Working with substances hazardous to health: A brief guide to COSHH These are specific to underground mining and tunnelling and already published in EH40/2005, which contains all updated limits. Heres what theyre for: Four-hour Control Limit this is a duty-of-care report: the employer is legally required to do these to demonstrate that individual workers are not exposed to asbestos above a certain limit, over a long period of time. The PCM-E concentrations were then used to calculate 8-h TWA exposure concentrations in an attempt to estimate the airborne concentration during a typical workday. Why not get in touch to book a free demonstration? Well-fitted, powered full-face masks give a protection factor of 40. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. If you find less asbestos than the LoQ (including none at all), you cant be sure of this so you say it was below the limit. C3 4.2ppm; T3 53 min. Specific short-duration activity (SSDA) this is the new kid on the block. Choose Yours, WIS Show: Step it up! We cannot make a judgment about the exposure at the facility after monitoring. Asbestos is a general name given to several naturally occurring fibrous minerals that have crystallised to form fibres. Calculating VOC Content of a Material. C is the concentration during any period of time, T, where the concentration remains constant. Working with substances hazardous to health: A brief guide to COSHH, (Examples of real life situations with COSHH), (Use these to get ideas for completing assessments). However, if you are going to expose an employee to an exposure level above the workplace exposure limit at any point during the shift, this exposure should be limited to one hour. Cn Tn)8 E is the equivalent exposure for the working shift. If you are able to specify that all personal monitoring tests are at least two hours long with a flow rate of two litres per minute, it will also automatically start to build up a library of results that could be used to estimate the balance of any exposure. In order to determine whether an employee is exposed above the regulatory limit for an air contaminant that does not have a ceiling value, use the following formula and Table Z-1 to determine the exposure for an 8-hour work shift: E is the equivalent exposure for the work shift. Where t1 is a portion of a time period (such as 0.25 hours) and c1 represents the levels of the substance or agent during that time-period. Similarly, if an employee is working a 4-hour shift, they can be exposed to higher levels within those 4 hours, than they would be if they were working the full 8 hours. EBITDA = 116 + 570 = $686 million. Essentially this test is to detect brief spikes in asbestos concentration, even when the average over four hours was acceptably low. Compliance with OSHA limits is the most common objective. It stands for Limit of Quantification, but what does that mean? Does this apply to me? This is related to the protection factor (PF) of the masks. To determine the eight hour TWA, you have to ask yourself if you believe: Obviously, I used the word believe. I could also say, in your professional judgment.This is due to the fact thatyou didnt monitor during this time and you have to use your judgment based on what you knowwasgoingon at the facility. Go to COSHH essentials for more guidance on this. Those of you whove read my articles before will know that trying to simplify personal monitoring for LARCs could be considered my thing. They are set to help protect workers' health. In order to calculate if the level is being exceeded or when action needs to be taken, you need to work out the time-weighted average of the employees exposed and compare this to the exposure limit set by the HSE. What if the exposure isn't always the same level? It is also applicable to short-term samples, such as a 15-minute TWA. Total Working Hours. First calculate the Noise Dose as: Dose = 100 x (C1/T1 + C2/T2 + C3/T3 + . Testing Lab Analysis: Mold, Asbestos, Bacteria, Legionella, USP. 01/23/1997 - Calculation methods used under the air contaminants. But how do you know if you are exceeding the exposure limit in your workplace? What is a substance hazardous to health? PCM NIOSH 7400 Method with OSHA 8 Hour Time Weighted Average (TWA. EH40 Workplace exposure limits is a publication which contains a list of all current GB WELs. G7 provides STEL based on second-by-second gas readings, with the overall calculation updated every minute. Often, when a personal monitoring test is booked, an analyst concludes that they cant do the four-hour test because the operative isnt in the enclosure that long. If . Find out more about workplace exposure limits (WELs) in COSHH workplace exposure limits (WELs) explained. In this case, the total weight is 13 + 23 + 4 = 40 hours. To calculate it, you divide the exposure by the PF to identify the likely level inside the mask. Simple. Harmful substances