Phoenix Journal · LEV & Air Quality
The fine mist thrown off a CNC machine looks like nothing, but it is one of the quieter causes of occupational lung disease in engineering. Coolant mist can trigger asthma and a scarring lung condition, and the fluid itself grows bacteria. Controlling it takes more than a fan.
Watch a CNC machine at work and the coolant flooding the cutting edge looks entirely benign. What you do not see is the fine mist thrown off as the fluid meets a fast-spinning tool, an aerosol of droplets small enough to reach deep into the lung and, in normal lighting, all but impossible to spot. Operators breathe it shift after shift, which is exactly why the HSE has run repeated campaigns after finding poor mist control in engineering firms using CNC machines.
Metalworking fluids go by many names on the shop floor: suds, coolant, slurry, soap, white water. Whatever they are called, the water-mix versions carry a specific set of health risks when their mist is inhaled. The recognised outcomes are occupational asthma, bronchitis, irritation of the upper airways and, more rarely but far more seriously, occupational hypersensitivity pneumonitis, a scarring inflammation of the lung once known as extrinsic allergic alveolitis.
That last condition is the reason coolant mist deserves respect it rarely gets. There have been documented outbreaks of hypersensitivity pneumonitis in British engineering plants, clustered around machines running contaminated water-mix fluid. It is uncommon, but when it strikes a workforce it does so in groups, and the damage can be permanent. A mist that looks like nothing is capable of taking someone's lung function for good.
No number to hide behind
Machinists sometimes ask what the exposure limit for coolant mist is, expecting a figure like the ones dust and fume carry. There is not one. Great Britain has no specific workplace exposure limit for metalworking fluid mist, which surprises people who assume a named hazard always comes with a named number. The absence is not a loophole; it changes the duty rather than removing it.
Without a limit to measure against, COSHH still applies in full. The employer has to prevent exposure where that is reasonably practicable and, where it is not, control it to as low a level as reasonably practicable using proper engineering rather than guesswork. New cases of asthma have been reported in machinists at mist concentrations below the old historical guidance figure, which is precisely why sitting near any remembered number is no defence. The honest target is the lowest mist the job allows.
Two things follow from having no number. First, control is judged by whether the mist is actually captured, so a dust lamp, smoke pen or aerosol monitor is worth more than a certificate in showing which machines leak. Second, because you cannot prove safety with a single reading, the fluid and its containment have to be managed continuously rather than signed off once and forgotten. The same care applies to the wash fluids from component-cleaning machines, which the HSE notes can be hazardous in much the same way as the coolant they rinse off.
The fluid is half the problem
Effective control starts by stopping mist escaping in the first place: enclose the machine head as far as the work allows and fit local exhaust ventilation designed to carry the aerosol away at source rather than let it drift into the room. But the second half is the fluid itself. Water-mix coolant is a living system that grows bacteria and fungi, and it is that microbial contamination that is most strongly implicated in the lung disease, so concentration, pH and bacterial levels have to be checked routinely, with dip slides the usual test. The fluid also harms through the skin, so gloves, overalls and clean rags matter alongside the extraction, because prolonged contact of unprotected hands and forearms with coolant is a recognised cause of dermatitis. Coolant mist is a textbook sensitiser, and the way it can quietly turn into asthma is the same story told in occupational asthma, causes hiding in plain sight, where an everyday workplace substance becomes a lifelong condition before anyone connects the two.
A legal duty, not a nicety
Where workers are exposed to metalworking fluid or its mist, health surveillance is not optional even when engineering controls are in place; it is a legal requirement under COSHH. In practice that means a respiratory questionnaire and skin checks run with an occupational health professional, plus a culture where operators are encouraged to report early symptoms rather than work through them, because the point of surveillance is to catch harm while it is still reversible. Setting this up properly is a programme rather than a one-off appointment, which is what building a health surveillance programme walks through, from baseline questionnaires to acting on the results and keeping the records the law expects.
Questions
There is no specific numeric workplace exposure limit for metalworking fluid mist in Great Britain. COSHH still applies in full, so the duty is to prevent exposure where you can and otherwise control it as low as reasonably practicable with LEV and mist enclosures.
Water-mix coolant grows bacteria and fungi, and contaminated fluid is strongly linked to hypersensitivity pneumonitis. Checking concentration, pH and bacteria with dip slides keeps the fluid, and the mist coming off it, far less harmful.
Yes. Where workers are exposed to coolant or its mist, health surveillance is a legal requirement even with controls in place. It usually means a respiratory questionnaire and skin checks, with an occupational health professional involved.
Phoenix Duct Clean · by the numbers
Mist you cannot see still reaches the lung. Phoenix examines and tests LEV on CNC machines to COSHH Regulation 9 and reports on capture at source, not just airflow. UK-wide.