Phoenix Journal · LEV Testing
Your lungs run two very different systems for clearing inhaled dust - one quick, one dangerously slow. Understanding both explains why capturing dust at source, and testing the extraction that does it, protects your team.
The biology behind the airflow
Your lungs are cleaning themselves right now, quietly and without any instruction from you - and understanding how that self-cleaning works is the clearest way to grasp why capturing dust at source matters so much.
Every breath a kitchen porter, a fryer or a joiner takes pulls in more than air. It pulls in whatever is floating in that air: flour, grease aerosol, wood dust, carbon from burnt fat, the fine grit that settles on every surface in a busy extraction system. The body has a remarkable set of defences for dealing with this, refined over a very long time. But those defences were built for a natural world, not for a workplace that generates the same particle, in the same spot, hour after hour. When the load is steady and heavy, the clearance machinery starts to fall behind. That gap - between what the lungs can shift and what the air keeps delivering - is the whole reason extraction and local exhaust ventilation exist.
The airway does not clear dust in one way. It uses two quite different systems, and which one deals with a given particle depends almost entirely on how deep that particle travels - which in turn depends on its size.
Line the inside of your windpipe and the branching tubes below it and you find a moving carpet of mucus. Beneath that mucus sit millions of tiny hair-like structures called cilia, and they beat in a coordinated wave, hundreds of times a minute, always sweeping upward towards the throat. Larger particles that land on this surface get caught in the mucus and carried up and out, to be swallowed or cleared without you ever noticing. Clinicians call it the mucociliary escalator, and it is genuinely quick: material that lands in the central airways is usually shifted within a day, often within a few hours.
This is the fraction that the air sampling world calls inhalable dust - the larger particles, up to around 100 microns across, that you draw in through the nose and mouth. Under UK workplace limits the inhalable dust ceiling sits at 10 mg/m³ as an eight-hour average. It is the coarser stuff you can sometimes see hanging in a shaft of light, and for the most part the escalator handles it well - provided it is working. Tobacco smoke, dehydration and certain irritants slow the cilia or thicken the mucus, and a slowed escalator lets more material sit for longer.
The smaller a particle is, the further it rides on the airstream before it settles. Once particles drop below roughly five microns they can slip past the escalator entirely and reach the alveoli - the tiny air sacs at the very end of the airway where oxygen crosses into the blood. There is no mucus down here and no cilia. There is nothing to sweep the particle back out. Instead the lung sends in cells called alveolar macrophages: mobile scavengers that engulf a particle, wrap around it and carry it away, either back up to the escalator or into the lymphatic system.
This second system is where the trouble starts. It is slow. Studies of healthy non-smokers show macrophages clearing roughly half of a settled dose within a few hours, but the other half lingers with an average residence time measured not in hours but in months - well over a hundred days for the stubborn remainder. That fine fraction, the part that reaches the gas-exchange region, is what the standards call respirable dust, and it carries a tighter limit of 4 mg/m³ precisely because it goes deep and stays long.
The numbers that matter
The two clearance systems run on completely different clocks, and the legal limits reflect it. When you compare the speed of the escalator with the sluggishness of the deep-lung scavengers, the case for stopping dust before it is ever breathed in makes itself.
Read those three figures together and the picture is stark. The dust the escalator handles is gone in a day. The dust that gets past it can sit in the lung for a season or more, and it is the very fraction that a limit set at 4 mg/m³ is trying to keep out of the air in the first place. Clearance is not a licence to let people breathe dust - it is a finite budget, and a steady workplace source spends that budget faster than the lung can top it up.
None of this defence machinery has an off switch, but all of it has a ceiling. The macrophage approach works beautifully for the occasional particle. Overwhelm it - keep delivering the same fine dust faster than the cells can carry it away - and you reach a state researchers call dust overload, where the scavengers are so full and so numerous that clearance actually slows down further. The particles that should have been removed instead settle in, and over years that retained load is what drives scarring, chronic inflammation and the occupational lung diseases that sit behind so much UK health-and-safety law.
Some workplace dusts make this worse in a way that has nothing to do with volume. Flour dust is the standout example in commercial kitchens and bakeries. It carries only an inhalable limit of 10 mg/m³ and no separate respirable figure, yet it is classed as a respiratory sensitiser - meaning that once a worker's immune system has reacted to it, even small future exposures can trigger asthma. For a sensitiser, clearance almost stops being the point. The duty under the Control of Substances Hazardous to Health Regulations is to reduce exposure as low as is reasonably practicable, not simply to sit under a number.
This is the exact logic behind local exhaust ventilation. Rather than trusting the lung to clear a particle after it has been breathed in, LEV captures the dust or the grease-laden aerosol at the point it is created and pulls it away before anyone inhales it. It is the one control that works with the biology instead of against it - it lightens the load the escalator and the macrophages ever have to carry. And like any engineered control it drifts out of tune: filters clog, ducts fur up, capture velocity at the hood falls away, and a system that passed comfortably a year ago can quietly stop protecting the people standing in front of it. That is why the regulations require a thorough examination and test of LEV at least every 14 months by a competent person, with records kept for at least five years. If a system does slip below the mark, it is worth understanding the common reasons behind a failure rather than simply resetting the clock.
Questions
Inhalable dust is the coarser fraction, particles up to around 100 microns, that you draw in through the nose and mouth and that the airways can usually clear within a day. Respirable dust is much finer, below roughly five microns, small enough to reach the alveoli deep in the lung. Because it penetrates further and is cleared far more slowly, respirable dust carries a tighter UK exposure limit of 4 mg/m³ against 10 mg/m³ for inhalable.
There are two systems. In the airways a moving carpet of mucus, driven by tiny beating hairs called cilia, sweeps larger particles up towards the throat - the mucociliary escalator - usually within 24 hours. Deeper in the lung there is no escalator, so scavenger cells called alveolar macrophages engulf the fine particles and carry them away, a process that can take months for the more stubborn fraction.
Flour dust is a respiratory sensitiser, which means that once a worker's immune system reacts to it, even small later exposures can trigger occupational asthma. It has an inhalable exposure limit of 10 mg/m³ and no separate respirable figure, but for a sensitiser the legal duty is to reduce exposure as low as is reasonably practicable rather than simply staying under a number. Good local exhaust ventilation is the practical way to do that.
Under the Control of Substances Hazardous to Health Regulations, local exhaust ventilation must have a thorough examination and test at least every 14 months by a competent person, and some higher-risk processes need it more often. The test checks airflow, capture velocity, duct velocity, filter condition and overall control effectiveness. Records must be kept for at least five years.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.