are used in nearly every business. eight-hour exposure (ppm), Substituting in the formula, we have: Example: Monitored 5 hours and the lab report says 100 ppm. What makes up the rest of the four hours can therefore come down to estimates and assumptions. In this situation, working sessions may be broken down to different exposure levels in your calculation. Enter 90 dBA, 2 hours and 30 minutes for location 1 and 85 dBA, 5 hours, and 30 minutes for location 2. However, counting efficiency is a function of filter loading, with lower loadings typically resulting in higher proportional concentrations [14-16]. The following formula must be used to determine the equivalent 8-hour TWA exposure limit for a mixture of air contaminants: Em is the equivalent exposure for the mixture. STEL/TWA values remain on the device and are also sent to Blackline Live. So heres my attempt to simplify the whole topic. The pictogram on the label is determined by the chemical hazard classification. The Analysts Guide was finally published in May 2021. CDM guides, tools and packs for your projects. (16) These predictions are coupled with an empirical factor that relates chrysotile asbestos ber to respirable dust concentrations to predict 8-h TWA asbestos ber concentrations. The 8-hour time-weighted average is a legal limit that should not be exceeded. I say new as despite it not being on the list before, it is also the test standard that most licensed asbestos removal contractors (LARCs) try to achieve. COSHH workplace exposure limits (WELs) explained. Threshold limit value (TLV) - time-weighted average (TWA) represents the time-weighted average concentration of a toxic substance over a normal 8-h workday and 40-h workweek, to which nearly all workers may be repeatedly exposed, every day, without adverse health effects. If youve got the computing skills, you can create a spreadsheet that will do all of that for you. This article was written by Emma at HASpod. All air tests have a LoQ which varies depending on the specifics of the test: how hard the pump was working (the sample rate), how long the test continued, and how many graticules were inspected. These are calculated from a worker's daily exposure to noise - they are not based on simple snap-shots of . Time-weighted average (TWA) is a method of calculating a worker's daily exposure to hazardous substances such as dust, fumes, chemicals, gases, or vapors. These negative values will not be applied to the TWA or STEL calculations. Exposure to a hazardous substance can happen in a variety of ways: The main route of entry for hazardous substances (and the hardest to control) is inhalation. By clicking sign up, you agree to receive emails from Safeopedia and agree to our Terms of Use and Privacy Policy. It is essentially a test of how effective the method was. Safeopedia is a part of Janalta Interactive. Example: Assume that Substance A has an 8-hour TWA limit of 100 ppm in Table Z-1. According to 1910.1000(b)(2), an employees exposure to a substance listed in Table Z-2 shall not exceed at any time during an 8-hour shift the acceptable ceiling concentration limit given for the substance in the table, except for a time period, and up to a concentration not exceeding the maximum duration and concentration allowed in the column under acceptable maximum peak above the acceptable ceiling concentration for an 8-hour shift.. workday, even if the 8-hour TWA is below the OEL. What Are All Those Gadgets On A Multitool? | Content Strategist and Consultant, Safeopedia provides a platform for EHS professionals to learn, collaborate, have access to FREE content, and feel supported. LoQ a favourite acronym of us exposure geeks. Note that the HSE example says 2hrs 45mins, but I am going to round down to make it easier, Next comes an hours break which we can assume is nil exposure. The term may also be hyphenated (time-weighted average exposure value). OSHA's PEL for noise exposure is 90 decibels (dBA) for an 8-hour TWA. Add together the time weights to get the total weight. If the person was performing the same task for the rest of the day and you would expect the same concentration all day, then you do not have to change your result. Be able to calculate TWA results from mu ltiple measurements : 2.3 Personal sampling . If you are struggling to wrap your head around personal monitoring, be reassured that it is a complex subject that often feels academic, with no practical benefit. Your result Xis the time on the report / 8 hours or 480 minutes = the 8 hour TWA. These are known as workplace exposure limits (WELs). The goal is to prevent or minimize the release of airborne asbestos fibres. Orinasal masks are rated at 20. [3] PEL can also be expressed as 100 percent "dose" for noise exposure. TWA 3.4ppm x 375 min 480 min TWA 2.7ppm I collected three [3] samples over the work day and the sum of the time equals the actual work/exposure time. Average Daily Dose (ADD) is generally expressed as mass of contaminant per unit body weight over time (e.g., mg/kg-day). Breaks during their shift may stop exposure altogether. Monitored 5 hours and the lab report says 100 ppm. In this case, you will obtain 1716 dB hours. TWA is the average exposure of a contaminant during a specified period of time, usually eight hours. Description. (ppm) (ppm) The longer they look, the more accurate the test. More specifically, that many analysts do what the guidance tells them without exercising a deeper understanding. