WL = ( ER X Radon Concentration ( pCi / L )/ 100
Long‐term lung cancer risk from residential radon exposure then is equal to the WL of the residence multiplied by the time of exposure . This calculation is known as the Working Level Month ( WLM ). The number of hours in a working month is 170 ‐ this is based on 8 hours a day for 21 working days per month . Lung cancer risk over time is calculated based on an individual ' s cumulative WLM value . WLM = ( WL X Hours of Exposure )/ 170 working hours in a month
Let ' s look at the increase in radon dose for some common radon levels for various number of hours spent at home
. Table 1 lists the annual radon exposure in working level months ( WLM ) of four common radon concentrations in homes : 1 ) 1 . 3 pCi / L ( the EPA ' s estimated national indoor radon average ), 2 ) 4.0 pCi / L ( the EPA ' s Radon Action Level ‐ 38 % of
Kansas homes test over this level ), 3 ) 8.0 pCi / L and 4 ) 20 pCi / L . The dose is calculated at four average daily exposure hours in the home : 1 ) 8 hours at home / day , 2 ) 12 hours at home / day , 3 ) 18 hours at home / day and 4 ) 24 hours at home / day . The calculations assume an
ER = 0.5 or that half the radon‐ released particles are in the air available to breathe .
If time at home for a full year increases from an average of 12‐hours / day to 18‐ hours / day , the annual radon dose in WLM from the radon levels in the home increases by 50 %, or by one half ! For every additional 1 hour per day spent at home at the same radon level , the annual radon dose increasess by about an average of 7 %.
Table 1 . Annual radon dose when spending 8‐ , 12‐ , 18‐ , or 24‐ hours in a home with 1 . 3 , 4.0 , 8. 0 , or 20. 0 pCi / L radon levels . ER = 0.5