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1.3 Respiratory Terminology Workplace Exposure Limit (WEL) Airborne concentration of a Hazardous Substance, aver- aged over a specified time period referred to as a Time Weighted Average (TWA). WEL Time Periods There are two reference periods for which WELs may be set; 8 hour Time Weighted Average (TWA) and 15 minute Short Term Exposure Limit (STEL). A substance may be assigned WELs at either one or both reference periods. ➣ 8 hour TWA - Some adverse health effects can occur after prolonged or accumulated exposure. The 8 hour TWA is set to restrict the total intake by inhalation over one or more shifts. ➣ 15 minute STEL - Some adverse health effects may be seen after short exposures. 15 minute STEL may be applied to control these effects. Immediatedly Dangerous to Life or Health (IDLH) The IDLH concentration of a substance is defined as “that which poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment”. The IDLH value repre- sents a maximum concentration from which a worker would escape within 30 minutes without any impairing symptoms or irreversible health effects. Odour Threshold The concentration of a substance at which the majority of individuals can smell or taste it. 1.4 Respiratory Protective Equipment (RPE) Selection Calculation Personal air monitoring information, when compared to the relevant WEL for that hazardous substance, helps to more accurately determine the required level of respirator protec- tion factor. For example: Woodworking ➊ Measured Levels (Wood Dust) = 60mg/m3 over 8 hours TWA. ➋ Workplace Exposure Limit (WEL) for wood = 5mg/m3 . ➌ Divide ➊ by ➋ =60/5=12. ➍ This figure of 12 is the level at which the hazard is above the WEL i.e. the hazard level is 12xWEL. ➎ Assuming all other control measures have been consid- ered, including the eight new principles of good practice, select a respirator with an Assigned Protection Factor (APF) greater than 12 (e.g. 3M 9332 which has an APF of 20). ➏ Ask yourself the further question “Do I need to lower actual workplace levels as far below the WEL as it is reasonably practicable?”; i.e. is this substance one of the group of substances that can cause cancer, sensitisation or heritable genetic change? In this case, wood dust is a carcinogen and therefore levels should be lowered as far below the WEL as is reasonably practicable. Therefore, if all other control measures have been considered, an even higher performing respiratory protection product should be contemplated. However, always remember that RPE should be the last resort and that one of the main principles of RPE selection should be that it is “sus- tainable to the job and the wearer”. Usage Chart for Various Hazards HAZARD MAX. CONCENTRATION FINE DUSTS Brick Dust, Iron, Coal Dust Concrete/Cement 4xWEL FINE DUSTS, MISTS e.g. Wood Dust, MDF Foundry Dust, Stone Dust, Steel, Pesticides (Water-based solids) 10xWEL FINE DUSTS/MISTS/ METAL FUME e.g. Glass Fibre, Mineral Wool Fibre, Cobalt, Cotton, Asbestos (low duration only) Silicia Quartz, Metal Fume Bacteria Spores, Wood Dust, MDF 20xWEL WELDING FUMES Welding Fumes only Where Ozone is produced 10xWEL 10xWEL (Ozone) PAINT SPRAY e.g. Acrylic, Cellulose 10xWEL or 1000ppm (whichever is lower) PAINT SPRAY (TWO PACK PAINTS) e.g. Epoxy Hardeners (Isocyanate Hardeners) ORGANIC VAPOURS (& PARTICULATES) e.g. M.E.K., Toulene, White Spirit (use only in well ventilated areas) 10xWEL or 1000ppm* 10xWEL or 5000ppm* *whichever is lower ACID GAS e.g. Chlorine, Sulphur Dioxide, Hydrogene Chloride Ammonia and Organic Ammonia derivatives 10xWEL or 1000ppm* 10xWEL or 1000ppm* *whichever is lower 44

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