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features Press Copyright Contact us Creators . Because the TWA is an average there could be times during an eight-hour shift when an employee is exposed to concentrations higher than the established PELs. How well are duty holders managing asbestos? Exposure to a substance is uptake into the body by: Workplace exposure limits (WELs) are GB occupational exposure limits approved by HSE. .Cn Tn)8 For other exchange rates the average level . 8 Where C1 is the concentration of the contaminant in air for exposure period 1 and T1is the exposure time in hours for exposure period 1 and so on If we want to work out the But its often the case that this doesnt last for four hours. Emma has over 10 years experience in health and safety and BSc (Hons) Construction Management. In this case, the total weight is 13 + 23 + 4 = 40 hours. The Health and Safety Executive (HSE) is getting ever closer to the release of its new analysts guide intended to help analysts and their clients comply with the Control of Asbestos Regulations 2012 and its Approved Code of Practice (ACoP). Thank you for subscribing to our newsletter! Fiber means a particle longer than 5 micrometers (m) with a length-to-diameter ratio of at least 3-to-1. The OEL for Asbestos There are two OELs for asbestos; the first is the eight hour time weighted average (TWA) TLV and the OSHA PEL of 0.1 fibers per cubic centimeter of air (f/cc) (reference 3 and 4). Orinasal masks are rated at 20. If you are searching for a substance by CAS number, make sure you put the hyphens in the right place. TWA is the average allowable amount of gas exposure during a given time interval. This is one of the most frequently asked questions by customers once they receive their report. According to 1910.1000(b)(1), an employees exposure to any substance listed in Table Z-2, in any 8-hour work shift of a 40-hour workweek, shall not exceed the 8-hour time weighted average limit given for that substance in Table Z-2. Sum the values that you obtained in Step 1. Required fields are marked *. TWA 3.4ppm x 375 min 480 min TWA 2.7ppm I collected three [3] samples over the work day and the sum of the time equals the actual work/exposure time. Its very hard to achieve meaningful results in these tests. Correct is better, my ego will heal someday.OSHA Extended Work Shift Interpretationhttps://www.osha.gov/laws-regs/standardinterpretations/1997-01-23www.timesdarkcaptains.comwww.gebco.orgIf you would like to support:https://paypal.me/TimesDarkCaptains?locale.x=en_USBTC Receive 19KgBgdEkPdwGkGZ7YMKRaLss7KBC74JTCLTC Receive Li3yQHKqKHD8mdTuuzEXyYe3XQnk9YEMfkETH Receive 0x27608BcC5C7a8cE2bAbB78882a64652206A3B99dDash Receive XyWJtXN9LwcoMVDBot1722LAsg3epGM9jsXRP Receive rah7XddCGB1uJ3nRSwc1sGGXeRtiTYxfDqBTC Cash qq2yqcqurnetne9r6vwn7x58vwt5lhh45y2fkcw8hv Since 81.25 ppm is less than 100 ppm, the 8-hour TWA exposure is acceptable. Z-1 describes limits for air contaminants, Z-2 describes limits for substances that have ceiling levels, Z-3 describes limits to the exposure of mineral dust. In a change from the existing analysts guide, youll see there are now four personal monitoring tests. PCM NIOSH 7400 Method with OSHA 8 Hour Time Weighted Average (TWA) TWA's (Time Weighted Average) are used for personnel sampling. You can check that exposure to hazardous substances is below their WEL by monitoring. Combining the asbestos exposure that occurred when mechanics performed clutch service, along with the duration and frequency of this task, the incremental contribution of this task to mechanics' 8-hr time-weighted average (TWA) asbestos exposures was 0.0016 flcc. Improving the depth and quality of personal monitoring will, I hope, help the HSE reduce exposure in our industry, and improve working conditions for us all. 2 hours 50 ppm toluene exposure, 4 hours of 120 ppm toluene exposure, and 2 hours of 100 . The bad news is that the HSE has stated a sample rate of four litres per minute. Why reinvent the wheel when there are so many ready-made safety observances to link up to? I hope this will solve the problem, but it doesnt get away from the lack of understanding surrounding the four-hour test, how to apply it, and how to calculate the results. Divide the value in Step 2 by the total weights in Step 3, to obtain 1716 / 40 = 42.9 dB. For example: DEQ Asbestos Awareness - Pima County Government Arizona | Tucson. The VOC of coating/colorant/adhesive is the same as the term "regulatory VOC," which is equivalent to the term "VOC, less water and exempts." The VOC of material is the same as the term "actual VOC," which . %PDF-1.4 % Now you will notice some differences between the values of formula#1 and #2. They are intended to be used as risk criteria for health risk assessment and risk management purposes . Add Row. