Laundered Shop Towels: 2 - A flaw in the model

Here is what Gradient concludes in their 2011 Study:

Metals on shop towels can get onto hands and then potentially be ingested, as evaluated in the 2003 report and as developed in this evaluation.

For typical use of 12 towels a day per person, exceedances of Proposition 65 limits, and US EPA and ATSDR toxicity criteria may occur for antimony, beryllium, cadmium, cobalt, copper, lead, and molybdenum.  Calculated intakes for these metals were up to 3,600-fold higher (based on maximum intake concentration for lead) than their respective toxicity criterion.

Notice the word "can."  Let's look at how that word is defined by Websters:

"be physically able to."

That word "can" is important because it is the basis behind their model, which is as follows:

• Laundered shop towels contain heavy metals - even after they have been washed.
• The heavy metals in/on the shop towels can get onto the hands.
• The heavy metals on the hand can get into the mouth.
• The amount of heavy metals entering the mouth may exceed California Proposition 65 limits, and EPA & ATSDR toxicity criteria.

Here is the graphic from the 2003 Gradient Study on laundered shop towels - their model:

2003 Gradient Study

There are two important assumptions made here by gradient.

1. The metals on/in the towel can be dislodged onto the hand
2. The metals on the hand will be transferred to the mouth each and every time a towel is handled.

These two assumptions are very important in evaluating the validity of the intake values used to determine the exceedance with Proposition 65, EPA, and ATSDR toxicity criteria.

For these posts I am only going to focus on lead since that is the one heavy metal with the greatest exceedance.

Let's look at the first assumption: The metals on/in the towel can be dislodged onto the hand.  Gradient is basing this on the findings from their 2003 Study on the same topic.  Here is what they base this transfer from the towel to the hand on:

For ingestion exposure via hand contact with the laundered shop towels, we estimated transfer of metals from laundered shop towels to hands based on empirical data regarding transfer of pesticide residues from surfaces to hands, data regarding the number of laundered shop towels used daily per person, as well as an estimate of the percentage of the towel surface area that would contact the hand.

The amount of metal transferred to the hand that could ultimately be ingested was based on a hand-to-mouth transfer efficiency, using methodology developed by the U.S. Consumer Products Safety Commission (CPSC) for evaluating exposure to dislodgeable residues on treated wood surfaces

Gradient is basing their intake values on a model that assumes the lead concentration they determined to be present in shop towels can be dislodged from the towel onto the hand.

It is reasonable here to challenge this assumption based on the following:

Is it reasonable to assume that a shop towel that has been washed in hot water, with a detergent, then dried under heat, can dislodge lead onto the hand?

Gradient is basing their model on a CPSC study that looked a dislodgeable reside and used the same value of dislodgement in their calculation for the "Towel to Hand" transfer rate.  You can read how Gradient justifies their value of "13%" by reading the paragraph on towel to hand transfer on page 9 of the 2011 Study.  Here are the studies they looked at:

2011 Gradient Study

The question is (and I think it appropriate) should a comparison be made using transfer rates involving pesticide residue and dislodgeable residues with what could come off of a towel that has been washed with soap, rinsed, and heat dried?

The basis of their model is that heavy metals on the rag can get onto the hand and into the mouth.  If the heavy metals are not transferred to the hand, exposure took place but transport into the worker did not.

Without the ability to show that a towel - washed in soap and dried under heat - can transfer the lead onto the hand, the model is not appropriate and the intake values calculated are erroneous.

This could have, with relative simplicity, been evaluated by Gradient.  If we are looking at the lead coming off the towel, soaking the towel in water or a saline solution would give some idea of the amount of lead that could be dislodged onto a wet hand.  Additionally, the towels could have been handled aggressively by a test subject and the hands swabbed to see what, if any, residue came off the towel.  Both of these methods would have derived a value of lead that would be available to be transferred to the mouth.

Just because you have exposure does not mean you will have a health risk. There must be intake.  In order for the Gradient model to be valid, the lead must be transported from the towel to the hand.  They have not shown this to take place, only showing that dust and pesticide reside can be transported from a soft surface to the hand.

I cannot agree with their findings based on this one condition alone.  But that would make for a pretty short series of posts if I stopped now.  And besides, where's the fun in that?

So let's assume that the necessary assumption that the metals can be transported from the towel to the hand does, indeed, take place.  The next question becomes" Is the transfer rate of 13% of the lead from the towel to the hand valid?

Once again we are back to square one.  If we assume that the towel can transfer the lead to the hand, then we also have to assume that the transfer rate is similar to that found with dust and residue based on the studies Gradient looked at (see Attachment A graphic above).  The value of "13%" was based on:

Several studies looked at multiple compounds and found different transfer efficiencies depending on the compound being evaluated.  Within each study, we averaged the various relevant transfer percentages; they were averaged separately for studies conducted with dry vs. wet hands.  In reviewing the current literature, transfer to moist hands (average 20%) is four times higher than transfer to dry hands average 5%).  Workers are likely to come into contact with RSTs with both dry and moist hands.  Therefore, we averaged the transfers to moist hands and dry hands separately before averaging the two averages, to equally weight the results from both categories.  This value (13%) is more than double the transfer efficiency used in the 2003 evaluation (5%). (1)

In order to move on, one must agree with Gradient's assumption that 13% of what ever is on the washed towel's surface can be transported onto the hand - based on the surface area of the towel the hand comes in contact with.  If you can live with this assumption, then their model holds and a "Towel to Hand" transfer rate (Tt/h) holds true as well.  If you find this particular assumption a bit hard to accept, well you can ignore the rest of these posts and throw their study into the trashcan.  In order for the assertion that lead exceeds an amount by 3600 times, the lead MUST leave the towel and attach to the hand.

That 13% transfer rate is critical in determining the intake they use to compare against Proposition 65, EPA, and ATSDR toxicity criteria.

I contend that the laundered shop towels will not transfer any heavy metals to the skin under normal shop towel use.  Additionally, I contend that any dislodgeable heavy metals, such as lead, that remains on the towel after washing in soap and drying would transfer onto the skin at a rate well below 1%.

Of course I have nothing to prove that contention with.  So it's Bowman "0", Gradient "1"

But not for long.  I have this little thing called "statistics" to help me out in my contention that there is no additional risk to a worker who uses a laundered shop towel.  Unless they were to maybe eat twelve towels a day....but that's for another blog post.

Next post: Laundered Shop Towels:  Is it mean to ask for a median?

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