Laundered Shop Towels: 5 - That's quite a load

I've tried to make a case in my last four posts about why the concentration of contaminants used in Gradient's equation are most likely higher than what would be normally found in a laundered shop towel.

Additionally, because Gradient used the actual maximum concentration they detected instead of a statistical maximum "exceedance ratio," the maximum intake value is calculated higher than is statistically possible.  I've dealt with a lot of contaminated rags in my career, and I rarely see heavy metal levels that high in dirty rags!  Those concentrations identified by Gradient are for laundered shop towels - clean ones!

Without seeing the actual analytical reports (which I have requested from Gradient) I have to assume that the values identified in Table 2 of their 2011 Study are actually what was found.

Let's look at lead for example.  If lead has a mean concentration of 100 mg/kg and a maximum concentration of 600 mg/kg, then I will assume that those values are accurate.

Once again, we need to assume that these values accurately reflect what would indeed be found on a laundered shop towel.  If you can't accept this assumption as valid, then you cannot proceed to the next step in the model they are using.  Gradient has calculated an intake based on a model that proposes that the laundered shop towel transfers its heavy metals to the hand and the hand then transfers them to the mouth.

With that model and equation you can calculate the uptake which is what they call intake.

Gradient has developed an equation to calculate the intake expected for a worker handling these laundered shop towels.  That equation looks like this:

The first parameter, Load, is based on the following assumptions (see study Table 2 notes):

• The laundered shop towel contains either a mean or maximum amount of the heavy metal in mg/kg
• The laundered shop towel weighs 0.0283 kg
• The laundered shop towel has a surface area of 2,268 square centimeters (cm2)  

The Load - in mg/cm2 - is derived from the following:

[mean or maximum concentration x 0.0283] / 2,268

For example, the average (mean) Load for lead is:

[100 mg/kg x 0.0283 kg] / 2,268 cm2 =  0.0012 mg/cm2

What that lead Load value represents is the following:

The contaminant is evenly distributed on the laundered shop rag so that each square centimeter (cm2) of cloth contains 0.0012 mg of lead.

This is important because the hand to mouth transfer is based on how much of the cloth will contact the hand (Ftowel) and how much of the contaminant will be transferred from the cloth to the hand (Tt/h).

There is another assumption here that is implicit.  That is, the contaminant is not spread evenly on each side of the cloth, but instead is assumed to be on whatever side that will come in contact with the hand.  There is nothing wrong with that assumption, but for that to be true, other assumptions will need to be true as well.

How much - and where - the contaminant is on the towel does not matter at this point.  What matters is the assumption that it is evenly distributed so that the three other parameters...

1. Ftowel = Fraction of towel in contact with hand (unitless);
2. Tt/h = Towel to hand transfer (unitless);  
3. HTE = Daily hand-to-mouth transfer efficiency (day-1)

...will work to derive an amount of metal entering into the worker's mouth.

You can read Gradient's Study to see why certain values were used by the authors. For example:

• Gradient "assumed," based on professional judgment, that the hand (single) would contact approximately 75% of the total surface area of a laundered shop towel, under typical laundered shop towel usage.

What Gradient basis their intake values on involves a worker coming into contact with 75% of the Load (Ftowel).  For the heavy metal lead, it would be calculated as follows::

• 0.0012 mg/cm2 x 2268 cm2 x 0.75 = 2.72 mg = Ftowel

What that Ftowel value represents is the amount of lead the worker is exposed to when his/her hand comes into contact with the laundered shop towel.  Exposure represents the total (mean or maximum) heavy metal available for intake.  Gradient's model now assumes that the hand WILL make contact with the mouth each time a laundered shop towel is handled.

Gradient assumes that even though the towel contains a mean total amount of lead (2.83 mg), only 75% of that lead will come in contact with the hands.  That is, only 2.72 mg is available to be transferred from the towel, based on the lead being evenly distributed on the laundered shop towel at 0.0012 mg/cm2.

The next part of the equation is where their assumptions really start to be stretched to the point of implausibility.  Gradient assumes that the hand (single) comes in contact with the 75% of the towel and 13% of the contaminant on the towel is then transferred to the hand.  This is what they call "towel to hand transfer," referenced as "Tt/h" in the equation.

There is nothing wrong with this logic, unless you take issue with the fact that these towels have been washed in hot water, with soap, and dried under a high temperature. To assume that 13% of whatever contaminants are in the towel can come off onto the hands is a bit of a jump here. (see post)

OK, so let's give them a 13% transfer from the towel to the hand - for now.  The next parameter, "daily hand-to-mouth transfer efficiency" (HTE) is where you really need to stretch your beliefs.

Gradient's model and calculation assumes that each worker that picks up a laundered shop towel will transfer 13% of what is on the towel onto the hand - and then - 13% of what is on the hand will be transferred into the mouth.

So for lead, that amount transferred to the mouth would be calculated as follows:

• 2.72 mg x 0.13 x 0.13 = 0.046 mg

That number, 0.046 mg, is the amount of lead Gradient's calculation determines the worker will ingest when they use one single laundered shop towel.

How did they come up with that second value of 13%?  Read their Study, but I'm going to tell you that its origination does not matter.  What matters is this:

How will the lead on the hands be transported into the mouth?

What other studies have calculated are not important unless they involve workers who use shop towels or some other contaminated material.  How does the hand transfer the contaminant to the mouth?  Gradient assumes that it takes place - and they assume it happens each and every time a worker uses a laundered shop towel.  They call this value in the equation the Hand to Towel Exchange ration (HTE)

Rule Number 5: Always make sure the model and equations reflect reality.

What Gradient says about their HTE value is this:

The HTE transfer is based on estimates of the amount of soil transferred by children from the surface of their hands to the mouth, where it is subsequently ingested, but is adapted for adults, based on a lower HTE value, to be consistent with the lower ingestion rate of adults. (Page 9)

Gradient's assumption is that because children transfer soil from their hands to their mouth, adults will do so as well - but at a lessor rate.  So 13% of the load on the hands will be transferred to the worker's mouth.

Once again I need to ask: How?  How does a worker get the lead off of their hand and into their mouth?

Next Post: Laundered Shop Towels: Finger Lickin' Good!


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