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. In regard to personal monitoring, the HSE seems to be asking why do we never get what we ask for?. The PCM-E represents the fraction of PCM total fibers that is estimated to be asbestos fibers, as opposed to other nonasbestos fibers such as cotton or wood fibers. While thousands of substances come under the COSHH regulations, around 500 have workplace exposure limits (WELs). For weather work at sea we care only about the true wind. ((3x18 mg.m-)+(1x55 mg.m-))/4 = 27.25 mg.m-*. As well as controlling exposure to substances hazardous to health, you need to be aware that legal limits have been set on the amounts of many of the substances that can be present in workplace air. I have attached the result obtained. predict 8-h TWA respirable dust concentrations from joint compound sanding activities. In this case, 2 lpm x 120 minutes is 240 liters. C 40 200. Workplace exposure limits. When workplace safety regulators set permissible exposure limits (PELs), the limit may be based on the average exposure that occurs during an 8-hour work day or a 40-hour work week. We will also explore a very basic application of statistics of. Suite 2525 Privacy Policy - There are several ways to define the wind. swallowing. This eBook covers: Record keeping best practices and exemptions Recordable injuries and illnesses vs. reportable injuries and illnesses When and how to report events OSHA Form 300, OSHA Form 300 A, and [], The Complex World of Flame-Resistant Apparel Flame-resistant (FR) apparel is crucial for workers in a variety of industries. The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. For fiber counting, as in asbestos analysis, the application is similar.. Hygienists need to calculate TWA s for all their sample results because the OSHA PEL. All Rights Reserved, download the Analysts Guide from the HSE, my review of last Novembers FAAM conference, Sample rate must be 1-2 litres per minute, The total volume of air samples must be 240 litres or more, Run an air test during the removal of asbestos insulating board (AIB). Having spoken to the authors I can assure you that they mean up to four. Back to the table. A listing of PELS can be found in OSHAs Standard 29CFR 1910.1000. Well-fitted, powered full-face masks give a protection factor of 40. Heres what I mean: If the exposure lasted 6 hours and you monitored for 6 hours, the result you have is the TWA for the 6 hours. Firstly it is better to think of the four-hour TWA as a calculation, not a test. According to research the total cost of workplace injuries, fatalities, and illnesses adds up to a whopping [], OSHA Record Keeping and Reporting Cheat Sheet Use this OSHA Record Keeping and Reporting Cheat Sheet to navigate the complexities of OSHA reporting. 0000001163 00000 n 4-hour TWA When considering personal exposure, results should be reported as a 4-hour, time weighted average. When this setting is turned on: Resetting peak readings using G7's Gas Options menu does not reset TWA or STEL. Time-weighted average calculations are used in situations where the time of exposure to a hazardous substance or the concentration of that substance varies. 4 hours of 2 fibers/cc asbestos exposure and 4 hours of 0.5 f/cc asbestos exposure b. only for the compound applied in the experiment) is included. OSHA created PELs to protect workers who operate in potentially hazardous environments. Thus, a worker may be exposed to a level higher than the TWA for part of the day (but still lower than the ceiling value) as long as he or she is exposed to levels below the TWA for the rest of the day. TWA, Dose and LEP,d Calculator Help. E = (Ca Ta+Cb Tb+. Click here to download it from the HSE website. Sorry for the inconvenience the HSE cracks down on site facilities, Revisiting the Control of Asbestos Regulations 2012. 0000003217 00000 n The caluclation can also consider particular variables, dose rates, and duration. If a substance has an 8-hour exposure limit, this actually means that the exposure in any 24 hour period is measured over 8 hours. I wanted to add my support to the HSEs renewed push on personal monitoring, so weve added a new TWA tool to the Assure360 solution. Risk assessment C is the concentration of a particular contaminant. (This is the most conservative approach.) HSE aims to reduce work-related death, injury and ill health. is calculated as: Unlike a Running Average, the TWA is an accumulated exposure dose, and thus the value can never . 