So, calculating TWA for maintenance would go something like this based on an 8 hour workday out in the silo equipment area in 570-6 missile site. Concentration of UDMH vapor is considered the same as the 236 minutes by the MSA on level six for the 8 hour time period with no breaks topside or travel time to and from the complex in the 8 hours. This is based on the readings taken on level 6 equipment area of the silo (DM STUDY 1983) and assuming it was the same throughout the silo. This would not assume the same thing in the launch duct for missile maintenance people. We know the UDMH vapor was way greater in the launch duct verses the equipment area.
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TWA = CT1+CT2+CTi / T1+T2+... Ti (DM BIOENGINEERING SHOULD HAVE USED THIS EQUATION TO BE CORRECT FOR JUST THE 8 HOUR TWA CALCULATION) So the real TWA actual measured exposure calculation for the Maintenance Members should have been:
(152µg/m3)(236 min)/236 min = 152µg/m3 converting to ppb (24.45)(152)/60.1 = 61.9 ppb TWA (8 hour only) actual calculated TWA using 236 minutes measured maintenance exposure at Missile site 570-6 in the equipment area .
This is 6 times the ACGIH TWA (10ppb) that is allowed today in 2015. This still does not account for skin intake which is worse than inhalation.
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Here are some other formulas for longer work periods.
Unusual Work Schedules not over 16 hrs/day
Overtime – extra hours – extra days
Compressed work week
- four 10 hour days
- three 12 hour days
Flex time
Daily Adjustment Formula
Adjusted TLV - TWA = TLV x ( 8 )/h x ( 24 - h )/16
Where h = # of hours worked per day
Weekly Adjustment Formula
Adjusted TLV = TLV x ( 40 )/h x ( 168 - h )/128
Where h = # of hours worked per week
Apply the formula that
recommends the greater
reduction
Brief and Scala Example:
Solvent has a TLV of 100 ppm
Worker works four 10 hour days.
What adjustment (if any) would you recommend?
Daily Adjustment Formula
Adjusted TLV = TLV x ( 8 )/10 x ( 24 - 10 )/16 = 70 ppm
Weekly Adjustment Formula
Adjusted TLV = TLV x ( 40 )/40 x ( 168 - 40 )/128 = 100 ppm (no adjustment)
• Daily formula gives 70 ppm
• Weekly formula gives 100 ppm
• Take lower of the two
• Adjusted TLV = 70 ppm
• Gives same protection as 100ppm for five 8 hour days
• Easy to use
• Only need to know hours worked
• Very conservative
• Gives more restrictive recommendations than other models
This work is in progress. As I read the 2003, 2005, and 2012 ACGIH manual I will continue to add more information.
"ACGIH in the 2003 TLVs® and BEIs® refers to the Brief and Scala model (found in Patty’s Industrial Hygiene) as a "tentative" guideline for industrial hygienist dealing with extended work shifts. The Brief and Scala model reduces exposure limits according to a reduction factor calculated by the formulas."
There are four steps to determining how much over exposed to the toxic fuels each of us were based on the DM AFB study of normal UDMH, NDMA, and Hydrazine concentrations in the silo and launch control center.
First, we must modify the 1995 UDMH 10 ppb TLV-TWA ACGIH allowed limits based on work schedule. The 10 ppb is based on a normal 8 hour work shift per day five days a week. There were very few jobs around the Titan II missiles that had that typical civilian work schedule (Bankers hours). The 10 ppb TWA on Hydrazine and UDMH must be reduced according to ACGIH guidelines.
Second, we must account for the other toxic chemical TLV-TWA's and adjust their limits.
Third, we must then account for all the other toxic breakdown chemical TLV-TWA's including the skin exposure which is worse than inhalation.
Fourth, we must figure out our actual exposure based on our travels throughout the Titan II missile Complex based on the DM Titan II complex toxic study of 1983. See botched DM study.