“Ventilation is the profound secret of existence.” — Peter Sloterdijk
Most people, when hearing about the legal limits of ethanol, will think of Blood Alcohol Concentration (BAC). Except for Utah, where the BAC limit for driving is 0.05%, the BAC limit in the United States is 0.08%. That applies to drivers that are 21 years old or older. For drivers under the age of 21 years, the legal limit in the United States is 0.02% or in some states, lower. That means that even without observable evidence to the contrary, drivers whose BAC exceeds these limits can be found to be intoxicated, per se.
Driving while intoxicated (DWI) is often the first safety concern associated with ethanol, because BAC above the legal limit impairs vision, hearing, and most importantly, judgment. At higher BAC, the effects are worse, with a BAC above 0.4% typically being fatal.
It’s not just drinking that can lead to safety concerns with ethanol, however. There are also limits on the amount of ethanol that should be in the workplace environment.
Particularly in terms of flammable limits and toxic exposure limits.
Flammable Limits of Ethanol
Most people understand that ethanol is flammable. But how flammable? There are several ways to judge the flammability of a material. Flash point and flammable limits are the most familiar.
Flash point is the temperature at which a flammable liquid generates enough vapor to form a flammable mixture with air. To understand flash point, then, it is important first to understand flammable limits.
Strictly speaking, combustion happens in the vapor phase. When ethanol burns, its molecules must first vaporize and mix with molecules of oxygen. If there is not enough ethanol vapor in the air, the ethanol will not burn. We call this being “too lean.” For ethanol, a mixture in air is too lean to burn when its concentration is less than 3.3% by volume, which is known as the Lower Flammable Limit (LFL) or the Lower Explosive Limit (LEL). If there is too much ethanol vapor in the air—in other words, not enough oxygen—the ethanol will not burn. We call this being “too rich.” For ethanol, a mixture in air is too rich to burn when it is higher than 19% by volume, known as the Upper Flammable Limit (UFL) or the Upper Explosive Limit (UEL).
The National Fire Protection Association has several standards that concern ventilation of flammable vapors. Generally, they require that ventilation be sufficient to keep flammable vapor concentrations less than 50% of the LEL with monitoring and interlocks. There is generally no need for LEL monitoring if the ventilation rate is sufficient to keep the flammable vapor concentration less than 25% of the LEL.
The amount of ethanol vapor that a solution releases into the atmosphere depends on its temperature and its concentration. Pure ethanol will release enough vapor to create a 3.3% concentration in air when it is 13 C (55°F). So, that temperature is the flash point of pure ethanol.
When ethanol is diluted with water, the mixture must be warmer before it will release enough vapor to create a 3.3% ethanol concentration in air. Ethanol at 80 proof (40% ABV), for instance, must be 26 C (79°F) to release enough vapor to create a 3.3% concentration in air. That means that the flash point of 80 proof ethanol is 26 C (79°F).
Even beer, when hot enough, can release sufficient ethanol to form a flammable mixture with air. The flash point of 10% ABV beer, as might be found in the fermenter of a bourbon distillery, is 49 C (120°F).
In the U.S., OSHA regulates processes using flammable liquids with a flash point lower than 100°F (37.8 C) with the Process Safety Management standard (29 CFR 1910.119). Ethanol mixtures at 35.5 proof (17.8% ABV) have a flash point of 100°F (37.8 C).
Toxic Exposure Limits of Ethanol
Long before ethanol vapor reaches flammable concentrations, it reaches concentrations where toxic exposure is a concern.
In the U.S., OSHA has established a Permissible Exposure Limit (PEL) for many substances. The PEL is the average concentration over an eight-hour shift above which workers must not be exposed. For ethanol, the PEL is 1,000 ppm (0.1%).
Again, the amount of ethanol released into the atmosphere is dependent on the temperature and concentration of the ethanol solution. Consider fermentation in an open‑top tank. Assuming a final concentration of 10% ABV, the ethanol vapor pressure above the liquid in the fermenter at 20 C (68°F) would be 0.029 psi, meaning a concentration of 1,940 ppm being released into the workspace. That is almost two times higher than OSHA’s PEL of 1,000 ppm for ethanol. For areas of spirits production processes exposed to higher liquid concentrations of ethanol, the ethanol in the atmosphere will be even higher. Obviously, the further from the source of the ethanol, the lower the ethanol concentration in the atmosphere will be, so distance helps. Adequate ventilation will do even more to lower the concentration.
Good Ventilation Will Keep Ethanol Within Safe Limits
For consumers, blood alcohol concentration is the limit of greatest concern. In most places, a BAC of 0.08% (800 ppm) will get a driver arrested.
Note: Breathalyzers work by correlating BAC to Breath Alcohol Concentration (BrAC) using the relationship
BAC = BrAC · 2251
So, a BAC of 0.08% is indicated by a BrAC of 0.355 ppm. It’s good to know that a drunk’s breath is neither flammable nor toxic.
For ethanol producers, however, safe levels of ethanol take on a whole new meaning. Exceeding those safe levels can result in hazards—both flammable and toxic—that must be addressed. Fortunately, good ventilation goes a long way to addressing both.
