Working Safely with Metal Working Fluids

Working safely with metalworking fluids

 During processes such as machining, grinding and milling, individuals working with metalworking fluids as part of their day-to-day role risk developing a host of respiratory illnesses, which is why it is important for organisations to encourage best working practices and ensure that exposure levels are as low as is reasonably practicable. Testing, inspection and compliance service provider.

Metalworking fluid – also known as cutting fluid – is a type of coolant used during metalworking processes such as machining, grinding and milling. They are typically neat oils or water-based fluids used to reduce the heat and friction between the cutting tool and the work piece, helping to prevent burning and smoking. The 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 work pieces.

What are the different types of cutting fluids?

There are three general types of metalworking fluids:

  1. Water-soluble oils (emulsions),
  2. Semi-synthetic fluids and synthetic fluids, and this includes oils, oil-water emulsions, pastes, gels, aerosols (mists),
  3. Air or other gases.


Is cutting fluid hazardous?

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

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.

What are the health risks associated with metalworking fluids?

Metalworking fluids are respiratory sensitisers’ and can lead to workers developing occupational asthma and other respiratory illnesses such as Occupational Hypersensitivity pneumonitis (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).

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.

A cutting fluid that is well maintained will be less hazardous than a fluid that has major contamination, and it is therefore important that routine checks are carried out.

  • 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 and 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 (e.g 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 therefore be checked by dip slide method. This will indicate if concentrations are light, medium or heavy, with concentrations of 10CFU/ml or greater requiring action to be taken.


COSHH requires that records of any inspections or checks that relate to control measures (e.g 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.

How can I assess exposure levels to metalworking fluid?

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. 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.

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;
  • Provide health surveillance such as skin and respiratory checks.


What are good practices to ensure the safe handling and disposal of metalworking fluids?

Control by 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.

Where installed, local exhaust ventilation should undergo routine checks and maintenance, all of which should be noted in a log book. 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.

Employees should also be encouraged to use tools such as brushes and swarf hooks so that direct contact with the fluids is avoided wherever possible.

PPE Provision

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 work wear that has become contaminated with metalworking fluids should be laundered at work. Observing good personal hygiene practices by washing skin before breaks and after work will help to remove any oil residues and reduce the risk of ingestion.

 Visual assessment and measurement

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, and this can literally come in the form of a simple torch with an intense beam of light. The lamp should be positioned on one side of the machine (e.g. left side 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 (e.g. right side of opening) by looking at an angle towards the beam without actually looking directly into the light. Slight adjustments to the viewing angle may be required to acquire a true visualisation of any mist clouds present.

Further information can be obtained from

HSE documents G402 and G403.

Contains public sector information licensed under the Open Government Licence v3.0.