Laundered Shop Towels: 6 - Finger Lickin' Good!

The basis for Gradient's model is that each laundered shop towel will transfer 13% of the load from 75% of the towels surface area onto the hand.  The hand will then be placed to the mouth and 13% of what is on the hand will be transferred into the mouth.  That second "13%" transfer is what Gradient calls a hand to mouth efficiency - "HTE" - value:

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)

Here is how Gradient came up with this value (excerpt from their 2003 study)

Daily Hand to Mouth Transfer Efficiency. To estimate the amount of metal on the hands that might be ingested via hand-to-mouth contact, we used a hand transfer efficiency, or HTE parameter, of 0.13. 

The HTE parameter quantifies the fraction of material on the hands that is likely to be transferred to the mouth and ultimately ingested. 

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.

Gradient used the median skin surface area data specific to a 1- to 6-year-old child and applied the soil AF derived from Roels et al. (1980) to estimate the average mass of soil on the hands for a 1- to 6-year-old child, which is approximately 145 mg for both hands. 

Gradient then combined the estimate of soil loading on the hand with an estimated soil ingestion rate to derive the hand transfer efficiency (HTE) value, which is an estimate of the fraction of the mass of soil adhering to the hands that would need to be ingested to yield the estimated daily soil ingestion rate. 

Read that last paragraph again.  I'll wait.  And while you are reading it, here is some music to set the mood.  With that premise, Gradient came to this:

A median soil ingestion rate of 38 mg/day for children ages 1 to 6 years was calculated based on a soil ingestion study conducted in Amherst, Massachusetts. 

This soil ingestion rate was divided by the hand soil-loading estimate for a child resident (for a child resident (approximately 145 mg on both hands), for a daily HTE value of approximately 0.26 hand loads per day.

If you are reading this and paying close attention you will see what they have done and why it should be viewed as inappropriate for this study.  If you want to read about the Amherst, Massachusetts, you can see a summary about it here.  Let's look at the first part of the abstract for the Calabrese & Stanek paper:

Sixty-four children aged 1-4 years were evaluated for the extent to which they ingest soil. [t]he present study included a number of modifications from the Binder et al. study. The principal new features were (1) increasing the tracer elements from three to eight; (2) using a mass-balance approach so that the contribution of food and medicine ingestion would be considered; 

See it? 

"contribution of food and medicine ingestion would be considered"

The "median soil ingestion rate of 38 mg/day" is based on the amount of soil consumed from ALL sources, not just from the hands.  Gradient has based the HTE on 38 mg/day of soil intake coming from the hands only.

For this to be true, the soil on the hands, 145 mg, would need to be placed on the hands - no more - no less - and no other soil consumed in the day.  To obtain an HTE of 0.26 all the soil intake had to come from the hands - both of them.

Gradient assumes that the child licks, touches, contacts the mouth with both hands so that 0.26 of the soil on both hands is transferred to the mouth.  The 38 mg/day is from all sources, including dust, mouth soil on surfaces, food, and contact with other sources throughout the day.

Gradient goes on to say:

In this report, we used half of 0.26 as the HTE value for adults, or 0.13. The smaller HTE value used for adults reflects the reduced hand-to-mouth behavior in people greater than 6 years of age. 

Using a smaller HTE for adults as compared to children is further supported by the USEPA soil ingestion rates: their recommended mean soil ingestion rate for adults is exactly one-half of the value for children less than 6 years of age.

I assume that the HTE is the same for one hand (their model) as for both hands.  Basically, as I understand it, Gradient assumes that 13% of what is on the hand will be transferred to the mouth.

You can see where they came up with that 13%.  It assumes that because 38 mg/day of soil is consumed for a child, and an adult consumes half that amount, it all had to come from the hand.

That's not true of course, soil consumption comes from many sources in a day, not just from the hands.  But I'll go with it for now...I'll assume that there is a transfer rate of 13% of the load on the hand into the mouth.

And once again I am perplexed to understand how that would happen.  What's the mechanics involved?

Kimberly-Clark tells us that "an average person touches their face 16 times per hour."  Does that mean a worker places one hand to his mouth each time he picks up a laundered shop towel?

And when the worker places his hand to his mouth, does he lick his fingers or rub his lips over the surface of the hand that contacted the towel?  And if this happens each and every time 12 towels are used per day, how much area of the hand would need to contact the mouth so that 13% of the load that is evenly distributed on the hand is transferred to the mouth?

For Gradient's model and equation to hold true, the hand must contact the mouth and 13% of what is on the hand must transfer to the mouth.  How?

Next post: Laundered Shop Towels: 7 - Fun with Graph Paper


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