If you're responsible for health and safety in your workplace, you've probably encountered the terms "workplace exposure level," "TWA," and "STEL." But what do they actually mean, and why do they matter for dust management? The simple answer: these are legal limits set by the Health and Safety Executive (HSE) to protect workers from harmful dust exposure. If you're not monitoring them, you're flying blind and potentially breaking the law. What is Workplace Exposure Level? A workplace exposure level (WEL) is the maximum amount of a harmful substance that a worker can be exposed to during their working day without suffering adverse health effects. For dust, it's measured in milligrams per cubic metre of air (mg/m³). The HSE sets these limits based on scientific evidence about how different dust types affect human health. Different dusts have different exposure limits because they pose different risks. For example, the WEL for respirable crystalline silica is far stricter than for general inert dust, because silica is significantly more harmful to the lungs. Think of it this way: the WEL is your compliance baseline. However being below a WEL does not by itself discharge the COSHH duty. Download the HSEs guidance on workplace exposure limits If you want to understand the levels in your workplace contact us about our silica air sampling services TWA: The 8-Hour Average TWA stands for Time-Weighted Average, and it's the most commonly referenced exposure limit. The TWA measures the average concentration of a substance over a standard 8-hour working day. Here's why that matters: dust exposure isn't constant. Workers might face high concentrations during certain activities (cutting, grinding, demolition) and lower concentrations during others. The TWA accounts for these fluctuations by averaging them out over the full shift. This recognises that workers can tolerate brief periods of higher exposure, provided the average stays within acceptable limits. For example, the current WEL for respirable crystalline silica is 0.1 mg/m³ as an 8-hour TWA. This means that if a construction worker is grinding stone for part of their shift but doing lower-dust tasks for the rest, the daily average should not exceed this level. The advantage of TWA is that it's practical for real-world work. Few jobs maintain constant dust levels. The disadvantage is that it can mask dangerous peaks, if a worker is exposed to very high concentrations for a short period, the TWA might still look acceptable. STEL: The Short-Term Exposure Limit STEL stands for Short-Term Exposure Limit, and it's your safety net for peak exposures. A STEL is the maximum concentration a worker should experience during any 15-minute period in a working day, and no more than four such periods should occur, with at least 60 minutes between them. The STEL exists precisely because the TWA can obscure dangerous peaks. Imagine a worker exposed to very high silica dust for 15 minutes whilst cutting stone, then lower levels for the rest of the day. The TWA might be within limits, but those 15 minutes could still cause significant lung damage. There should be a maximum of 4 STEL's in a single working day and there must be at least 60-minutes between each STEL period. Not all dusts have a STEL, but those that do (like respirable crystalline silica) reflect a recognition that short, intense exposures carry genuine health risks that the 8-hour average doesn't capture. Reducing personal exposure to inhalable dust with Cleanspace Respirators