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Metalworking Fluids & Oil Mists - The Complete Guide


Does your business or occupation involve using machines that require metalworking fluids or oil mist – do you know the potential risks to your employee’s health if they are exposed to them on a frequent basis?

The HSE are currently running a campaign of checks to identify if organisations are controlling this risk from metalworking fluids; this article will increase your awareness and give you a better understanding of what is required to comply with the guidance. 

What are metalworking fluids and oil mists?

Metalworking fluids (MWFs) - sometimes referred to as suds, coolants, slurry or soap – are water-based fluids used during the machining of metals to provide lubrication and cooling, and to help carry away debris such as swarf and fine metal particles.They can also help to improve machining performance and prolong the life of the cutting tool, as well as providing corrosion protection for workplace surfaces.

Oil mists form when high pressure fuel oil, lubricating oil, hydraulic oil, or other oil is sprayed through a narrow crack, or when leaked oil connects with a high temperature surface, vaporises, and meets low air temperature. This can happen while the fluids interact with the moving parts during the machining processes. Some oil mist particles are smaller than the eye can see but the danger is still as big!

Health risks of MWFs & Oil Mists

If the mist from oil or MWFs (applied by continuous jets, sprays or via a hand dispenser) are either inhaled, touch unprotected skin, enter wounds or broken skin, or get access to the eyes or mouth - they could affect employee health and lead to a number of varying illnesses.

Exposure to such fluids can cause irritation of the skin, occupational asthma, bronchitis, irritation of the respiratory tract, general breathing difficulties or in extreme cases, can see individuals develop a serious lung disease called extrinsic allergic alveolitis.

Metalworking fluids that contain water or water-mixes are particularly vulnerable to becoming highly contaminated with harmful bacteria and other micro-organisms.

What precautions should be put in place?

Before any precautions are put in place, a suitable and sufficient workplace exposure risk assessment should be undertaken. This will detail current and safe exposure levels alongside any cause for concern in relation to staff health and safety.

Other areas to consider:

  • Use splash guards, to control splashing and misting.

  • Minimise the production of mist and vapour by controlling the volume and rate of delivery of the fluid to the cutting edge of the tool.

  • Install enclosures or ventilation to remove / control any mist or vapour produced.

  • Inform staff of a recommended time delay before opening the doors on machine enclosures - to ensure that all mist and vapour has been removed by the ventilation before exiting.

  • Ensure employees know the reporting process for any damaged or defective splash guards, ventilation hoods or other control equipment.
  • Open workroom doors and windows to improve natural ventilation.

  • Don’t use compressed air to remove excess fluids from machined parts, plant or other equipment.

  • Conduct regular health surveillance checks.

Exposure Limits

In 2005, the Health & Safety Executive withdrew the exposure limit for Metalworking Fluids (set at 1.0mg/m3) due to outbreaks of ill health at a manufacturing facility within the UK. It was found that exposures were successfully being controlled below the exposure limit set at the time, however, workers were still experiencing occupational ill health.

The National Institute for Occupational Safety & Health (NIOSH) have set a Recommended Exposure Limit (REL) of 0.5mg/m3, however, levels of exposure below this standard have been found to cause ill health which means that the standards set by various bodies have no relevance to the risk of health.

In the absence of a relevant exposure limit, more emphasis should be put on controlling exposures to MWF’s via good controls and working practices which minimise exposures as low as is reasonably practicable.

It should be noted, that although there may not be a specific exposure limit for the MWF itself a review of the datasheet for the specific product being used should be carried out to identify the need to conduct exposure monitoring for any harmful ingredients with published exposure limits e.g 2-(2-Butoxyethoxy)ethanol, Monoethanolamine etc

Oil mists are much more straight forward and have an exposure limit currently set at 5mg/m3.

Routine exposure monitoring can ascertain current exposures, identify where further control measures are necessary and allow for comparison against previous assessments to identify improvements.  

Ongoing Biological Monitoring Requirements

The bacterial contamination of fluids and associated machinery and pipework should be monitored and controlled. Direct means of measuring bacterial contamination should be used in conjunction with other checks on fluid quality, e.g. fluid concentration and pH.

Microbiological dip slides are a simple way of checking bacterial contamination. Dipslide checks should be carried out once a week and the frequency reduced only when it can be demonstrated that your fluid quality management is effective.

A dip slide consists of a plastic carrier coated with a sterile culture medium, which is dipped into the liquid to be tested. It is then incubated to allow microbial growth and the resulting colonies are estimated by reference to a chart to indicate the level of bacterial contamination. Results are expressed in terms of colony-forming units per millilitre (CFU/ml) of fluid.

The following values indicate what can be regarded as good, reasonable and poor standards of fluid management, and what action should be taken. Monitoring should be used to confirm your standard of control, as well as indicating increased levels of bacteria at an early stage.

  • <10x4 CFU/ml Good control. Bacteria are being maintained at low levels. Regular checks and actions to maintain the fluid quality should continue. 
  • ≥10x4 to <10x6 CFU/ml Reasonable control. Review and take action to check the quality of the metalworking fluid and adjust fluid parameters to those recommended by the supplier. If bacterial growth continues despite these adjustments, add biocide at the dose recommended by your supplier.
  • ≥10x6 CFU/ml Poor control. Immediate action should be taken in line with the risk assessment. Normally draining and cleaning should take place




In Summary

To ensure you are compliant with the guidance you need to implement the following actions: 

  1. Carryout an initial assessment of potential exposure and review the current controls you have in place.
  2. Implement any engineering and administrative controls.
  3. Make an assessment of exposure levels as a baseline and for comparison against exposure limits where appropriate.
  4. Implement a weekly monitoring regime to check the fluid quality (pH, concentration and temperature) is maintained in accordance with the manufacturers guidance.
  5. Carryout weekly biological contamination checks using dipslides

All test monitoring records should be kept for a minimum of 5 years.


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