# EPA 8 hour TWA?



## assuraance2884 (Mar 14, 2011)

I have a home with formaldehyde in the air at .15 parts per million. The air sample was taken over a 45 minute period. How would an 8 hour Total Weighted Average (TWA) be calculated for this home given spending 20 hours a day, seven days a week, in the home? Thank you, posting it forward.


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## 1002richards (Jan 29, 2006)

Is this your homework by any chance?


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## assuraance2884 (Mar 14, 2011)

How many students do you know who have specialized knowledge in Indoor Air Quality? How many students do you know who can even spell formaldehyde? How many people do you know who know what a TWA is? If you have 4,000 post were all of them so impertinent? Please don't waist our time with a reply like you did.


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## 1002richards (Jan 29, 2006)

I was not trying to waste your time but people do try to get their homework done by others. I asked a reasonable question in a polite manner, so there's no need to be rude. I'm still curious though, if it's not homework then what is it?

Good luck in your quest.


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## assuraance2884 (Mar 14, 2011)

I am trying to determine if this house is higher or below the OSHA Permissible Exposure Limit (PEL) 8 hour Time Weighted Average (TWA) of .75 ppm. When this home goes up for sale, the answer to the math will determine if the PEL has to be included in Michigan Disclose Act. I an not a lawyer and am not looking for a legal response. I can work the problem with several different answers.

.15 pp in 50 minutes =
.15 is to 50 as x is to 60 minutes
.15 x 60 / 50 = .144 ppm per hour
.114 x 8 hours =1.15 ppm This is above the OSHA PEL of .75 ppm, but is the answer a time weighted average.

Being senior citizen the actual time in the house is closer to 20 hours per day. So would it be correct to use:

.114 x 20 hours = 2.28 ppm. but is this a time weighted average. Sources on the internet give formulas that divide by 8. Is a time weighted average a "per hour" number?


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## valis (Sep 24, 2004)

okay, you are looking at (according to your original post):

.15 ppm (a) over 45 minutes (b), or .75 of an hour (c). Seeing as how .75 is 3/4 of .100, divede .15/3 and tack that on to the end. 

so you are looking at .2 ppm over 60 minutes. The rest is just math, unless I'm missing something.



And take it easy; you'd be astonished at how many homework questions we get here. While we love teaching, unless the student learns anything, then I reckon the lesson is lost.

And furthermore, my math is a bit rusty; I was an English major, but this still seems pretty straightforward.

thanks, 

v


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## assuraance2884 (Mar 14, 2011)

Perhaps this is the wrong forum, but the Time weighted average has not been addressed. Is a TWA a common mathematical formula other than in indoor air quality? If a TWA an accumulation of exposure: .15 + .15 + .15 ...? Or some other calculation?


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## paisanol69 (Sep 7, 2005)

assuraance2884 said:


> I have a home with formaldehyde in the air at .15 parts per million. The air sample was taken over a 45 minute period. How would an 8 hour Total Weighted Average (TWA) be calculated for this home given spending 20 hours a day, seven days a week, in the home? Thank you, posting it forward.


....a TWA is a government standard based on an 8 hour workday, for either airborne contaminants, or noise.



> Time-Weighted Average (TWA)
> TWA is the concentration of a contaminant averaged over a workday (usually 8 hours long). It's measured in a workplace by sampling a worker's breathing zone for the whole workday.
> 
> To determine the level of exposure, the TWA reading is compared with a standard such as the Threshold Limit Value (TLV) or Permissible Exposure Limit (PEL). If the reading exceeds the standard, overexposure occurred.
> ...





> Example TWA
> Below is a graph of concentration over an 8-hour day at a hypothetical workplace. During this day, the TWA and the Ceiling value A concentration of a toxic substance in air that should not be exceeded at any time. were not exceeded:
> 
> Excursions of concentration above the TWA line were balanced out by periods when concentrations were below the line
> ...


However, it should be noted that a TWA cannot be used in a situation where say, a 15 minute average exceeds the ceiling value of the contaminant:



> Short-term exposure limit (STEL)
> A STEL is a 15-minute TWA concentration that may not be exceeded, even if the 8-hour TWA is within the standards.
> 
> TWA-STEL are given for contaminants for which short-term hazards are known. For the rest, an excursion factor of 3 often has been used: STEL should not exceed 3 times the TWA limit.





> I am trying to determine if this house is higher or below the OSHA Permissible Exposure Limit (PEL) 8 hour Time Weighted Average (TWA) of .75 ppm. When this home goes up for sale, the answer to the math will determine if the PEL has to be included in Michigan Disclose Act. I an not a lawyer and am not looking for a legal response. I can work the problem with several different answers.
> 
> .15 pp in 50 minutes =
> .15 is to 50 as x is to 60 minutes
> ...


so..in summary:

You must take an 8 hour reading of the ppm formaldehyde in the air in the home.
If during that sampling time, the PPM of the formaldehyde does NOT EXCEED the ceiling value of the contaminant-


> I have a home with formaldehyde in the air at .15 parts per million


you state that the maximum ceiling of formaldehyde in the air is TWA .75 PPM.(PEL) If that is correct, and you perform an 8 hour sampling of the home, and you never exceed that .75PPM at any time during the sampling period, then you do not have a problem.

The TWA for that 8 hour sampling period becomes your TWA, irregardless if you live in the house for 20 hours a day, or 1 hour a day, since it is exactly what it says it is.._a time weighted average_.

There is no need to calculate for 20 hours of exposure!


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## 1002richards (Jan 29, 2006)

assuraance2884 said:


> I am trying to determine if this house is higher or below the OSHA Permissible Exposure Limit (PEL) ....


Thanks for explaining, not something we have to get to grips with in the UK (for now ...)!

Richard


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## assuraance2884 (Mar 14, 2011)

Thanks guys, I'm off the net and happy to have a satisfactory explanation. Final post reply.


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