Source and Control

Individuals working with metalworking fluids as part of their day-to-day role risk developing a host of respiratory illnesses. SOCOTEC says best practice reduces that risk

Metalworking fluid – also known as cutting fluid – is a type of coolant used during metalworking processes such as machining, grinding and milling. It is typically neat oils or water-based fluids used to reduce the heat and friction between the cutting tool and the workpiece, helping to prevent burning and smoking. As well as providing lubrication and cooling, metalworking fluid also helps to carry away debris such as swarf and fine metal particles. It helps to improve machining performance and prolong the life of the cutting tool, as well as provide corrosion protection for the surfaces of workpieces. There are three general types of metalworking fluid: water-soluble oils (emulsions), semi-synthetic fluids and synthetic fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists), and air or other gases. These may be made from such elements as petroleum distillates, animal fats, plant oils, water and air.

Metalworking fluids are respiratory sensitisers and can lead to workers developing occupational asthma and other respiratory illnesses such as occupational hypersensitivity pneumonitis (OHP, also known as extrinsic allergic alveolitis), so the Control of Substances Hazardous to Health (COSHH) Regulations require exposures to be reduced to as low as is reasonably practicable (ALARP). Earlier this year, between January and March, the HSE made visits to industries involved with fabricating metals to ensure that suitable control measures that minimise exposure to metalworking fluid were in place.

The most likely route of exposure to metalworking fluid is by inhaling the mist or aerosol produced during metalworking processes, or when it comes into contact with the skin and eyes. However, exposure may also result through cuts and abrasions or through ingestion.

Where controls have been introduced, employees should be provided with suitable information, instruction and training so that they can use the controls effectively and are able to identify any failures. Workers should be informed of the health effects associated with exposure to metalworking fluid, as this will make it easier for significant issues to be identified early. Employers should also be committed to maintaining controls and ensuring they are used correctly. Health surveillance should be provided where there is the potential for workers to regularly inhale MWF mists and there is a reasonable likelihood that asthma or dermatitis may develop. For further guidance see HSE documents G402 and G403.

Risk of infection

Metalworking fluid systems that contain water or water-mixes can become highly contaminated with harmful bacteria. Under suitable conditions, it is common for pseudomonas, mycobacterium, fungus, stenotrophomonas, brevundimonas and bacillus to grow.     

  • Check the appearance and odour: any subtle changes to the colour and/or odour of the fluid may indicate contamination. The fluid should be sampled into a clear bottle then left to settle ideally overnight. Solids settling to the bottom of the container may indicate the presence of significant swarf/fines in the fluid     
  • Temperature: machines where temperatures of water mixed with metalworking fluids are high (above 30°C) will stimulate microbe growth. Higher temperatures for neat oil can affect viscosity, cooling and misting properties     
  • Concentration: an increase in MWF concentration may arise as a result of evaporation, increasing skin and respiratory irritation, while a drop in concentration may increase the risk of microbial contamination and corrosion. The concentration of the mix can be measured with a refractometer – a visual blurring of the sampled fluid may indicate tramp oil contamination     
  • pH: bacteria and fungi produce acids, which reduce pH. A sharp drop in pH (eg more acidic) normally results in further microbial growth, which can also lead to corrosion. A higher pH can lead to increased skin irritation, making it incredibly important that the pH be maintained within the supplier’s recommended range     
  • Tramp oil: concentrations above 2% tramp oil may increase the risk of dermatitis and encourage microbial growth. One method of checking tramp oil is by sampling into a clear container, leaving overnight and carrying out a visual inspection on the surface the following day. A glistening layer on top of the fluid will indicate the presence of oil, which should be removed by an oil skimmer or vacuum     
  • Microbial concentrations: microbial accumulation is a significant health hazard and should be checked by dip slide method. This will indicate if concentrations are light, medium or heavy, with concentrations of 104 CFU/ml or greater requiring action to be taken.  

COSHH requires that records of any inspections or checks that relate to control measures (eg LEV, tramp oil, bacteria, MWF concentration and pH) are kept in paper or electronic format and retained for five years. Further guidance on how to maintain MWF can be found in section 5.0 of the Good Practice Guide for Safe Handling and Disposal of Metalworking Fluids (UKLA).

Assessing exposure

Assessing a worker’s exposure to metalworking fluid is not as simple as taking an air sample and comparing it with a limit, as there are currently no HSE workplace exposure limits (WELs) available. While there have been exposure limits in the past, the cause of respiratory health effects due to exposure to metalworking fluid is quite complex. This is due to the fact that the fluids may contain a number of different components, including additives such as detergents, biocides and corrosion inhibitors, as well as contaminants such as bacteria and endotoxins. Therefore, respiratory and skin problems may result from exposure to one component or a combination of the above.

Other issues with using air sampling as the sole means for exposure assessment is that some cutting fluids – such as boron – no longer have markers present, and these have been used previously to measure exposure. Moreover, measuring total inhalable dust (gravimetric analysis) may not always give a true picture of exposure from the metalworking fluid if there are other components present in the air, such as welding fume.