0000001412 00000 n A second HSE chart helps show how you square the circle: Charts by kind permission of Sam Lord HSE. If you monitored for 12 hours, its a 12 hour TWA; 6 hours, then a 6 hour TWA, etc. Our Paperless app has now been enhanced so that, along with the test result, it will collect three more data points: The Assure360 database will do everything else for you. According to OSHA, when dealing with contaminant exposures to achieve compliance in the workplace, administrative and/or engineering controls must first be determined and implemented whenever feasible. Practical EHS Tips, News & Advice. We will email you when we have something important to share. Em is the equivalent exposure for the mixture. TWA values can be customized in G7's configuration profile, where each individual gas has a customizable TWA period (between 4 and 16 hours). Take our COSHH awareness elearning course and get your certificate today. Lab Request Forms for Chemical Capacity Testing, HCS-501 Instrument Description & Specifications. Here is a review of the formulas used to calculate PELs. Divide the value in Step 2 by the total weights in Step 3, to obtain 1716 / 40 = 42. Testing Lab Analysis: Mold, Asbestos, Bacteria, Legionella, USP. The difference with shorter exposure is you can adjust the workplace exposure limit, providing this does not exceed any other exposure limit such as a short term (15-minute) exposure limit. Example: Employee works for 2.5 hours in 90 dBA and 5.5 hours in 85 dBA. The most common type of workplace exposure limit (WEL) is the 8-hour limit, or long term exposure limit. Although the minimum sampling duration for measuring an 8-hour TWA is not specified in the asbestos standards, several other OSHA substance-specific standards (e.g., arsenic, benzene, cadmium, and coke oven emissions) require full-shift personal sampling to monitor at least seven (7) continuous hours of an employee's work shift in order to be ACGIH/EH40: TWA is calculated by taking the total exposure divided by a fixed interval regardless of exposure time. 0000000516 00000 n Time weighted average exposure values may also be referred to as time weighted average (TWA). Where the exposure limit is exceeded, action must be taken to bring exposure within the set limits for the particular substance(s). WELs are concentrations of hazardous substances in the air, averaged over a set period of time. Levels of 90-140 dBA are included in the noise dose. They follow a World Health Organisation method which assures a level of accuracy that we would all agree is important when we are talking about peoples long-term health. Related content For some reason I went with 2.7. Instead, they do the 10-minute test, the LoQ for which is so high as to be next to useless. The follow-up question is usually, how do I convert my result to an 8 hour TWA? For example, if a worker is exposed to different doses of a chemical vapor for different amounts of time, a TWA calculation can help a safety professional determine the average level of exposure. 6.7ppm; Tl 210 min. To illustrate the formula, values have been inserted to the contaminants below. breathing fume, dust, gas or mist. But at this stage I should give you a spoiler alert: later in this post Ill explain that Assure360 can do it all for you! Professional judgment is Safeopedia Inc. - The 8-hour TWA is a legal limit that should not be exceeded. STEL is the limit of a toxic gas concentration that you can be continuously exposed to for a short time interval without suffering adverse health effects. PM 10 and PM 2.5 are not used for any application. If the car was going at a constant speed for 8 hours and you watched the car go 55 MPH for 6 hours, then you know the 8 hour MPH will be 55. What you might not realise is that this is inside the RPE that you have provided. 2 hours at 75 ppm Because Em is less than unity (1), the exposure combination is within acceptable limits. 182 0 obj <> endobj xref 182 11 0000000016 00000 n The shorthand equation for doing this is simple: Your result X is the time on the report / 8 hours or 480 minutes = the 8 hour TWA. This true wind is the speed and the direction of the wind relative to the fixed earth under the ocean. Copyright 2023 Blackline Safety Corp. All rights reserved. View Full Term. Example risk assessments (Use these to get ideas for completing assessments) Indeed, now that SSDAs are on the list we will be able to remove one of the biggest problems to completing personal monitoring: analysts. In order to assist you in preparing an up to date COSHH Assessment, the most effective way of assessing exposure levels to substances in the workplace is by portable air sampling.A popular way of doing this is by drawing a known quantity of air through a sorbent tube, which is then analysed for contaminants known to be used in the process. Every