During the coffee roasting and large scale grinding of beans a compound called diacetyl is released into the air along with other volatile compounds. Media reports have raised concerns about diacetyl exposure levels from the process of roasting and grinding coffee in these commercial settings. To ensure worker safety, there are well-established safety protocols and procedures that should be followed to monitor exposure and reduce risk. However Independent research published in 2015 confirms that airborne concentrations of diacetyl in coffee processing “are far below the concentrations that are expected to cause even minimal responses in the human respiratory tract.”

What is diacetyl?

Diacetyl is an organic compound, created naturally during certain cooking and fermentation processes giving off a distinct buttery flavour and aroma, it's what gives butter its flavour. It's also present in cheese, milk, whiskey, wine, beer, and roasted coffee. Until recently, artificial diacetyl was used in some processed foods to impart this flavour, and is still applied to flavoured e-cigarettes.

What are the concerns for coffee drinkers?

For those grinding and drinking coffee in their own home the risks are virtually zero. The U.S. Food and Drug Administration (FDA) says diacetyl in foods and beverages is safe to consume.  (in fact, regular consumption is linked to many wellness benefits).

So, are coffee workers safe?

Scientific evidence does not support a link between diacetyl exposure in coffee production and obstructive lung disease, including an extremely rare lung disease, bronchiolitis obliterans (also known as “popcorn lung” as it was initially found in a popcorn factory where workers were exposed to high concentrations of the artificial liquid chemical). Every coffee roasting facility is different. The conditions affecting diacetyl exposure vary widely, from ventilation to equipment to production process. Levels can also vary in different areas of any given facility. Currently, the U.S. government has issued no regulatory standard, such as an Occupational Exposure Limit (OEL), for levels of diacetyl. The numbers proposed by various governmental and scientific organisations in the US vary widely:

  • National Institute for Occupational Safety & Health (NIOSH): 5 parts per billion (ppb) over an 8-hour workday, or 25 ppb during a short-term, 15-minute exposure.
  • American Conference of Government Industrial Hygienists (ACGIH): 0.01 parts per million (ppm), which is 10 ppb, over an 8-hour workday, or 0.02 ppm, which is 20 ppb, during a 15-minute exposure.
  • Toxicology Excellence for Risk Assessment (TERA): 0.2 ppm, which is 200 ppb, over an 8-hour workday.

Thankfully most commercial coffee roasting machines are generally “closed systems” that are built to channel the natural byproducts of roasting through closed pipes to exhaust stacks. Even the “cooling carts,” where coffee beans cool after roasting (as the name implies), are designed to minimise potential exposure. A system of fans and holes in the bottom of these trays draw the air down through the beans, and then out via exhaust pipes to the outside. However we here at Caffeine have noticed an unprecedented expansion of small roasters in sheds and garages, these small batch operations may not be aware of the potential risks to themselves or their employees. We encourage all companies large or small to learn more, follow suggested best practices (see the resources listed below), and take action to the correct course of action.


Respiratory Health and Safety in the Flavor Manufacturing Workplace, Flavors & Extracts Manufacturing Association (FEMA)

Best Practices: Engineering Controls, Work Practices, and Exposure Monitoring for Occupational Exposures to Diacetyl and 2,3-Pentanedione, U.S. Centers for Disease Control (CDC)Diacetyl and Food Flavorings, NIOSH Science BlogResearch: Naturally occurring diacetyl and 2,3-pentanedione concentrations associated with roasting and grinding unflavored coffee beans in a commercial setting, Toxicology Reports, 2 (2015), pp. 1171-1181 The NCA Statement on Diacetyl