For an employer to ensure that exposure levels are as low as is reasonably practicable, relying on exposure monitoring alone is not adequate. A wider approach is required and should include the following:     

  • Routinely check and maintain metalworking fluids     
  • Install, use and maintain local exhaust ventilation (LEV)     
  • Establish clearance times inside the machine     
  • Add a suitable delay on the machine door after cutting (based on the clearance time)     
  • Ensure fluid delivery and cutting speeds are suitable     
  • Use alternatives to compressed air line guns to blow swarf and fluid     
  • Wear suitable PPE to protect the skin and eyes     
  • Visually assess the suitability of control by dust lamp and/or real-time particulate measurement     
  • Provide suitable information, instruction and training; and     
  • Provide health surveillance such as skin and respiratory checks.

Local exhaust ventilation (LEV)

Machines using metalworking fluid should be equipped with LEV to draw mists away from the operator. Some machines may come equipped with this, whereas others require LEV to be fitted retrospectively. Effective LEV will provide the machine with adequate negative pressure so that air is drawn inside, preventing the escape of mist when the machine is opened. HSE guidance states that inward air speed should be at least 0.5 ms-1 at the machine opening. Machines with low fluid delivery that generate lower mist concentrations may require air changes of around six per minute, whereas high fluid delivery machines generating high levels of mist may require air changes of up to ten per minute. As the LEV will also operate when the machine is closed, it is important that adequate replacement air is provided through suitable vents.

Contaminated air extracted from the machine will need to be cleaned via a suitable particulate filter. These can come in the form of particulate filters such as HEPA after filters, which work by centrifugal force and return oil back to the machine. An alternative is mechanical air purification, which contains various static filters – including HEPA – but this may require frequent maintenance.

Airflow indicators should be placed close to the hood so the operators can detect any drop in performance. A pressure gauge will also be required on the filter to help identify when maintenance or changes are required. A clearance time will have to be established to determine how long it takes for mist to clear from inside the machine by filling the machine with smoke and timing how long it takes to clear. The clearance time will be the delay required before the machine can be opened by the operator.

Where installed, local exhaust ventilation should undergo routine checks and maintenance, all of which should be noted in a log. Every system is required to undergo a thorough examination and test at least every 14 months as per the requirements of Regulation 9 of the COSHH Regulations.

Within the machine, the pressure and flow of metalworking fluid delivery and the cutting speed of the tool should be suitably set to reduce mist generation to a minimum. Splash guards should also be added to reduce fluids from escaping. The use of compressed air to blow swarf and debris from machines after cutting is a significant source of exposure, as it can generate a mist and cause fluids to splash onto the skin. There is also the issue of elevated exposure to noise when open air lines are used. Alternatives to manual compressed air lines include coolant wash guns, vacuums and compressed air fixed inside the machine.

If none of the above are practicable, then lowering the operating pressure of the compressed air line will help to reduce the mist generated. Pressures of 2.1 bar (30 PSI) have been shown to remove liquid and debris without increased mist generation. Employees should also be encouraged to use tools such as brushes and swarf hooks so that direct contact with the fluids is avoided where possible.

Further protection

Even with controls in place, suitable PPE in the form of protective gloves, eye protection and overalls that cover the skin should be provided. For example, nitrile 0.4mm single-use gloves will be adequate for general use, but thicker gloves or gauntlets may be required for sump cleaning. Overalls and other workwear that has become contaminated with metalworking fluids should be laundered at work. Washing skin before breaks and after work will help to remove any oil residues and reduce the risk of ingestion.

Studies have shown that the most significant exposures arise when CNC machines are opened immediately after cutting has been completed. A useful assessment tool is to visually observe the process with the aid of a dust lamp. This is a simple qualitative tool for making fine particle clouds visible or enhancing the visibility of partially visible clouds, in the form of a torch with an intense beam of light. The lamp should be positioned on one side of the machine (eg left of opening) with the light shone across the front opening of the machine. This should then be viewed by someone on the opposite side of the machine (eg right of opening) by looking at an angle towards the beam without looking directly into the light. Slight adjustments to the viewing angle may be required for a true visualisation of any mist clouds present. For HSE guidance see

Although air measurement alone cannot be relied upon to provide an assessment of adequate control, it can help to identify control failure. For example, direct reading instruments that count particles in the air are useful instruments for identifying leaks when the machines are opened, as well as any other fugitive sources. They provide instantaneous measurements that should not be relied upon as exact concentrations, but rather as a tool to compare concentrations in locations.

Whether used on their own or combined with other occupational instruments, air monitoring will allow an occupational hygienist to provide specific advice concerning the effectiveness of controls and help employers to prioritise improvements.

SOCOTEC’s team of occupational hygienists are fully trained to help employers and their staff comply with occupational regulations and help eliminate the risk to health within the workplace, including LEV assessments, workplace exposure monitoring and control measure evaluations. For more, visit



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