways that are quite different, and all of them are unsafe.
Because so many people consume this drug, it is common to hear diverse rationalizations of its use, and excuses for allowing it to be present in the bodies of persons engaged in safety-of-life tasks such as operating vehicles and piloting airplanes. Unfortunately this is a dangerous practice. Just because alcohol is legal up a certain limit does not make that limit (or any presence of alcohol) safe.
If a pilot cannot bring himself to abandon the use of this drug, at the very least he should take care to ensure that his blood levels of ethanol are zero within the limits of measurement while he is flying. Not 0.01, or 0.04, or 0.08, and not just 8 or 12 hours "from bottle to throttle," but truly zero. A pilot who flies intoxicated simply because he is "legal" has a poor sense of priorities: not only is his judgment significantly impaired as he flies, but his judgment in a broader sense is extremely questionable.
Indeed, if a pilot considers alcohol so important that he is willing to take these risks and cannot countenance total abstinence before flight and zero tolerance for alcohol during flying, he may have a much more serious problem with alcohol than he realizes, which itself is a good reason to disqualify him from flying.
There are many pilots who take their flying seriously and truly do make sure that their blood alcohol is zero when flying (indeed, many of these pilots simply do not drink at all, just to be on the safe side). They would sooner shoot themselves in the head than risk having booze on board in any measurable concentration in the cockpit. Their personal standards are higher than those of any law, and uncompromising. I'm not worried about them. I'm worried about the pilots who seem to think that arresting a pilot for being intoxicated is a bad thing, and who have a long list of excuses and rationalizations for being under the influence of alcohol on the flight deck. Some of the pilots in this latter category will become the subjects of accident investigations. Unfortunately, they may take a lot of other people with them when they go, and they are a disgrace to the profession.