Exposure to diacetyl vapour in food and drink manufacture

Health and Safety Executive - Safety alert

Department name: Engagement and Policy Division

Bulletin number: EPD01-2023

Issue date: 01/23

Target audience: Employers, and the self-employed, in food and drink manufacture who use diacetyl. 


Exposure to vapour from diacetyl, often used as a flavouring and a by-product of coffee roasting, can lead to severe and irreversible lung disease. Even if diacetyl is present at low concentrations within mixtures or flavourings, exposure to its vapour may be above safe workplace exposure limits (WELs).

Outline of the problem

HSE scientific studies show that heating diacetyl above certain temperatures significantly increases airborne concentrations and the potential for exposures above safe workplace limits.

Risk in coffee manufacture

Exposure levels during bean roasting and grinding can exceed WELs. The amount of diacetyl generated naturally during bean grinding is temperature dependent.  Concentrations are significantly greater if the roasted beans are ground when still warm (around 40°C) and reduced if the beans are cooled between roasting and grinding down to room temp (around 16-20°C).

Risk in flavour manufacture 

Airborne concentrations and the potential for exposures above safe workplace limits is significantly increased if flavour mixtures containing diacetyl, even at low concentrations (below 5%) are heated, added to hot processes or spray dried.

Risk of exposure can occur during

  • opening of diacetyl or flavouring containers
  • decanting and weighing
  • mixing
  • spray drying to produce powdered mixtures 
  • packaging
  • cleaning of vessels or spillages        

Hazardous substance workplace exposure limit (WEL)   

Diacetyl (CAS: 431-03-8), also known as 2,3-butanedione, is a naturally occurring organic compound but is also manufactured synthetically. Diacetyl vapour can be generated as a by-product during the roasting and grinding of coffee beans and may also be present during brewing of some beers. Synthetic diacetyl is classified as a hazardous substance. It is toxic if inhaled, can cause skin irritation and eye damage by contact and harmful if swallowed.

WELs for diacetyl were published in EH40/2005 Workplace Exposure Limits. Limits are at 20 parts per billion (ppb) or 0.02 parts per million (ppm) over an 8-hr time weighted average (TWA) and 100 ppb or 0.10 ppm over a 15-min TWA period. Suppliers' safety data sheets for diacetyl or for mixtures containing diacetyl should list these WELs.  the Control of Substances Hazardous to Health (COSHH) Regulations 2002 (as amended) require employers to ensure work-related exposure is assessed, prevented or adequately controlled so that it is below the WELs.

Action required

Assess the risk

If your processes include the use of diacetyl, food flavourings that contain diacetyl, or are likely to produce diacetyl, then you must carry-out a risk assessment. Your risk assessment will help you to identify the hazards associated with the potential for exposure, understand who might be harmed and how, evaluate the risks and decide on precautions.

Check the safety data sheet

If diacetyl is not mentioned on a safety data sheet for food flavourings (which are likely to contain it) you should contact the supplier to confirm if it is present or not


If there is a potential for diacetyl exposure use sampling and analysis to verify whether exposure is likely to be above the WEL.


Substitute to a safer alternative product.  Substitutes should not contain compounds similar to diacetyl such as 2,3-pentanedione. 

Control risk of exposure

If substitution is not a viable option (for example if diacetyl is a natural by-product), then strict controls must be implemented:

  1. Keep the flavouring at a low temperature (below 4°C) as this will significantly reduce vaporisation
  2. Enclose the process and use extraction, to control diacetyl vapour emissions at source
  3. For coffee manufacture, cool the coffee beans (to at least below 20ºC) pre-grind.
  4. For diacetyl flavouring manufacture and use, add the flavouring at the last stage of production and via an enclosed or automated system
  5. Where the above controls do not reduce exposure below the WEL, you should consider providing suitable personal protective equipment (PPE), including suitable respiratory protection equipment (RPE).

Health surveillance

If there is a reasonable likelihood that workers may be harmed by diacetyl you must introduce a health surveillance programme. Your risk assessment will help you decide if this is required. A health surveillance programme should be devised in consultation with an occupational health provider.

Carbon Monoxide (CO)

Carbon monoxide is also known to be a by-product in coffee processing. Control of diacetyl through enclosure and extraction will also help control CO emission. HSE recommends that as part of a risk assessment process, you carry out sampling to establish whether any further controls for CO might be necessary.


Relevant legal documents

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