Working level month exposure

Working level month (WLM) An exposure ot I WLM can be taken to be received by a person working in a Radon daughter concentration of 1 WL for 170 hours. 

Working Level Month (WLM)—A unit of exposure to radon daughters corresponding to the product of the radon daughter concentration in Working Level (WL) and the exposure time in nominal months (1 nominal month =170 hours). Inhalation of air with a concentration of 1 WL of radon daughters for 170 working hours results in an exposure of 1 WLM. 

A survey of the radon concentrations in a representative sample of more than 2000 dwellings in the UK has been completed and provisional results are now available. On average, concentrations are 29% lower in bedrooms than in living areas. The mean radon concentration weighted for room occupancy is 22 Bq m 3. Assuming an equilibrium factor of 0.35 and a mean occupancy of 75%, the mean annual exposure in UK homes is assessed as 0.08 Working Level Months (WLM) and the mean annual effective dose equivalent as 0.43 mSv. 

An equilibrium factor of 0.35, derived from measurements made during the local surveys, has been assumed to typify conditions in UK dwellings. This value has been used to convert the average radon concentrations measured in the national survey to potential alpha-energy concentration of radon decay-products. On average, persons in the UK spend 75% of their time in their homes and 15% of their time elsewhere indoors (Brown, 1983). The occupancy factor of 0.75, together with an equilibrium factor of 0.35, results in an annual exposure of 1.3 10"5 J h m"3 (0.0037 Working Level Months,... 

A working level month assumes exposure only for the working period of a month, 170 hours, whereas exposure may occur for all of the month. 

In the uranium mining industry, the Working Level (WL) is defined as a concentration of decay products having PAEC equal to that of decay products in equilibrium with 100 pCi l-1 (3.7 x 103 Bq m-3) of radon. The Working Level Month (WLM) is defined as exposure to decay products equivalent to 1 WL for 170 h, this being the nominal number of hours worked per month in a mine. The WLM is still used in discussions of the epidemiology of lung cancer in relation to exposure to radon and its decay products. 

Most cancers from radon were produced by radon daughter decay products (polonium - 3 isotopes bismuth - 1 isotope and lead - 3 isotopes). ERR (excessive relative risks) of cancers varied, ranging from 0.002 to 0.08 per working level months (WLM) of 170 h of exposure. One WLM is about 200 pCi per liter in a home and 300 pCi per liter in an imdergroimd mine. Relative risk increases from 1 to 10 at 2,500 WLM and 16 at twice that exposure level (5,000 WLM) in the Colorado cohort (Harley, 2008). 

The greatest risk was at 5-15 years after initial exposure later risk fell to 0.5 of that level and in those aged 55-64 to 0.4 of that level. Cancer fatalities were thought to be 0.0003 per working level month. Smoking may have had a modest effect. For example, there are 15,000 lung cancer cases in people exposed to radon and radon daughters, 10,000 in those who have smoked and 5,000 in those who have never smoked (Harley, 2008). 

Several different dosimetric models have been developed to calculate radiation dose due to radon exposure taking into accoimt all the various parameters involved. Such models are generally complex and beyond the scope of this text. In most epidemiological studies, dose figures for the exposed population are not available and such studies are carried out using exposure level — in terms of working level months (WLM) — rather than radiation dose... 

ERA recommends that all homes should be monitored for radon. If testing shows levels less than 4 picocuries radon per liter of air, then no action is necessary. For levels above this, follow-up measurements should be taken. If follow-up levels are 20 picocuries radon per liter of air or higher, the home owner should consider some type of procedure to decrease indoor radon levels. The Mine Safety and Health Administration (MSHA) uses a standard of 4 Working Level Months (WLM) per year for people who work in mines. (Working Level Months combine the amount with length of exposure.) You will find more information on guidelines and standards in Chapter 7. 

An additional unit of measurement used to describe human exposure to radon and radon progeny is the Working Level Month (WLM), which expresses both the intensity and duration of exposure. One WLM is defined as the exposure of a person to radon progeny at a concentration of 1.0 WL for a period of 1 working month (WM). A working month is assumed to be 170 hours. The S.l. unit for WLM is joule- hour per cubic meter of air (J-h/m ) 1 WLM is equal to 3.6x10 J-h/m. ... 

The cumulative exposures for miners have been based on the unit working level month (WLM),... 

The radon concentration indoors in terms of radiation dose exposure is expressed in WL (Working Level, the radiation level of 100 pCi per litre or 3700 Bq per m of Rn in equilibrium with its decay products). Effects of radon are given in terms of WLM (Working Level Months), which is the exposure at 1 WL for one working month, or 170 h. Since there are 365 X 24 = 8760 h per year and 80% of them (7000 h) are spent indoors, the annual time of exposure is 7000/170 = 41 working months . A world-wide representative value adopted by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 1977) is that 1 pCi per litre or 37 Bq per m is the average Rn concentration indoors, with an equilibrium factor (ratio of Rn decay product concentration to their concentration in radioactive decay equilibrium with Rn gas) of 0.5. This corresponds to an average concentration of 0.005 WL or 5 mWL. The annual indoor exposure is then 0.005 WL x 41 WM = 0.205 WLM. 

Several studies have reported antibodies being produced in response to an HDl inhalation exposures. In one study, 149 men and 1 woman were selected to prospectively evaluate any clinical signs of illness associated with HDl exposure and, by blood antibody production, to both HDl and HDl trimer. These workers were employed in a factory that spray-painted trucks with paint containing HDl and HDl trimer. Questionnaires were distributed that asked about clinical symptomology, blood samples were taken periodically for antibody determination, and the ambient air in their work environment was sampled for HDl and HDl trimer concentrations over an 18-month period. Serum samples were analyzed via an ELISA for antibodies to HDl and/or HDl trimer. Mean levels of exposure for both HDl and HDl trimer were found to be extremely variable. For the HDl monomer, the levels were between <0.08 and... 

Then, from the worker exposure data, an average and a maximum exposure level for each type of work activity involved with pesticide use is taken into consideration. The number of workers in each job category, the total yearly body dose (which may be derived from number of hours/days/months exposure in performing the job), and the total dermal absorption is all taken into consideration. Residue levels (including degradation products) found in treated crops or a particular product are used to assess consumer risk. 

Tb allium, which does not occur naturaHy in normal tissue, is not essential to mammals but does accumulate in the human body. Levels as low as 0.5 mg/100 g of tissue suggest thallium intoxication. Based on industrial experience, 0.10 mg /m of thallium in air is considered safe for a 40-h work week (37). The lethal dose for humans is not definitely known, but 1 g of absorbed thallium is considered sufficient to kHl an adult and 10 mg/kg body weight has been fatal to children. In severe cases of poisoning, death does not occur earlier than 8—10 d but most frequently in 10—12 d. Tb allium excretion is slow and prolonged. For example, tb allium is present in the feces 35 d after exposure and persists in the urine for up to three months. 

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