Carbon Dioxide: Silent but Deadly

“I had thought fermentation was controlled death.”  — Michelle Zauner

I enjoy touring distilleries. I love seeing the process of making whiskey, but I’m usually taken aback when we get to the fermentation step. Occasionally, the fermenters are enclosed and vent to the roof. Most fermenters that I see, however, are open top vats. This is true whether the distillery is a small craft distillery or a major producer. I’m not concerned about contaminants or foreign organisms falling into the fermenters; the subsequent distillation will take care of that.

No, it’s the carbon dioxide being released into the fermentation room that concerns me. It’s not because of the role carbon dioxide plays in global warming, either. It’s because carbon dioxide is an asphyxiant. Workers and visitors are exposed to elevated levels of carbon dioxide, and those levels might not be safe.

Fermentation and Carbon Dioxide

As a refresher, yeast consumes sugars and converts them to ethanol and carbon dioxide. The conversion is

HCO-(HCOH)₄-CH₂OH →  2 H₃C-CHOH + 2 CO₂

That means that for every molecule of ethanol produced, fermentation also produces a molecule of carbon dioxide. For bakers, the molecules they care about are the CO₂ molecules, because it is the CO₂ produced by the yeast that makes their bread rise. The ethanol is a byproduct that gets evaporated out of the bread during baking.

The molecules that brewers and distillers care about, on the other hand, are the ethanol molecules. The carbon dioxide is just a byproduct that is released into the atmosphere.

Exposure to Carbon Dioxide

We are all exposed to carbon dioxide. Continuously. The air we breathe in contains carbon dioxide. The level of CO₂ in the atmosphere is currently 425 ppm (0.0425%), but climbing. The level in our lungs when we exhale is 4% (40,000 ppm).

The Occupational Safety and Health Administration (OSHA) has established a Permissible Exposure Limit for carbon dioxide of 5,000 ppm (0.5%). That means that workers should not be exposed to environments that contain more than twelve times the CO₂ normally found in the atmosphere.

The National Institute of Occupational Safety and Health (NIOSH) has established the level of carbon dioxide that is Immediately Dangerous to Life and Health (IDLH) as 40,000 ppm (4%). This is based on acute toxicity and means that people should not be exposed to atmospheres containing more CO₂ than is in the breath we exhale.

For every proof gallon of ethanol produced (a proof gallon is the amount of ethanol contained in one gallon of 100 proof alcohol, also known as is 50% ABV), fermentation produces 27.46 standard cubic feet of CO₂.

Is that a lot? Does that mean that CO₂ levels in fermentation rooms exceed the PEL or IDLH? Not necessarily.

Venting and Ventilation

Fermentation does not happen instantaneously, so there is not a sudden release of 27.46 scf per proof gallon into the fermentation room. Fermentation occurs over the course of days. In distilleries, the fermentation time typically ranges from two to ten days, at least based on the distilleries we’ve visited; in breweries, fermentation time typically ranges from two to eight weeks. However, the fermentation rate is not constant throughout. The most vigorous carbon dioxide generation occurs a few days into the fermentation.

It is at this peak rate that the CO₂ level will be highest. Whether it is too high depends on venting and ventilation. If the fermenters are enclosed and vented outdoors, the level is unlikely to be too high, except immediately at the discharge of the vent pipe.  A good engineering practice is to have the vent pipe discharge at an elevation of 10 feet (3 meters) above the working surface.  This will allow natural outdoor convection to disperse the CO₂. If the fermenters are indoors and release directly into the fermentation room, then good ventilation will make an important difference.

In a room without adequate ventilation, the CO₂ will accumulate, perhaps to dangerous levels. In a room with adequate ventilation, the CO₂ will likely be diluted with fresh air and kept at safe levels.  The National Fire Protection Association (NFPA) considers six air changes per hour to be adequate ventilation.

CO₂ is Not a Simple Asphyxiant

It is important to keep in mind that, despite frequent assertions to the contrary, carbon dioxide is NOT a simple asphyxiant.  A simple asphyxiant can be fatal, not because it is toxic, but because it does not contain oxygen. The normal concentration of oxygen in air is 20.95%. OSHA has established a safe lower limit for oxygen as 19.5%. A simple asphyxiant, like nitrogen, helium, argon, neon, krypton, or xenon, will be unsafe if they dilute the oxygen in what we breathe to less than 19.5% and will be fatal if they dilute the oxygen to less than 6%. There is no PEL for any of these gases—they are harmless as long as they do not dilute the oxygen to unsafe levels.

Carbon dioxide is different.  While a mixture of 20.95% oxygen and 79.05% argon is perfectly safe to breath, a mixture of 20.95% oxygen and 79.05% carbon dioxide will result in fatal carbon dioxide poisoning. If enough carbon dioxide is released into the atmosphere to reach the CO₂ IDLH of 4%, the oxygen in that atmosphere will only be diluted to 20.11%. That means that there is still plenty of oxygen in the atmosphere even as the CO₂ reaches dangerous levels. An oxygen monitor will not detect levels of CO₂ as they become too high.

A Project to Quantify

Of all the hazards associated with beverage alcohol, CO₂ probably gets the least notice. Perhaps that is because personnel move in and out of high CO₂ environments frequently, enough that they are getting plenty of fresh air. Perhaps it is because the CO₂ generation rate in fermenters is low enough that the ventilation keeps it diluted, whether or not the ventilation has been deliberately engineered to provide adequate ventilation. As far as we can tell, however, if workers and visitors are not being exposed to unsafe levels of carbon dioxide, it is typically not because anyone is consciously monitoring and controlling CO₂ levels.

Here at Bluefield, we are beginning a project to look at CO₂ levels in beverage alcohol production.  We will be looking at CO₂ generation rates as a function of time, and measuring CO₂ levels in and around the fermenters. Our plan is to share our findings at the Global Congress on Process Safety and with the readers of our blogs. Then, we’ll be able to either advise that CO₂ exposure is not an issue requiring attention, or to advise that it is a concern, at least in terms of the OSHA PEL, and make recommendations accordingly.

We’ll keep you posted.