year, almost 2.78 million workers die from work-related illness and accidents. Fibers < ca. time-weighted average (TWA) A method of calculating a worker's daily exposure to a hazardous substance (such as chemicals, dusts, fumes, mists, gases, or vapors) or agent (such as occupational noise), averaged to an 8-hour workday, taking into account the average levels of the substance or agent and the time spent in the area. The last three hours there was zero exposure. Average 8-h TWA asbestos exposures were estimated to range from 0.010 to 0.062 f/cc. There are three different regulatory bodies that have different ways of calculating TWA: OSHA, ACGIH and EH40. What are 'Dark Factories,' and Do They Really Exist? Note This is not required in an area that is enclosed to prevent the release of respirable asbestos fibres and negative pressure is used in accordance with Regulation 477. So. Every test will be given its four-hour TWA value, and every duty-of-care report will provide the information. The answer is your result is most definitely a TWA. An additional group of PELs were developed for cancer-causing substances. Is COSHH important in your workplace? In my example below Ill round this up to one hour to simplify the maths. Control of Substances Hazardous to Health Regulations 2002 (COSHH), as low as is reasonably practicable (ALARP), Go to COSHH essentials for more guidance on this, the substance will be assessed and analysed by relevant specialists, including an economic assessment, the Workplace Expert Health Committee (WHEC) will act as independent experts to review the scientific basis for any WEL proposed, there will be a public consultation on the updates. Depending on the regulatory body your organization follows, your TWA value may be calculated one of two ways: OSHA: TWA is calculated based on an 8-hour moving window. 120 minutes), the sample volume is determined by multiplying the flow rate by the sampling time. Where substances have been classified as carcinogens, mutagens or asthmagens, to comply with the Control of Substances Hazardous to Health Regulations 2002 (COSHH), exposure must be controlled to as low as is reasonably practicable (ALARP). Instead, they are zeroed out when applied to the calculation so that the false negative readings do not inappropriately lower the result. No representation is made that the information or references are complete or remain current. But while the former is bad enough, the latter is quite soul destroying. The window is fixed at 8 hours and cannot be changed in Blackline Live. For example, where an employee works an 8-hour shift and is exposed to a substance for 3 hours at 0.15mg.m-3 and for 2 hours as 0.20mg.m-3 the time-weighted average can be calculated by: Not everyone works 8-hour shifts. As promised, we need to dive into them. b. By clicking sign up, you agree to receive emails from Safeopedia and agree to our Terms of Use & Privacy Policy. Updated Daily. L is the exposure limit for the particular contaminant found in the Z tables. 0000001999 00000 n Where there is evidence to support the introduction of a new/revised WEL, HSE will follow its own approach for GB workplaces: HSE has considered the evidence on all outstanding EU limits and will introduce a revised limit for 3 of them. C is the concentration of a particular contaminant measured in the workplace. Just as with everything else we do, a small tweak to the front end allows the system to do the heavy lifting for you saving hours. Yes! And we still must not exceed the 8-hour WEL that is given. Safe Work Practices and Safe Job Procedures: What's the Difference? 6.7ppm; Tl 210 min. Delete Row. The mission of EHS On Tap is to provide clear, relevant, actionable information on topics that matter to EHS professionals in podcast form through engaging and insightful interviews with experts and thought leaders. It will check the rules to make sure the test qualifies, and it will automatically do the calculation. Typically, this is eight hours, but short-term exposure periods of 15 minutes are also assigned limit values. The threshold limit value (TLV) is believed to be a level to which a worker can be exposed per shift in the worktime without adverse effects.Strictly speaking, TLV is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH). The more air you sample and the harder you examine it, the more sensitive the test is. Shift 1 1.5 4.2 Total fibers/cc Asbestos fibers/cc Ratio of STEL-TWA to long-term TWA: A WEL is the maximum concentration of a hazardous substance in the air that people breathe, averaged over a specific period of time, referred to as a time weighted average (TWA). Asbestos means chrysotile, cummingtonite-grunerite asbestos (amosite), crocidolite, anthophylite asbestos, tremolite asbestos, and actinolite asbestos. Workplace exposure limits that apply under the COSHH regulations are subject to a time-weighted average (TWA).