Phoenix Journal · LEV Testing
Workplace exposure limits look like simple numbers, but each one is the end of a careful scientific process. Here is who sets them, the evidence they rest on, and why the figures matter for your controls.
LEV testing · the science behind the numbers
Every time you read a figure in a COSHH assessment - a number in parts per million, a milligram limit for a dust or fume - you are looking at the end of a long scientific process, not a round number someone picked for convenience.
In Great Britain those figures are called Workplace Exposure Limits, or WELs, and they set the legal ceiling for how much of a hazardous substance is allowed in the air you breathe at work. They matter to any business that generates airborne contamination, from a fabrication shop cutting steel to a busy commercial kitchen where cleaning chemicals, grease vapour and combustion by-products all share the same air. Understanding how these limits are arrived at helps you read your own assessments with a clearer eye - and it explains why a control measure such as local exhaust ventilation is judged against a specific number rather than a vague sense of "fresh enough".
This piece walks through who sets WELs, the evidence they lean on, and why two substances that seem equally unpleasant can end up with limits that differ by a factor of thousands.
A WEL is the maximum concentration of an airborne substance, averaged over a set reference period, to which workers may be exposed. The list of current limits lives in a single Health and Safety Executive publication, EH40/2005, which is revised periodically and carries legal weight under the Control of Substances Hazardous to Health Regulations. If a substance appears there, meeting its limit is not optional guidance - it is a duty.
Each limit is expressed against one or both of two time frames. The long-term exposure limit uses an eight-hour time-weighted average, which reflects a normal working shift and is designed to guard against harm that builds up over months and years. The short-term exposure limit uses a fifteen-minute average and exists to catch substances that can do damage in a brief, sharp burst - an irritant that scorches the airways, say, even when the daily average looks acceptable. Figures are given in parts per million for gases and vapours, and in milligrams per cubic metre - mg/m³ - for dusts, mists and fumes.
The eight-hour figure is not a simple snapshot. Because real work is rarely steady, exposure is worked out as a time-weighted average, so a short spell at a higher concentration is balanced against quieter periods, all referenced back to a standard eight-hour day. This is why proper air monitoring matters: a single reading tells you very little, whereas a pattern of readings across a shift tells you whether you are genuinely inside the limit.
WELs are not set by the HSE alone, nor by industry. The scientific groundwork rests with independent expert committees. For many years the Advisory Committee on Toxic Substances advised on limits; today the standing body of independent scientists and clinicians is the Workplace Health Expert Committee, which reviews the evidence linking a workplace hazard to ill health and gives the HSE an impartial, authoritative opinion to build policy on. Working under an independent chair, its role is to weigh the science, not to negotiate a figure that suits any particular sector.
The evidence itself comes from several directions at once. Epidemiology - real health data from workers who have been exposed over time - is the most directly relevant, though it is often incomplete or confounded by other factors. Animal toxicology fills gaps that human data cannot ethically or practically cover. Mechanistic studies explain how a substance actually causes harm at the cellular level, and where the numbers allow, formal risk modelling estimates how likely harm becomes at a given concentration. A limit is rarely built on a single study; it is a judgement drawn from the weight of all of it.
How that evidence is turned into a number depends on the kind of harm involved, and this is the single most important idea to grasp. For a substance with a threshold - one where there is a level below which no adverse effect is seen - scientists identify the highest exposure that produced no observed adverse effect, the NOAEL, and then divide it by uncertainty factors. Those factors deliberately build in caution: one to allow for differences between test animals and people, another for the natural variation between individuals, since a young fit worker and someone with an existing respiratory condition do not respond alike. The resulting limit sits comfortably below the point where harm was ever detected.
Substances with no safe threshold are treated very differently. Genotoxic carcinogens - agents that damage DNA, where in principle a single molecule carries some risk - cannot be handled by finding a "safe" level, because none exists. Here a WEL is set as a risk-based value: a concentration judged to carry an acceptably small excess risk of disease across a working lifetime. Because there is no floor, the duty does not stop at the limit. Exposure must be reduced as low as is reasonably practicable, and the WEL is a backstop rather than a target. This is exactly why the reclassification of all welding fume, including mild steel, as a carcinogen in 2019 - following evidence from the International Agency for Research on Cancer - changed enforcement expectations so sharply: general ventilation was no longer accepted, and suitable engineering control such as extraction became the expectation for indoor work.
WELs are not fixed forever. As new studies land, as classifications shift under retained and updated European directives, and as the expert committee revisits older assumptions, the list is amended. The 2020 revision of EH40, for instance, brought in new and revised limits arising from the Carcinogens and Mutagens Directive, tightening the position on several substances. Proposed changes typically go through public consultation before they are adopted, so employers and specialists can test the practical implications before a figure becomes law. The practical lesson is simple: always work from the current edition, and treat an old printout in a folder with suspicion.
It is worth being honest about what a WEL is not. Meeting the limit does not prove your workplace is safe in any absolute sense. The number is a legal ceiling built on the evidence available at the time, with caution baked in - but sensitisers can trigger occupational asthma in susceptible people at exposures well below the published figure, which is why some substances carry a specific "Sen" notation and why the honest goal is always the lowest reasonably practicable exposure. If you want to understand how repeated low-level exposure translates into lasting harm, our explainer on how work exposure causes occupational asthma sets out the mechanism in plain terms.
This is where the limit meets the real world of ventilation. A WEL only becomes useful once you can show, with monitoring, where your actual exposures sit against it - and that is the entire point of testing an extraction system rather than simply installing one and trusting it. If readings drift towards the limit, or a control fails and exposures spike, you are into investigation territory; our guide on how to investigate a workplace exposure incident covers the steps that follow. For most businesses the practical takeaway is that the science behind these limits is only worth anything if the equipment meant to keep you under them is proven to work, on a schedule, with the results written down.
Questions
The long-term exposure limit is averaged over an eight-hour reference period and guards against harm that accumulates over a working life. The short-term exposure limit is averaged over just fifteen minutes and catches substances that can cause damage in a brief, high burst, such as an irritant that harms the airways even when the daily average looks acceptable. A substance may carry one limit, the other, or both.
Not necessarily. A WEL is a legal ceiling built on the evidence available when it was set, with caution deliberately built in, but it is not a guarantee of zero harm. Respiratory sensitisers can trigger occupational asthma in susceptible people well below the published figure, and for carcinogens there is no safe threshold at all. The correct duty is always to reduce exposure as low as is reasonably practicable, not simply to sit at the limit.
New epidemiological and toxicological evidence emerges, substances get reclassified, and the independent expert committee revisits older assumptions. Changes usually go through public consultation before they become law - the 2020 revision of EH40, for example, introduced new and revised limits from the Carcinogens and Mutagens Directive. Because of this you should always work from the current edition of EH40 rather than an older copy.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.